The Nature and Nurture of Sex and Gender - Becoming Gendered: Biological and Social Factors

The Psychology of Sex and Gender - Jennifer Katherine Bosson, Joseph Alan Vandello, Camille E. Buckner 2022

The Nature and Nurture of Sex and Gender
Becoming Gendered: Biological and Social Factors

Caster Semenya is a world-class track athlete and Olympic gold medalist for South Africa.

Source: Getty Images / Lukas Schulze / Stringer

Test Your Knowledge: True or False?

· 3.1 Sex (whether a person is male, female, or intersex) is shaped entirely by biological factors.

· 3.2 Genetically female (XX) individuals who get exposed to high levels of androgens in utero can have male-typed genitalia at birth and are sometimes raised as boys.

· 3.3 All female individuals have an XX pair of chromosomes, and all male individuals have an XY pair.

· 3.4 From twin studies, we know that gender identity is determined primarily by socialization (nurture).

· 3.5 In the past decade, neuroscientists identified structural sex differences in the brain and linked them definitively to specific psychological sex differences.


Nature Versus Nurture or Nature and Nurture?

· Gene-by-Environment Interactions

· Epigenetics

· The Microbiome

How Do Nature and Nurture Shape Sex Differentiation?

· Typical Sex Differentiation

o Chromosomes and Genes

o Hormones and Anatomy

· Intersex Conditions

o Journey of Research: Unlocking Genetic and Hormonal Contributions to Sex

o Chromosomes and Genes

o Hormones and Anatomy

o Debate: Should Intersex Individuals Be Allowed to Compete in Athletic Competitions?

How Do Nature and Nurture Shape Sex Assignment and Gender Identity?

· Optimal Sex

· Gender Identity

· Gender Confirmation Procedures

What Do Sex Differences in Brain Structure Reveal?

· Sex Differences in the Brain

· Equating the Brain With “Nature”

· Neuroscience or Neurosexism?

How Do Theories of Sex Differences Account for Nature and Nurture?

· Evolutionary Psychology

· Biosocial Constructionist Theory


Students who read this chapter should be able to do the following:

· 3.1 Explain how nature and nurture interactively contribute to the development of sex and gender.

· 3.2 Explain how chromosomes, genes, and hormones shape sex differentiation in both typical and atypical (intersex) cases.

· 3.3 Analyze the biological and sociocultural factors that shape sex assignment and gender identity.

· 3.4 Evaluate evidence for sex differences in the brain and the prevalence of neurosexism.

· 3.5 Examine the roles of nature and nurture in theories of the origins of sex differences.


In 2009, a teenage South African runner, Caster Semenya, burst onto the world track scene when she won gold at the World Championships in the 800-meter race. Caster quickly gained notice not only for her dominance in track but for her appearance as well: Because she has a masculine, muscular appearance, rumors spread that she might not be a woman. Pierre Weiss, the general secretary of the International Association of Athletics Federations (IAAF), expressed doubts about Caster’s sex: “She is a woman, but maybe not 100%” (Longman, 2016). A day after Caster won the gold, the IAAF ordered her to undergo a sex verification test, and she was forced to withdraw from competition for almost a year based on her test results. Although Caster’s results were never officially published, leaked reports suggested that she had internal testes, no uterus or ovaries, and unusually high levels of testosterone and other male hormones. In short—if the leaked reports are to be trusted—Caster Semenya is intersex in a world that largely recognizes only two sexes. By intersex (not to be confused with transgender; see Chapter 1), we mean that the biological components of sex do not fit either the typical male or the typical female pattern.

Intersex Individuals for whom the biological components of sex (chromosomes, hormones, and internal and external genitalia) do not consistently fit either the typical male pattern or the typical female pattern.

What is sex verification testing? Just like it sounds, sex verification tests seek to determine a person’s sex through various indicators. The earliest method of athletic sex verification, spurred by suspicions that some Soviet and Eastern European female athletes were actually men, simply involved physical examinations to assess whether the athletes looked male anatomically. By the late 1960s, sex verification testing was more advanced and involved examining an individual’s pattern of sex chromosomes (XX, XY, etc.). More recently, testing shifted to include an examination of testosterone levels (K. Thomas, 2008). Each of these testing methods has shortcomings, however, because atypical cases exist. As discussed in Chapter 1, sex is complexly determined and consists of many components, including genes, hormones, and internal and external anatomy. While these components usually line up as consistently male or female, this is not the case for intersex individuals like Caster Semenya. Despite the predominant cultural tendency to view sex as a binary, sex actually falls along a continuum from consistently female to consistently male. In fact, biologist Anne Fausto-Sterling (1993) long ago argued for the recognition of five or more biological sexes. For instance, a person can be male at the chromosomal (XY) level but have female-typical genitalia and developed breasts and identify as a woman. And non-intersex individuals, whose internal and external attributes generally align as male or female, also show great within-sex variability in characteristics such as facial and body hair, breast size, and muscle tone.

In Caster Semenya’s case, based on her sex verification test results, the IAAF required her to take medication to lower her testosterone levels to the female-typical range before returning to competition (Engber, 2016). Although a court upheld this decision in May 2019, Semenya appealed and was issued a temporary suspension of the IAAF regulations (Lovett, 2019). Depending on the outcome of her appeal, Semenya—despite winning Olympic gold medals for South Africa in 2012 and 2016—may have to choose between giving up the sport she loves or taking hormone-suppressing drugs that have unknown side effects.

The question of how to determine a person’s sex resonates beyond the world of sports. Recent changes to athletic sex verification policies reflect our evolving understanding of the biology of sex and gender (see the chapter debate titled “Should Intersex Individuals Be Allowed to Compete in Athletic Competitions?”). Today, we are witnessing a rapid redefinition of what it means to be female or male and a growing recognition that these labels alone do not adequately capture nature’s range of outcomes. Ultimately, these issues are both biological and sociocultural in nature.

Hijras are third-gender individuals in India who are assigned male at birth but live as women or as nonbinary. They are considered sacred in Hinduism but tend to occupy a low social status.

Source: ©

Cultures vary in the extent to which they accept more than two categories of sex and gender, with some granting intersex and transgender individuals a special social status (Lang & Kuhnle, 2008). As discussed in Chapter 1, hijras—who may be intersex or transgender—are considered sacred within Hinduism and constitute a legally recognized third sex category in India. Kathoeys (translated loosely as ladyboys) are third-gender individuals in Thailand who may be transgender women, effeminate gay men, or intersex. The fa’afafine in Samoa are third-gender individuals who are assigned male at birth, display female-typical traits and preferences, are valued for their loyalty to family and community, and tend to partner romantically with men. Note that some of the third-gender identities presented here blur the boundaries between gender identity (e.g., as man, woman, nonbinary, or genderqueer) and sexual orientation (e.g., as heterosexual, gay, lesbian, bisexual, or pansexual). Although gender identity and sexual orientation are different things, people commonly assume some overlap between them, which is a theme we will revisit throughout this book.

In this chapter, we will examine the biological and sociocultural pieces that shape sex and gender identity, with a particular focus on how nature (or biological factors such as genes, chromosomes, and hormones) interacts with nurture (or social and cultural factors such as environments, life experiences, and socialization). We end the chapter by discussing how major theories about the origins of sex differences account for the roles of nature and nurture.


When a 1997 NBC poll asked people to choose the main reason for the difference between men and women, 53% chose nurture, 31% chose nature, 13% chose both, and 3% were unsure (Applegate, 1997). How would you respond to such a poll? Which factor—nature or nurture—do you think has a stronger influence on sex differences and similarities? On gender identity? On masculinity and femininity? Why?


The “nature versus nurture” question has a long history. As early as 350 BCE, Greek philosophers such as Plato and Aristotle tackled questions about whether people’s traits reflected nature (biological factors) or nurture (sociocultural factors). While many great thinkers came down solidly on one side versus the other, scholars today generally find it overly simplistic to ask questions about nature or nurture. In reality, both nature and nurture shape sex and gender in powerful ways, and we see an increasing consensus that nature and nurture jointly shape sex and gender (W. Wood & Eagly, 2013).

Gene-by-Environment Interactions

Biology and the environment interact in ways that make them impossible to separate. Genes are the basic units of heredity—passed down from parents to offspring—that carry the instructions for shaping the offspring’s characteristics. Environmental factors influence whether and how genes get expressed as actual physical and psychological traits. A gene-by-environment interaction occurs when a genetic tendency emerges only under certain environmental circumstances or when an environment shapes traits or behavior only for individuals with a particular genetic makeup. Gene-by-environment interactions can be passive, evocative, or active as children develop (Scarr & McCartney, 1983). In passive gene-by-environment interactions, parents create certain rearing environments that cannot be separated from their own (and thus their child’s) genetic makeup. For instance, consider parents who are genetically skilled at reading and who both pass along reading skills to their children genetically and create reading-rich environments for them. With evocative interactions, an individual’s genetic tendency evokes specific treatment from others, such as a boy with an active temperament who elicits rough-and-tumble play from his parents and peers. Finally, with active interactions, an individual’s genetic tendency guides her to choose certain environments, such as a genetically shy person who deliberately chooses quieter environments than her more extroverted sibling chooses. In each of these cases, a biological difference (nature) influences an environmental factor (nurture). Conversely, nurture can influence nature. For example, when a child experiences more active and vigorous play at a young age, it can alter the wiring of the brain, strengthening neural connections that would otherwise not be strengthened. As another example, when girls experience environments of higher family stress, they tend to have an earlier age of menstruation (Colich, Platt, Keyes, & Sumner, 2020). Here, the environment (nurture) alters the individual’s neural circuitry or hormones (nature).

Genes Basic units of heredity passed down from parents to offspring, consisting of specific sequences of DNA (deoxyribonucleic acid) that carry instructions for the offspring’s characteristics.

Gene-by-environment interaction When a genetic effect on a trait or behavior emerges only under certain environmental circumstances or when the environmental effect on a trait or behavior depends on a person’s genetic makeup.

Avshalom Caspi and his colleagues found an interesting gene-by-environment interaction when they investigated the roots of depression (Caspi et al., 2003). In a large longitudinal study, they measured people’s stressful life experiences and whether or not they had a genetic marker of depression, a variation of the serotonin transporter gene (referred to as 5-HTT). There were no direct links between stressful life experiences (e.g., childhood maltreatment) and later depression, or between the presence of the 5-HTT gene and later depression. Instead, it was individuals who displayed the risky 5-HTT gene and also experienced high levels of stress who had a higher prevalence of depression. These findings support the diathesis-stress model in which people with a genetic predisposition for a disorder only develop the disorder when they experience certain stressful environmental circumstances.

But how do gene-by-environment interactions relate to sex differences? We will address this topic more specifically in other chapters of this book. For instance, in Chapter 13 (“Gender and Psychological Health”), you will read about how girls experience higher rates of depression than boys, a difference that emerges around the onset of puberty. One explanation for sex differences in depression is that girls’ pubertal hormones (e.g., estrogen and progesterone) interact uniquely with interpersonal and social stressors during adolescence to produce depression (Hilt & Nolen-Hoeksema, 2014). In Chapter 7 (“Cognitive Abilities and Aptitudes”), you will read about the biopsychosocial model, which rejects the nature/nurture dichotomy and argues that biology (genes and hormones) and environment (culture and learning experiences) mutually influence each other in shaping sex differences and similarities in various cognitive abilities (Halpern, 2012).


Further evidence of the interconnectedness of nature and nurture is found in epigenetics. Whereas genetics is the study of genes (the basic units of heredity) and how traits are inherited, epigenetics is the study of the biological mechanisms that guide whether or not certain genes get expressed or activated (Allis & Jenuwien, 2016). Epigenetic marks are molecular structures that sit on genes and instruct them to activate or deactivate. Although every somatic cell (e.g., cells that make up organs, blood, bones, and connective tissue) in the human body contains identical gene sequences, only certain genes are activated in particular cells, leading them to become very different types of cells (e.g., hair, muscle, or eye cells).

Genetics The study of genes (the basic units of heredity) and how physical traits are inherited.

Epigenetics The study of the biological mechanisms that guide whether or not certain genes get expressed.

The functioning of epigenetic marks can be influenced by the environment, either in the uterus or after birth. This phenomenon can help to explain why identical twins, who are (typically) exact genetic copies of each other, nonetheless differ in subtle ways and become increasingly different over time as their environments diverge: Epigenetic marks may activate different genes in the cells of identical twins. Some research shows that epigenetic marks can be transmitted from mother to offspring (M. M. McCarthy et al., 2009). For example, a mother’s diet, sleep patterns, or stress levels during pregnancy may affect not only her own epigenetic environment (the biochemical changes around the genes that can cause the genes to turn on or off) but also the epigenetic environments of her offspring. In other words, a mother’s environmental experiences may alter which genes will be expressed in her offspring, thus illustrating the complex interaction between nature and nurture.

Importantly, life experiences often vary by sex and gender, and these different experiences can sometimes cause epigenetic changes in the brain (Cortes, Cisternas, & Forger, 2019). Consider alcohol consumption: Around the world, men drink alcohol more frequently and more excessively than women (see Chapter 12, “Gender and Physical Health”). Furthermore, chronic alcohol consumption can alter epigenetic factors and produce changes in brain DNA (Tulisiak, Harris, & Ponomarev, 2017). This suggests that gender-linked behavior can influence people’s genes.

The Microbiome

Exciting research at the frontier of nature—nurture interactions examines how the bacteria and other microbes that live in human bodies can influence physical and mental health, memory, attention, and even attraction and mating (Smith & Wissel, 2019). The microbiome is the system of 10 trillion microbial microorganisms—including bacteria, fungi, and viruses—that lives inside the human body, primarily in the large intestine, and communicates with the brain along the gut-brain axis. A number of factors influence the composition and diversity of the microbiome, from the local environment to diet to specific behaviors. For instance, being delivered vaginally versus by cesarean section can influence infants’ bacteria. Kissing is another factor that can influence the microbiome: Mouth-to-mouth kissing exchanges tens of millions of bacteria, and people who kiss one another a lot come to have more similar microbiomes (Kort et al., 2014).

Microbiome The complex system of microbial microorganisms that lives inside the human body.

Gut-brain axis Bidirectional communications that take place between the brain and the gastrointestinal tract.

Around puberty, sex differences in the composition and diversity of the microbiome begin to emerge. These differences may be caused by differences in hormones, drug and medication exposure, or diet (Kim, Unno, Kim, & Park, 2020). Might sex differences in the composition of the microbiome account for psychological sex differences? It is too early to know for sure, but mounting evidence indicates that the health and diversity of the microbiome can influence anxiety and depression (Lach, Schellekens, Dinan, & Cryan, 2018), which women report experiencing more commonly than men. The microbiome may even play a role in sex differences in autism (Kushak & Winter, 2018).

The study of the gut-brain axis is in its infancy, and there is still much we do not know about how diets and environmental factors influence and are influenced by the microbiome. However, psychologists have an important role to play in the years ahead in understanding how tiny, invisible bacteria and other microbes may influence our lives. In sum, although biological and sociocultural factors are conceptually independent, they interact in ways that make them difficult to disentangle, as research on the microbiome illustrates. In the sections that follow, we will further explore how biological and sociocultural factors operate to shape sex and gender.

Sex differentiation The complex processes that unfold as sex-undifferentiated embryos transition into individuals with male, female, or intersex internal and external genitalia.


The processes that ultimately create our sex begin at the moment a sperm fertilizes an egg. Sex differentiation refers to the complex series of processes that unfolds as the sex-undifferentiated embryo transitions into an individual with male, female, or intersex gonads and genitalia (see Figure 3.1; C. A. Wilson & Davies, 2007). Although much of sex differentiation occurs prenatally, further differentiation occurs during puberty. In this section, we outline the stages of typical sex differentiation and we then address several intersex conditions in which the components of biological sex (genes, hormones, and anatomy) do not align as consistently female or male.

Typical Sex Differentiation

What determines a person’s biological sex? This turns out to be a complicated question. Sex is not merely a person’s external genitalia, as early methods of sex verification testing in sports revealed. Nor is it simply the sex chromosomes. Sex chromosomes initiate the processes of sex differentiation, but they provide an incomplete picture when viewed alone. Biological sex is a product of chromosomes, genes, hormones, internal sex organs, and external genitalia. In most cases, these various factors align in a consistent manner to produce a biologically female or male person. Let’s examine what happens at each stage in typical cases.


Figure 3.1 The Biological Components of Sex Differentiation

Chromosomes and Genes

The human body has two major categories of cells: somatic cells (e.g., organ, blood, bone, and connective tissue cells) and reproductive cells (egg and sperm). All cells contain chromosomes, which are organized units of genes. Every somatic cell in the human body has 23 pairs of chromosomes, with each parent contributing one set of 23 chromosomes. In contrast, reproductive cells contain 23 unpaired chromosomes. In each cell, all but one chromosome or chromosome pair are autosomes (non-sex chromosomes), which contain genes that code for all attributes other than sex (e.g., eye color, hair color, and height). The remaining chromosome or chromosome pair is an allosome (sex chromosome), which contains genes that code for sex (see Figure 3.2). Each sperm cell contains either an X or Y allosome, whereas egg cells always contain an X allosome. Thus, at the point of fertilization, either the X or Y provided by the sperm pairs with the X chromosome provided by the egg (Griffiths, Wessler, Carroll, & Doebley, 2015). In typical development, the XY pairing produces a male individual, and the XX pairing produces a female individual. At the genetic level, female and male are thus defined by the presence or absence of a Y chromosome. More precisely, a single gene on the Y chromosome called SRY regulates genital and testicular development, thereby causing a fetus to develop as male, although we will note some exceptions in a moment.

Chromosomes The organized units of genes inside the cells of all living organisms. Somatic cells in the human body have 23 pairs of chromosomes, and reproductive cells have 23 unpaired chromosomes.


Figure 3.2 Chromosomes of the Somatic Cells


Unlike all the other chromosomes in the human genome, the Y (“male”) chromosome does not have a duplicate partner. This makes it the most vulnerable chromosome, as its genes have to fend for themselves in the event of harmful mutations. In fact, over millions of years of evolution, the Y chromosome has lost almost all of its genes. Whereas most chromosomes contain thousands of genes, the stubby little Y contains fewer than 100 genes, including the SRY gene, which guides male sex development (C. A. Wilson & Davies, 2007).


Suppose that technology allowed parents to choose the sex of their offspring easily and reliably. Would this practice be morally justifiable? What if parents have a family history of a sex-linked hereditary disease, and choosing a child’s sex could protect the child? What if parents already had several children of one sex and wanted a child of another sex? What if they lived in a culture that highly valued boys over girls? Would some of these reasons be more morally acceptable than others? Why or why not?

Hormones and Anatomy

Gonads (ovaries and testes) are sex organs that produce sex cells (egg and sperm) and sex hormones (estrogen and testosterone). Until about the sixth week of prenatal development, the gonads of female and male human embryos do not differ by sex. Instead, all embryos contain an undifferentiated internal reproductive structure called the genital ridge. At about the sixth week after conception, the SRY gene initiates the development of the male gonads (testes). In the absence of the SRY gene, the female gonads (ovaries) develop. By about the eighth week of gestation, the gonads begin producing hormones, chemical substances that regulate bodily functions such as digestion, growth, and reproduction. In genetic males, the testes produce androgens, such as testosterone, which initiate the biological masculinization process in the male internal genitalia (testes, seminal vesicles, and vas deferens). In genetic females, the ovaries do not produce many hormones prenatally, and the genital ridge develops into female internal genitalia (cervix, uterus, ovaries, and fallopian tubes) in the absence of androgens (Hines, 2004).

Gonads The sex organs (ovaries and testes) that produce sex cells (egg and sperm) and sex hormones (estrogen and testosterone).

Genital ridge The precursor to female or male gonads (ovaries or testes). It appears identical in genetic female and male embryos.

Hormones Chemical substances in the body that regulate bodily functions such as digestion, growth, and reproduction.

Genitalia Internal and external reproductive organs. For females, these include the cervix, uterus, fallopian tubes, and ovaries (internal) and the labia and clitoris (external). For males, these include the seminal vesicles, vas deferens, and testes (internal) and penis and scrotum (external).

Genital tubercle The undifferentiated embryonic structure that becomes the clitoris or the penis.

Just as all internal genitalia originate from the same undifferentiated structure, so too do the external genitalia (see Figure 3.3). Until about the 12th week of gestation, the external genitalia of human embryos consist of a genital tubercle (a small bump between the legs) with a small opening, surrounded by a swelling of skin on either side. After 12 weeks, in most cases, the genital tubercle and swellings develop into either a penis and scrotum or a clitoris and labia (M.-H. Wang & Baskin, 2008). The presence of androgens typically leads to the development of male-pattern genitalia, and their absence typically leads to the development of female-pattern genitalia.


Figure 3.3 Prenatal Development of Genitalia

We should clarify a common misperception about hormones. People often view testosterone as a male hormone and estrogen and progesterone as female hormones. In fact, almost everyone produces all of these hormones, but the amounts produced differ by sex. From about 8 to 24 weeks of gestation, male fetuses have higher concentrations of testosterone than female fetuses. Then, between the 24th week of gestation through birth, the testosterone levels of male fetuses drop and remain as low as those of female fetuses (Hines, Constantinescu, & Spencer, 2015). Immediately after birth, boys’ testosterone levels and girls’ testosterone and estrogen levels surge briefly for about 6 months, a period sometimes called “mini-puberty” (Copeland & Chernausek, 2016). Throughout the rest of childhood, boys and girls do not differ much in their hormone levels until puberty, when boys begin to produce higher levels of testosterone. After puberty, testosterone levels peak in young adulthood, by which point men have testosterone concentrations many times higher than women. In contrast, while estrogen and progesterone are involved in reproductive system development, menstruation, and pregnancy (discussed further in Chapter 12, “Gender and Physical Health”), they play little role in sex development before puberty (Hampson & Moffat, 2004).

Mini-puberty A period from birth to about 6 months in which boys experience surges in testosterone and girls experience surges in testosterone and estrogen.

Beyond sex differentiation and reproductive development, hormones play a role in some sex-linked traits. For example, prenatal androgen levels may predict levels of masculinity and male-typed occupational preferences in adulthood (Manning, Trivers, & Fink, 2017). A less obvious connection also exists between hormones and cognitive functions, such as verbal and visual-spatial abilities, which we discuss further in Chapter 7 (“Cognitive Abilities and Aptitudes”). For instance, the quality of babbling at 5 months is positively correlated with estrogen levels and negatively correlated with testosterone levels in both girls and boys during mini-puberty (Quast, Hesse, Hain, Wermke, & Wermke, 2016). Furthermore, the sex difference in visual-spatial tasks (e.g., navigating through a three-dimensional space) that favors male over female mammals seems to be influenced by estrogen levels. For example, male rodents learn to navigate through complex mazes faster, on average, than their high-estrogen female counterparts, whereas female rodents low in estrogen perform just as well as male rodents (Galea, Kavaliers, Ossenkopp, & Hampson, 1995). This sex difference is likely due to estrogen’s effects on the hippocampus, an area of the brain related to learning and memory (Duarte-Guterman, Yagi, Chow, & Galea, 2015), which is generally larger and denser in male than female mammals.

Intersex Conditions

Even though all individuals vary in sex-related physical attributes (e.g., hormone levels and genital size), some individuals show greater degrees of variation than others. In most cases, the various biological components of sex (chromosomes, genes, hormones, and internal and external genitalia) line up to produce typical female or male babies, but in approximately 1%—2% of cases, babies are born intersex (Arboleda, Sandberg, & Vilain, 2014; Fausto-Sterling, 2000), making it about as common as having red hair. An intersex individual has inconsistency across the biological components of sex, showing some combination of male and female components (e.g., an XY individual with female external genitalia). The medical community refers to intersex conditions as differences of sex development (DSDs).


We know that genes and hormones guide the development of biological sex, but how do we know the precise mechanisms by which these processes unfold? People have speculated and experimented on how animals and plants become differentiated by sex for thousands of years. In the fifth century BCE, the Greek philosopher Anaxagoras reasoned that sex must be determined by the sperm, which is partially correct, but the rest of his logic fell short of reality: Anaxagoras thought that sperm produced in the right testicle would create male children, and those produced in the left testicle would create female children (Mukherjee, 2016). It was not until the discoveries of genes (dating back to Gregor Mendel’s work in the mid-19th century) and hormones (a term first used in 1905 by physiologist Ernest Starling) that scientists began speculating about their roles in guiding biological sex development in sexually reproducing species.

In 1905, biologist Nettie Stevens conducted groundbreaking research on mealworm beetles that led to the discovery of the role of genes in sex determination. While analyzing male beetle sperm, she discovered that chromosomes in the sperm came in two different sizes, large (later called X chromosomes) and small (later called Y chromosomes). Stevens then observed that fertilizing an egg with the large chromosome (X) sperm created female offspring, whereas fertilizing an egg with the small chromosome (Y) sperm created male offspring. She concluded that sperm must contain the genes that code for biological sex, meaning that male members of the species determined biological sex (Brush, 1978). This pattern was later confirmed in other animals, including humans.

If genes carried by sperm provide the initial blueprint for biological sex, hormones get the ball rolling. Some of the earliest work on hormonal regulation of sex began in the late 1940s with the work of Alfred Jost, who removed the testes of fetal male rabbits and found that the rabbits appeared female at birth. In other experiments, Jost reversed this procedure by transplanting testes into female rabbit fetuses and found that these rabbits appeared male at birth. Jost’s research showed that the Y chromosome instructs undifferentiated gonads to develop into testes and produce hormones, such as testosterone, that are critical to the development of male sex characteristics (including internal and external sex organs). In the absence (or inactivity) of these hormones, fetuses will typically develop as biologically female (Jost, Vigier, Prepin, & Perchellet, 1973).

More recent advances in molecular biology highlighted the complexity of chromosomes and genes in the development of biological sex. For instance, in the early 1950s, scientists predicted that certain genes on the Y chromosome must be responsible for initiating the masculinization of fetal gonads; they called these genes the testis-determining factor. By the 1990s, geneticists confirmed that the SRY gene on the Y chromosome did, in fact, produce fetal gonad masculinization, at least in most cases. Very rarely, the SRY gene can show up on an X chromosome in the father, which causes an XX fetus to appear male. At other times, the SRY gene does not work properly on the Y chromosome, and an XY fetus develops largely as female.

To complicate matters, hormonal anomalies can sometimes occur with otherwise typical female (XX) or male (XY) babies. Although scientists studied intersex individuals prior to the 20th century under the labels true hermaphrodites and pseudohermaphrodites (Holmes, 2016), a more complete understanding of genetic and hormonal causes of intersexuality emerged much later. For example, in androgen insensitivity syndrome, first identified in 1953, cells fail to “hear” masculinizing androgens, leading a genetic (XY) male to appear female at birth (Hines, Ahmed, & Hughes, 2003). Conversely, a genetic female (XX) can be exposed to atypically high levels of masculinizing hormones prenatally, due to a malfunctioning adrenal gland (a condition called congenital adrenal hyperplasia). Genetic females with this condition may have male-appearing genitals, and they are sometimes raised as boys (de Jesus, Costa, & Dekermacher, 2019).

Determining what makes someone female or male at the biological level can be quite complicated. As we learn more about biological sex differentiation, a more nuanced picture emerges, with sex appearing less categorical than previously believed. With further advances in genetics, molecular biology, and endocrinology, the future holds the promise of an increased understanding of biological sex and intersexuality in all their varieties.

Differences of sex development (DSDs) Conditions present at birth in which sex development varies from the norm in terms of chromosomes, gonads, or anatomy.

Examining DSDs not only gives visibility to the variety of forms that sex can take, but it also sheds light on how biological and social factors contribute to sex and gender. For example, how does gender identity develop in someone with an unpaired X chromosome? What about in someone with an XXY chromosome pattern? And what about in a genetic male (XY) whose body cannot respond to androgens? Or a genetic female (XX) who produces unusually high concentrations of androgens? By examining the different patterns that emerge across a variety of intersex conditions (see Table 3.1), we can draw inferences about the relative influences of biological and sociocultural factors in shaping gender identity.


In Greek mythology, the God Hermaphroditus, son of Aphrodite and Hermes, had both female and male features. For this reason, Edwin Klebs (1876) referred to intersex individuals as hermaphrodites, a term that is now considered outdated, imprecise, and offensive when referring to humans. This term technically refers to nonhuman species, such as earthworms and some types of snails and fish, that have both female and male functioning reproductive organs.

Table 3.1

Intersex conditions can result from atypical variations occurring at either the chromosomal or hormonal levels. Note that the links between the specific type of atypicality (chromosomal vs. hormonal) and the individual’s most likely gender identification are complex and depend on a variety of factors.

*The “O” indicates that the second sex chromosome is missing, partially missing, or structurally altered.

Chromosomes and Genes

Four deviations from the typical XX or XY chromosome pattern have been widely studied (see Table 3.1). Typically identifying as female, individuals with Turner’s syndrome have only a single X chromosome in the allosome in each cell (Powell & Schulte, 2011), whereas those with triple X syndrome have an XXX chromosomal pattern (Otter, Schrander-Stumpel, & Curfs, 2010). Typically identifying as male, individuals with Klinefelter syndrome have an XXY chromosomal pattern in each cell, whereas this pattern is XYY in individuals with Jacob’s syndrome (J. L. Ross et al., 2012).

What do these atypical conditions tell us about the role of chromosomes in sex? It appears that the presence of a Y chromosome strongly predicts having a male appearance and gender identity, while the absence of a Y chromosome strongly predicts having a female appearance and gender identity. However, developing a female gender identity does not require two X chromosomes, as we see in Turner’s syndrome and in transwomen who are not intersex. As we will see in the next section, other factors, such as hormones, can override these genetic influences in contributing to gender identity development.

Hormones and Anatomy

Occasionally, due to genetically transmitted disorders, fetuses experience atypical levels of sex hormones in the womb by having either unusually high concentrations of androgens or an inability to process androgens (see Table 3.1). In congenital adrenal hyperplasia (CAH), the body overmanufactures androgens. Genetic females (XX) with CAH have internal female reproductive organs but tend to have more male-appearing external genitalia. Typically assigned female at birth, XX individuals with CAH often undergo feminization surgery of their genitalia in infancy, although such surgery raises serious ethical questions. While most individuals with CAH who are assigned female at birth usually identify as female, they tend to show less satisfaction with their sex assignment, more gender fluidity, and more male-typical behaviors and interests than do girls without CAH (Hines, Brook, & Conway, 2004). Although XX individuals with CAH are usually raised as girls, those with more prominent masculinization are sometimes raised as boys, especially in developing countries such as India and Pakistan where many lack the means for early diagnosis and treatment (de Jesus et al., 2019).

Let’s revisit the case of Caster Semenya from the chapter opener. The diagnosis that presumably applies to Semenya is hyperandrogenism, a medical condition characterized by an excess of androgens in the female body. Note that hyperandrogenism can result from CAH, but it may also reflect other causes. Therefore, we cannot know for sure whether Caster was born with CAH, but her androgen levels in adulthood appear consistent with this disorder. And consistent with what we know about CAH, Semenya exhibited male-typical play preferences in childhood: She spent her time playing only with boys, and she loved soccer (“Who Is … Caster Semenya,” 2019). In fact, girls with CAH play with male-typical toys more than their sisters do but less than their brothers do. This pattern emerges even though parents often encourage more female-typical toy play in daughters with CAH than in daughters without CAH (Pasterski et al., 2005).

In the rare condition of complete androgen insensitivity syndrome (CAIS), the cells of the body do not respond to the influence of androgens. For genetic females (XX), the insensitivity to androgens in utero does not disrupt sex development, but for genetic males (XY), it does. Because testosterone (an androgen) directs the masculinization of genitals in fetuses, genetic males with CAIS typically appear female at birth and express a female gender identity. By developing a gender identity inconsistent with their genetic sex in a relatively straightforward manner, CAIS XY individuals illustrate the importance of external appearance and assigned sex in the identity formation process. Likewise, partial androgen insensitivity syndrome (PAIS), characterized by a partial inability of cells to respond to androgens, only disrupts sex development for genetic males (XY), whose genitals may resemble an enlarged clitoris or a small penis, depending on the degree of androgen insensitivity (Deeb, Mason, Lee, & Hughes, 2005).

Belgian model, Hanne Gaby Odiele, was born with androgen insensitivity syndrome. She identifies openly as intersex and advocates for intersex rights.

Source: Ovidiu Hrubaru / Alamy Stock Photo

These different conditions suggest that hormones sometimes override the influence of chromosomes in guiding both sex assignment at birth and the development of gender identity. That is, due to atypically high or low testosterone exposure, people can develop a gender identity at odds with their chromosomal sex. But hormones cannot tell the whole story. Consider the fact that genetic females with CAH and genetic males with PAIS can develop an identity as either girls or boys. In other words, among infants with CAH, their parents’ decision about how to raise them can lead them to internalize a gender identity that is inconsistent with either their chromosomal sex (CAH XX individuals raised as male) or their hormone levels (CAH XX individuals raised as female). If this sounds complicated, it is, and we will explore the implications for gender identity in more detail shortly.

Individuals who do not fit cleanly into the sex binary live in all cultures, and cultural responses to intersexuality vary widely. For example, consider the guevedoces of the Dominican Republic (Knapton, 2015). Because of a rare genetic disorder that is unusually prevalent in an isolated Dominican village, about 1 in 90 genetic boys lacks an enzyme that produces testosterone. As a result, these individuals do not undergo male sex differentiation in utero, and their testes remain hidden inside their bodies. Because they appear to have a vagina at birth, they are assigned female at birth and raised as girls. But when they enter puberty at around age 12 and experience a surge of male hormones, their testicles descend, and they grow a penis (guevedoces translates to “penis at 12”). The villagers usually welcome and celebrate this transformation, perhaps due to the higher social status of men in Dominican society. Rather than stigmatizing the guevedoces, Dominicans’ cultural narrative recognizes their unique experiences and identities. See Table 3.2 for examples of other cultures that have unique third-sex or third-gender categories to accommodate and give meaning to the lives of intersex, transgender, and sexual minority (e.g., LGB) individuals.

Unlike the cultures described in Table 3.2, the United States and many other Western cultures tend to exclude the experiences of intersex, transgender, and nonbinary individuals from predominant narratives. This can have implications for violence. Though intersex and transgender individuals are at an increased risk for violence in any culture (see Chapter 14, “Aggression and Violence”), the stigma and violence that they face is generally higher in countries where they experience more social and legal exclusion (Badgett, Nezhad, Waaldijk, & van der Meulen Rodgers, 2014).

Table 3.2

Many non-Western cultures around the world recognize third-sex or third-gender categories that accommodate the lives and experiences of intersex, transgender, and sexual minority individuals. The social status of these third-sex and third-gender categories can vary widely, from “accepted but may face stigma” to “respected and valued.”

Source: Adapted from Lang and Kuhnle (2008).

1The term Berdache is generally considered offensive today but has historical significance.


The case of Caster Semenya, highlighted in the chapter opener, raises questions about fairness in sports. Many would agree that it is unfair for men to compete against women in athletic competitions for which speed and strength generally give men a significant advantage. But what about when a person’s sex does not fit neatly into the sex binary? Should intersex athletes be allowed to compete along with members of the sex with which they identify, or does their atypical physiology give them an unfair advantage? Note that this debate only affects female-identified athletes with intersex conditions. Officials do not conduct sex verification testing on male athletes because male-identified intersex athletes competing as men would not be seen as having an unfair competitive advantage. Let’s examine both sides of the debate.


Almost all sports are divided by sex, and these sex-based divisions make sense. If women competed against men, men would have an advantage. Men are larger and stronger than women, on average, and they typically run faster, jump farther, and throw harder and farther. These sex differences are substantial: Recall from Chapter 2 that sex differences in physical traits are among the largest known sex effects (J. R. Thomas & French, 1985). Thus, if women have biological anomalies that push them into the male range, it creates a disadvantage to the great majority of women who lack these features.

Performance-enhancing drugs such as artificial testosterone are banned in sports—and for good reason. Bicyclist Lance Armstrong was stripped of his seven Tour de France titles and banned for life from competitive cycling in 2012 after testing positive for several steroids. The home run records of baseball stars Mark McGwire and Barry Bonds are similarly tainted by their use of steroids and human growth hormones, actions that keep them locked out of the Baseball Hall of Fame.

Just as the public would find it unacceptable for an athlete to elevate his or her testosterone levels artificially, atypical conditions that naturally elevate testosterone levels create the same unfair competitive advantage. If an intersex woman has testosterone levels much higher than her opponents, as Caster Semenya apparently does, then the same logic should apply.


By definition, elite athletes have atypical physiologies. They are bigger, stronger, faster, and more agile than average humans. Legendary NBA basketball player Shaquille O’Neal’s remarkable physique (height, 7 feet and 1 inch; weight, 325 pounds) gave him an obvious competitive advantage. No one would deny that O’Neal’s towering frame helped him dominate his opponents. Similarly, the atypical body of Olympic swimmer Michael Phelps—uncommonly long torso and tremendous arm span—is acknowledged as a primary reason for his exceptional swimming abilities. Tennis star Serena Williams has a famously muscular physique and one of the fastest serves in the game. These athletes’ unique bodies clearly play a substantial role in their athletic victories. And yet, some people question whether an intersex athlete’s atypical physiology (say, a testosterone concentration several times higher than her competitors) renders her performance problematic. If intersex athletes have a unique advantage because of their physiology, how does this differ from the unique physical gifts of O’Neal, Phelps, and Williams?

Moreover, research to date does not clearly show how much competitive advantage, if any, naturally elevated testosterone levels provide to female athletes (Longman, 2016). Even if testosterone did confer a competitive physical advantage in women, the exclusive focus on hormone testing in female athletes presents a bias. Women and men both have testosterone, and men also vary from each other in their testosterone levels. Given this, why have officials only felt compelled to regulate the testosterone levels of female athletes? Should we not also regulate those of male athletes to ensure that men with very high levels of testosterone do not have an unfair competitive advantage?

In fact, the larger issue may stem from gender role norms about how much masculinity people accept in women: The concerns may have less to do with supposed competitive advantages and more to do with challenges to conventional notions of femininity that some intersex female athletes represent. In reality, people come in all shapes and sizes, and the physiology of men and women exists on a spectrum. Naturally occurring, elevated levels of testosterone should thus not preclude women from competing in athletic events.

Which side of the debate seems more compelling to you? Which points do you find most and least convincing? Why?


How do we identify the sex of a fetus or infant? The most common method is very simple—we look at the genitals. This can be done at birth or prenatally with sonograms that allow for a visual inspection of a penis or labia between the third and seventh months of pregnancy (G. F. Cunningham et al., 2014). Another method, known as noninvasive prenatal testing, can determine a fetus’s genetic sex (and test for chromosomal abnormalities) by drawing blood from the mother about 9 weeks after conception (Chandrasekharan, Minear, Hung, & Allyse, 2014). The typical method of assigning sex based on a visual inspection of the genitals highlights a potential clash between nature and nurture. As discussed, while nature offers us a continuum of sex configurations (Fausto-Sterling, 2000), most—though not all—cultures choose to dichotomize sex as either male or female based on a simple glance at the genitals. But what happens when a visual inspection of the genitals does not offer a clear male/female answer?

Optimal Sex

Optimal sex refers to the binary sex that doctors and parents perceive as the best option for a newborn whose genitalia appear atypical at birth. Although many scholars refer to this concept as optimal gender, we use the term optimal sex instead because it better reflects how we define sex in this book, as referring to categories of sex (e.g., male, female, or outside the binary). Prior to the 1950s, the scientific community lacked consensus on how to treat newborns whose genitals could not easily be categorized into the sex binary. Then, psychologist John Money and his colleagues at Johns Hopkins University developed the optimal sex policy, which proposed that intersex infants should be socialized as either boys or girls beginning in the first 18 months of life (Money, Hampson, & Hampson, 1955). Money believed that gender identity was largely a product of socialization and that social factors could override any role that biology played in gender identity. He also advocated for early corrective surgery when the genitals were not clearly female or male, followed by hormone treatments to ensure typical hormone levels for the assigned sex. The optimal sex policy thus prioritized the goal of creating a physical appearance consistent with assigned sex. In this way, the scientific community viewed intersex children as a problem that needed solving, and the favored solution involved modifying them to fit into the sex binary. The infant’s own inability to provide consent received little attention (Reis, 2019).

Optimal sex The binary (male or female) sex perceived to be most advantageous to assign to a newborn whose genitalia appear atypical at birth.

Social and medical opinions on this issue have changed over time. Professionals today increasingly reject the optimal sex policy on ethical grounds and instead recommend to parents of intersex infants that they postpone unnecessary surgeries and hormone treatments until children are old enough to understand their situation and consent (or not) to treatment (Harris, 2016). And in 2018, California became the first U.S. state to pass a resolution condemning (though not legally disallowing) unnecessary surgeries on intersex infants (Miller, 2018). In part, these changes reflect the work of intersex advocacy groups, who seek to educate the public and medical community about the experiences and rights of intersex individuals. For instance, some intersex people feel traumatized by the experience of unwanted surgeries that took place in their infancy, without their awareness or consent (Lambda Legal, 2018). Not only do such surgeries violate intersex individuals’ rights to bodily autonomy, they can cause damage to the tissues and nerves involved in sexual responses, leaving some individuals with reduced ability to experience orgasm or sexual pleasure. Moving forward, it is important that intersex children and their parents receive adequate psychological counseling, and that physicians receive adequate training about the complexities of intersex conditions (Ernst, Liao, Baratz, & Sandberg, 2018).

The evolving response that society has to intersex individuals reveals an important point about sex and gender: Even though biological factors do play a role, sex is also socially and culturally constructed. The moment the sex of a fetus or infant becomes known, social values and beliefs enter the process. When doctors recommend surgery to alter an infant’s genitalia, they reflect and reinforce a cultural worldview that recognizes only two categories of sex. The movement away from such procedures and the increasing cultural visibility of intersex and transgender individuals in the United States and around the world suggest that cultural constructions of sex and gender as binaries are changing.

Gender Identity

Let’s return to John Money’s assumption that nurture is stronger than nature in determining individuals’ gender identity. Money assumed that a child born with visually atypical genitals could be raised as either female or male, contingent upon strong and consistent socialization by parents and the surrounding environment. How accurate is this belief? To what extent do intersex individuals who get assigned either female or male at birth come to embrace versus reject their assigned sex? The evidence is mixed. On the one hand, the majority of genetically female (XX) individuals with CAH who are assigned female at birth do tend to develop a female gender identity (Hines et al., 2004). On the other hand, XX individuals with CAH raised as boys tend to show more distress about their assigned sex and higher rates of gender transition (from male to female) than their counterparts raised as girls (de Jesus et al., 2019). Furthermore, the famous case of David (born Bruce) Reimer, a genetic boy raised as a girl, offers some vivid evidence that socialization cannot always override biological factors in shaping gender identity. In 1965, a surgeon accidentally removed most of Bruce’s penis while circumcising Bruce and his identical twin brother. Since surgical reconstruction of the penis was impossible, Bruce’s parents followed John Money’s advice and had Bruce reassigned as female. This meant surgically constructing a vagina for him and giving him hormonal and psychological treatments (Mukherjee, 2016).

For years, Money claimed that Bruce (raised as Brenda) developed a typical female gender identity and adjusted well, but this was far from true, as chronicled in John Colapinto’s (2000) book, As Nature Made Him: The Boy Who Was Raised as a Girl. In reality, Brenda never felt like a girl. She cut up dresses she felt forced to wear, preferred her brother’s toys over her dolls, and stood while urinating. At age 14, upon learning the truth about his botched circumcision in infancy, Reimer adopted a male gender identity, took the name David, and sought testosterone treatments. Although David Reimer married as an adult, he suffered from depression and anger issues and ultimately committed suicide at age 38, almost 2 years after his identical twin brother died of an overdose (Mukherjee, 2016).

Brenda Reimer as a child and David Reimer after assuming a male gender identity.

Source: REUTERS/Str Old

David Reimer’s story suggests that he and his twin brother may have had struggles that went beyond David’s traumatic gender upbringing, which underscores the difficulties of generalizing from a single case. That said, Reimer’s story indicates that gender identity cannot necessarily be shaped entirely by socialization, especially in individuals who undergo typical sex differentiation in utero. A study of intersex individuals—genetic males (XY) who were androgenized normally in utero but raised as female due to the absence of male-typical genitalia at birth—corroborates this conclusion (Meyer-Bahlburg, 2005). Although the majority (78%) developed a female gender identity, 13% experienced symptoms of gender dysphoria, or clinically significant levels of distress arising from a mismatch between assigned sex at birth and one’s felt sense of gender. Moreover, 22% of the sample ultimately transitioned to a male gender identity, a rate that is almost 37 times larger than the estimated rate (0.6%) of transgender people in the United States (Flores, Herman, Gates, & Brown, 2016). On the one hand, these findings offer partial support for Money’s beliefs because the majority of the sample developed the female gender identity that they were socialized to adopt, despite their genetic maleness. On the other hand, the fact that 22% of the sample transitioned to a male identity seems to suggest the reverse—that chromosomes and hormones can override socialization in the development of gender identity. Also noteworthy is the fact that in a comparison sample of intersex XY babies raised as boys, none changed gender identity, and only one experienced possible gender dysphoria (Meyer-Bahlburg, 2005).


What do you think about the practice of doing medical procedures (e.g., hormone treatments and genital reconstructive surgery) on intersex infants and children before the age of consent? Are these procedures ethical? What are their pros and cons? Do the pros outweigh the cons, or vice versa? Why?

Another way to assess the relative roles of biology versus socialization in gender identity is to estimate its genetic heritability. Using twin studies, researchers can compute heritability estimates that quantify the extent to which genes versus nongenetic factors shape a given trait or tendency. A heritability estimate is a statistic that specifies the proportion of total population variance in a given trait that is due to genetic differences among the people in the population. Heritability estimates (signified by h2) can range from 0% to 100% (or 0.0—1.0). An h2 of 0 would indicate that genetic differences among people account for none of the population variance in a trait, and an h2 of 100% (or 1.0) would indicate that genetic differences among people account for all of the population variance in a trait. For example, if the h2 for creativity is .42, this means that genes explain 42% of the population variance in creativity. Put another way, 42% of creativity differences among people are attributable to genetic differences among them.

Heritability estimate A statistic that specifies the proportion of total population variance in a given trait that is due to genetic differences among the people in the population. Heritability estimates (signified by h2) can range from 0% to 100%.

Heritability estimates are calculated by comparing the similarity of monozygotic (identical) twins to the similarity of dizygotic (fraternal) twins. Monozygotic twins share 100% (or close to 100%) of their genes in common (Machin, 2009), while dizygotic twins share, on average, 50% of their genes. Thus, to the extent that monozygotic twins are more similar to one another than dizygotic twins are on a given trait, we can estimate the extent to which genes shape the trait. Using this logic, one twin study reported a heritability estimate of 62% for gender identity (Coolidge, Thede, & Young, 2002). In a more recent study, Heylens et al. (2012) selected 23 monozygotic twin pairs and 34 dizygotic twin pairs in which one twin was transgender and then measured the gender identity (transgender or cisgender) of the other twin. The monozygotic twins had transgender identities in common in nine pairs (39%); in contrast, the dizygotic twins had no cases in which both twins shared a transgender identity. With heritability estimates closer to 50% than to 100%, these studies suggest that biological and social factors both contribute substantially to gender identity. If genes predominantly determined gender identity, you would see greater similarity in the gender identities of identical twin pairs and thus higher heritability estimates. For the sake of comparison, a meta-analysis of heritability estimates of major personality traits such as neuroticism (the tendency to experience negative emotions) and extraversion (the tendency toward positive emotion and sociability) yielded an overall estimate of 40% (Vukasovic´ & Bratko, 2015). Therefore, what little data we have on the heritability of gender identity indicate that it is somewhat more heritable than major personality traits are.

As with research on gender identity, twin studies also illuminate the role of genes in masculine and feminine attributes. For instance, monozygotic twins are more similar to one another on masculine and feminine traits than are dizygotic twins, resulting in heritability estimates of 33%—35% (Verweij, Mosing, Ullén, & Madison, 2016). These estimates mean that genetic differences among people explain between 33% and 35% of the population variance in masculine and feminine traits, with social and environmental factors explaining the remainder (65%—67%) of the population variance. Studies with children document similar moderate genetic influences on sex-typed preferences for toys (dolls and toy guns) and activities (dressing up and playing soldier; Iervolino, Hines, Golombok, Rust, & Plomin, 2005).

Biological factors—genes and hormones—clearly contribute to both gender identity and sex-linked attributes, although they cannot explain the full picture. Just as we have discussed throughout this chapter, nature and nurture interact and influence each other in ways that make them difficult to separate. Particularly relevant in the interpretation of findings from twin studies, identical twin pairs tend to be treated more similarly than do fraternal twin pairs, which confounds nature and nurture. Note that we will address the specific factors that contribute to the development of sex-typed traits and preferences further in Chapter 4 (“Gender Development”). Similarly, we will address the nature and nurture of sexual orientation in detail in Chapter 9 (“Sexual Orientation and Sexuality”).

Monozygotic (identical) twins share 100% (or close to 100%) of their genes. Therefore, if they are more similar to one another on a given trait than dizygotic (fraternal) twins are, it suggests that genes exert an influence on the trait.

Source: © Source

Gender Confirmation Procedures

As discussed, controversy surrounds the hormone treatments and surgeries used by doctors and parents to bring the bodies of intersex infants and children in line with their assigned sex. In a different context, transgender individuals sometimes voluntarily seek surgeries and other procedures to bring their physical bodies into greater alignment with their gender identities. These gender confirmation procedures include surgery, hormone treatments, psychological therapy, and voice and communication therapy. Transmen (female-to-male, or FtM, individuals) who undergo surgical procedures may opt for phalloplasty (the lengthening of the urethra and construction of a penis using grafted tissue) or metoidioplasty (the enlargement and separation of the clitoris to form a penis). FtM individuals may also elect to have a mastectomy (removal of breast tissue). Transwomen (male-to-female, or MtF, individuals) who undergo surgical procedures may seek vaginoplasty (the surgical construction of a vagina) and breast augmentation. Although transgender people have regularly altered their appearance to satisfy their gender identity, options for safe and medically sound surgical alterations to the body are relatively new. The first vaginoplasty was performed in 1931 (Munro, 2017), and the first phalloplasty was conducted in 1946 (Kennedy, 2007). Still, many transgender individuals, especially people of color and low-income people, continue to face difficulties in accessing medical care to assist them in the process of transitioning (Strousma, 2014). One study of transgender adults in Massachusetts found that almost 25% were unable to access needed hormones or surgery, especially if they were low income, had low educational attainment, or had limited insurance coverage (White Hughto, Rose, Pachankis, & Reisner, 2017).

Gender confirmation procedures Procedures (including hormone treatments, surgeries, speech therapies, and psychotherapies) that transgender individuals sometimes seek to bring their physical bodies into greater alignment with their psychological identities.


Individuals who transition from their assigned sex to their felt gender identity have often been (and sometimes still are) referred to as transsexual, but not all individuals who transition identify with this term. In this book, we use the terms transwomen or MtF (male-to-female) individuals and transmen or FtM (female-to-male) individuals to signify people who undergo a formal (physical or psychological) transition from their assigned sex to their felt gender. Furthermore, some people dislike terms such as sex change or sex reassignment surgery because “change” is not an accurate way to describe an identity that many transgender individuals have always felt. While some prefer the term gender confirmation surgery, others opt for genital reconstructive surgery since it simply describes the procedure with no added meanings about sex or gender being reassigned or confirmed. Gender confirmation procedures, in contrast, refer to a broad range of treatments that bring the body into greater alignment with gender identity, including surgeries, hormone treatments, voice and communication therapies, and psychotherapies.

The decision of whether or not to get surgery can be challenging. Genital reconstructive surgery is irreversible, and the surgery itself can be painful, with a difficult recovery. This level of physical body alteration may also be inconsistent with people’s values or personal preferences. Finally, the cost is prohibitive for some. Not surprisingly, income is a predictor of the decision among transgender individuals to get genital reconstructive surgery (Beckwith, Reisner, Zaslow, Mayer, & Keuroghlian, 2017). Thus, some transgender individuals who want to alter their bodies physically opt instead for hormone treatments, and these treatments are also often used as a preliminary stage in the transition process, preceding any surgical procedures. Relatively less expensive feminizing or masculinizing hormones (e.g., estrogen and testosterone, respectively) can be administered in many forms (injections, pills, patches, and implants) and effectively stimulate the development of secondary sex characteristics such as increased breast tissue in MtF individuals and facial and body hair in FtM individuals. In terms of mental health, one review of longitudinal studies found that both MtF and FtM individuals showed significant decreases in anxiety and depression symptoms 3—12 months after initiating hormone treatments (White Hughto & Reisner, 2016). Still, other transgender individuals opt out of both surgery and hormone treatments for a variety of reasons such as cost, personal beliefs, and ease of access.

Former Thai boxing champion Nong Toom, a transwoman who used her boxing earnings to pay for gender confirmation surgery, had her life story portrayed in the film Beautiful Boxer.

Source: Getty Images / STEPHEN SHAVER / Staff


If one goal of the transgender movement is to push society beyond a binary conceptualization of sex as either male or female, then how do MtF and FtM individuals fit into this picture? Do you think that fewer people would transition from male to female or from female to male if society moved beyond a binary system of sex? Why or why not?


Some of the physical sex differences that we have discussed so far, including chromosomes and hormones, are understood fairly well. In contrast, identifying sex differences in the brain is a less straightforward task. While there appear to be some (small) sex differences in the brain, little is known about what causes them and what they signify. Neuroscientists debate questions of how to understand and interpret sex differences in the brain, with some contending that sex differences in the brain matter greatly (Cahill, 2006) and others that the findings are overstated (Fine, Joel, & Rippon, 2019).

Sex Differences in the Brain

If you placed a pile of human brains on a table, would an expert be able to sort them by sex based on visual inspection alone? Not likely. One analysis of the brain scans of over 1,400 people between the ages of 13 and 85 concluded that male and female brains are structurally very similar in terms of cortical gray matter (neuron cell bodies and dendrites in the cerebral cortex), cortical white matter (myelinated axons in the cerebral cortex), and connective tissue (Joel et al., 2015). When results were averaged across all brains, the researchers did identify a number of regions that differed in size between female and male brains. However, approximately 92% of the individuals in the sample did not follow this sex-typical pattern entirely. Instead, most people showed what Joel and her colleagues (2015) called a “unique mosaic” of male-typical and female-typical patterns in the brain. This suggests that the structure of the brain does not cleanly differ along the sex binary (Phillips et al., 2019).

Magnetic resonance imaging (MRI) An imaging procedure that uses magnetic fields and radio waves to create high-resolution images of brain structures.

Functional magnetic resonance imaging (fMRI) A brain imaging technique that uses magnetic fields and radio waves to map brain activity.

Since the late 1970s, brain imaging techniques have become much more sophisticated and include technologies such as magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI). While MRI creates vivid, high-resolution images of brain structures (size, shape, and form), fMRI measures brain activity by detecting changes in blood flow in the brain, thereby allowing insight into brain functions (processes that underlie cognition, behavior, emotion, or perception). These and other sophisticated imaging techniques led to a proliferation of research on sex differences in the brain. One finding is that male brains are, on average, about 11% larger in volume than female brains, as measured in total volume and not corrected for body size differences (Ruigrok et al., 2014). While this sex difference is not obvious to the naked eye, computers can easily detect it. When researchers fed information about the gray matter volumes of 1,300 female and male brains into a computer, using machine learning techniques, the program was able to distinguish the brains by sex with over 93% accuracy (Anderson et al., 2019). There are also sex differences in specific regions of the brain, with some areas larger or denser in male brains and some areas larger or denser in female brains. For example, women show greater volume and density than men in the frontal pole cortex (responsible for strategic planning and decision-making). And men often show greater volume and density than women in the left hippocampus (part of the limbic system that regulates memory, learning, and emotion) and in the left amygdala (part of the limbic system involved in processing and expressing emotion, especially fear; Lotze et al., 2019).

Machine learning A method of data analysis that trains computers how to detect patterns, and learn from data, with minimal human intervention.

How do these structural brain differences arise? Some evidence suggests that they are set in motion prenatally by the production of gonadal hormones that shape fetal brains, and they continue to develop during the phase of brain plasticity in adolescence. Plasticity (or neuroplasticity), which refers to the brain’s ability to reorganize and adapt physically in response to life experiences and environmental factors, is especially high during adolescence. During this time, the brain undergoes several major developmental changes that are regulated internally by pubertal increases in sex hormones (including testosterone, estrogen, progesterone, and estradiol) and that are also shaped by factors such as individuals’ patterns of eating, sleeping, and caffeine and tobacco use (Arain et al., 2013). But the (perhaps) more interesting question concerns the precise manner in which structural sex differences in the brain relate, if at all, to psychological sex differences. And when it comes to this question, the fact is that psychologists do not yet have an answer. Neuroscientists do not fully understand the connections between brain structure and brain function, and hypotheses about human sex differences in brain functions tend to be speculative (Fine et al., 2019). Given this, consider this important question: If the size or structure of various brain regions does not reliably predict meaningful behavioral or psychological differences between women and men, then what is the value of searching for sex differences in the brain?

Plasticity (or neuroplasticity) The ability of the brain to reorganize and adapt physically throughout life in response to life experiences and environmental factors.

Equating the Brain With “Nature”

Several challenges confront researchers who study sex differences in the brain. Recall from Chapter 2 (“Studying Sex and Gender”) that bias can enter a study at any point in the research process. In the case of brain research, some neuroscientists exhibit a bias toward equating the brain with nature. Upon documenting sex differences in the brain, they assume that these differences must result from innate biological differences between female and male individuals. Consider what might be problematic about this assumption. In particular, think back to our discussions of the malleability of the brain and the interconnectedness between nature and nurture. Dynamic systems theory (DST) is relevant here (Fausto-Sterling, Coll, & Lamarre, 2012). This theory proposes that sex differences in the brain, body, and behavior are small to nonexistent at birth, and grow larger over time through dynamic interactions between caregivers and infants. The gender expectations of caregivers shape how they treat their infants (e.g., vocalizing more in response to female infants), which, in turn, shapes the brain development of their infants, leading to larger sex differences over time. This process emphasizes the brain’s plasticity, which renders it susceptible to modification via life experiences. If boys and girls are exposed to different gender-stereotyped toys, games, and media when they are young, this may enlarge or create sex differences in the brain, and these brain differences then grow over time as the environments of boys and girls become increasingly different (Eliot, 2018). Therefore, the observation of sex differences in brain structures does not necessarily imply a stronger role of nature than nurture in causing these differences.

The environment continues to shape neural structures and sex differentiation past infancy and childhood. Furthermore, just because hormones—a biological factor—contribute to sex differences in brain structures, it does not mean that biology is the sole driver of these effects. In fact, social factors can alter hormone levels. As we discussed in Chapter 1, performing male-typed and female-typed behaviors can change people’s testosterone levels. For example, women who role-played having power over someone (i.e., firing a subordinate) subsequently showed increases in testosterone relative to a neutral condition (van Anders, Steiger, & Goldey, 2015), and men who soothed a simulated crying infant showed a decrease in testosterone relative to those who listened to but did not soothe the infant (van Anders, Tolman, & Jainagaraj, 2014). These findings led Sari van Anders and her colleagues to theorize that adult sex differences in testosterone levels may partially reflect sex differences in the types of behaviors that women and men are routinely socialized to perform.

Neuroscience or Neurosexism?

Neuroscience is the scientific study of the nervous system, including neurons and the brain. Although powerful neuroimaging technologies (such as fMRI) opened the door for neuroscientists to peer into the brain in new ways, these technologies, like any tool, can be misused, and the images they produce can be misinterpreted. In her book Delusions of Gender, Cordelia Fine (2010) introduced the concept of neurosexism, which occurs when people interpret the findings from neuroscience research in ways that reinforce gender stereotypes without valid supporting evidence. When researchers catalog structural sex differences in the brain without linking these to meaningful functional differences, they may perpetuate inaccurate gender stereotypes. Moreover, as we noted, a focus on structural sex differences in the brain can imply that these differences stem from biological factors, which is not necessarily true. Thus, neurosexist research practices can reinforce essentialist beliefs that men and women have inherent, unique, and natural attributes that make them two qualitatively different sexes, which then fuels the popularity of these binary beliefs in the media (Bluhm, 2013b; Fine, 2013). To combat this, the subfield of neuroethics encourages responsibility in neuroscience research. Neuroethics prompts neuroscientists to reflect systematically on their perspectives and research practices and to consider the social, legal, and ethical implications of their findings (Clausen & Levy, 2015).

Neurosexism Interpreting the findings from neuroscience research in ways that reinforce gender stereotypes without valid supporting evidence.


As noted earlier, scholars have often favored either a biological or a social-environmental account of human development. Reflecting this either/or approach, some theories of sex differences focus more on biological influences (e.g., newborn temperaments or activity levels), and some focus more on environmental influences (e.g., parental socialization). Increasingly, however, gender researchers embrace interactionist models that acknowledge the roles of both types of factors. In this section, we outline two theories that attempt to explain the ultimate origins of sex differences and similarities: evolutionary psychology and biosocial constructionist theory. Each theory examines why people of different sexes exhibit certain traits, gender roles, and sex-related preferences. Because we will return to these theories repeatedly throughout this book, it is worth considering them in some detail. Note that although both theories consider nature and nurture, they differ in the emphasis placed on each factor.

Evolutionary psychology A theoretical approach that explains much of human thought and behavior in terms of genetically heritable adaptations that evolved because they helped ancestral humans survive and reproduce.

Biosocial constructionist theory A theory that explains how biological differences between women and men lead to sex-based labor divisions in society, which then shape the development of role-relevant skills and gender stereotypes.

Natural selection The evolutionary process by which heritable features that increase the likelihood of an organism’s survival get passed down through genes.

Sexual selection The evolutionary process by which heritable features that increase the likelihood of successful mating get passed down through genes.

Evolutionary Psychology

Using Charles Darwin’s theory of evolution as a guiding framework, evolutionary psychologists assert that humans’ physical, behavioral, and psychological attributes today are products of what our ancestors did to survive and reproduce hundreds of thousands of years ago. People vary genetically across many attributes, and sometimes these variations give some people advantages in terms of survival and reproduction. When this happens, advantageous variations more frequently get passed down genetically to future generations. But how does evolution shape sex and gender differences? According to the logic, sex differences should arise in domains in which women and men faced different adaptive problems in our evolutionary past (Buss & Schmitt, 2019). An adaptive problem is any environmental condition that challenges an organism’s ability to survive or reproduce. Some adaptive problems affect people equally regardless of sex, and thus we would not expect sex differences to evolve in response to these challenges. However, women have faced unique challenges when it comes to pregnancy, birthing, and nursing offspring, and men have faced the unique challenge of paternity uncertainty, which is the problem of not knowing with certainty whether a given child is one’s biological offspring. Therefore, according to evolutionary psychology, women and men should have evolved different psychological and behavioral tendencies in adaptation to these different challenges. It is important to note that psychologists’ use of evolutionary theory to explain sex differences is controversial, stimulating much critique and debate (Eagly & Wood, 2013). We will first describe the main tenets of this perspective and then summarize the debate.

Natural selection is a process whereby heritable features increase or decrease an organism’s likelihood of survival, and sexual selection is a process in which heritable features make an organism more or less likely to reproduce and pass on its genes. According to evolutionary psychologists, sexual selection takes two main forms. First, in intrasexual selection, an individual may have a feature that gives it a competitive edge over other same-sex members in the contest for access to mates. For example, male bighorn sheep use their large curved horns to head-butt rivals in an attempt to gain status and access to mates. Second, in intersexual selection, an individual may have a feature that gives it an advantage by increasing its attractiveness to the other sex. For example, peacocks evolved elaborate colorful plumage presumably because peahens select the peacocks with the biggest, most elaborate feathers as mates. You can thus think of these two evolutionary processes in this way: Intrasexual selection produces weaponry for besting one’s competitors, while intersexual selection results in attractive ornamentation for enticing mates.

Intrasexual selection The process by which heritable features get passed down because they give an animal a competitive advantage in contests against same-sex animals for access to mates.

Intersexual selection The process by which heritable features get passed down because they give an animal an advantage by increasing its attractiveness to other-sex mates.

Let’s return now to the notion of sex differences in adaptive challenges. According to evolutionary psychology, one adaptive challenge faced by ancestral men was gaining access to mates. Whereas most ancestral women had a high likelihood of reproducing, ancestral men displayed much greater variance in reproductive success: High-status men at the top of social hierarchies often had multiple mates, while those at the bottom of status hierarchies sometimes did not mate at all (M. Wilson & Daly, 1992). To adapt to this challenge, men should have evolved qualities that helped them during intrasexual competitions, including large physical size, physical strength, and competitive and aggressive tendencies. These qualities helped men to win competitions against other men, and men who won such competitions would have had more mating opportunities with women (Janicke, Häderer, Lajeunesse, & Anthes, 2016).

On the other hand, women (and female members of many other species) faced an adaptive problem by having to invest more in reproduction and parenting than males do. For women, successful child-rearing requires, at the very least, a 9-month commitment of energy, nutrients, and physical resources during pregnancy, plus an additional 1—3 years of lactation and nursing. Compare this to the brief investment required of men—potentially as brief as a single act of sexual intercourse—in order to produce a child. According to parental investment theory, the member of the species who invests more in producing offspring and parenting will generally be more selective when choosing mates because they have more to lose from bad choices (Trivers, 1972). How does this relate to intersexual selection? Across cultures, women, on average, demonstrate a stronger preference than men do for high-status, resource-rich mates (Buss & Schmitt, 2019). Evolutionary psychologists propose that women evolved a preference for such mates because mates with higher social status and more resources can provide better for offspring. Like the peacock’s flashy tail, men’s displays of status and resources are like ornaments that advertise their ability to provide for a mate and offspring.

Parental investment theory Theory proposing that the sex that invests more in parenting (usually female) will be more selective in its choice of mates and will prefer mates who have social status and resources.

At the same time, men across cultures demonstrate a stronger preference than women do for physically attractive and younger mates (Buss & Schmitt, 2019). Women’s physical attractiveness and youth signal health, good genes, and fertility, and these qualities convey a greater likelihood of carrying healthy offspring to term. Moreover, because of paternity uncertainty, men should have evolved a heightened tendency toward sexual jealousy because men who jealously guarded their mates against rivals had a lower risk of supporting offspring who did not carry their genes (for more on this topic, see Chapter 10, “Interpersonal Relationships”). So according to evolutionary psychologists, beauty, youth, and signs of sexual fidelity are ornaments that women can use to indicate their mating potential to men.

The horns of a bighorn sheep (left) are the product of intrasexual selection, allowing it to compete against other sheep in head-butting status competitions for access to mates. The peacock’s elaborate plumage (right) is a product of intersexual selection, serving as an enticing ornament to attract peahens.

Source: ©;

As noted, however, the evolutionary psychology perspective suggests that sex differences in mate preferences should only emerge for qualities relevant to sex differences in our ancestors’ adaptive problems. And, in fact, the qualities that both sexes rank as most essential in a mate, including emotional stability, dependable character, and mutual love, show little evidence of sex differences (Boxer, Noonan, & Whelan, 2015). Moreover, evolutionary psychologists argue that environmental and cultural cues interact with evolved tendencies to shape mate preferences (Buss & Schmitt, 2019). In other words, while genes (nature) are the driving force of evolution, the human mind is responsive to local environments (nurture) and varying contexts. For example, in cultures characterized by higher levels of gender equality, sex differences in preferences for a high-status mate with earning potential are smaller (Eagly & Wood, 1999). This may reflect the fact that the reproductive success of women in more gender-egalitarian cultures is not as contingent on their ability to secure a resource-rich mate as it is in cultures where women have less access to education, jobs, and financial security.

Evolutionary psychology is both an incredibly influential and an incredibly controversial approach. One criticism of it centers on the scientific merit of the theory, in that its tenets tend to be speculative and difficult, if not impossible, to test empirically (Gannon, 2002). After all, we cannot go back 200,000 years, randomly assign some women to mate with high-status men and others to mate with low-status men, and observe how their offspring fare. Despite being speculative about the conditions of the distant past, however, the theory still generates testable predictions about present-day behavior, and we will consider many of these predictions throughout this text. Evolutionary psychology is also criticized for reinforcing the sex binary and for being heteronormative in its emphasis on heterosexual mating preferences and reproductive sex. And yet, evolutionary psychologists have contributed several hypotheses regarding the origins and maintenance of same-sex sexual activity and lesbian and gay sexual orientations, which we will consider in depth in Chapter 9 (“Sexual Orientation and Sexuality”).


Evolutionary psychologists and feminist psychologists often do not see eye-to-eye about sex differences, as each camp occupies a different location on the nature—nurture continuum. Evolutionary psychologists criticize feminist psychologists for overstating sex similarities and ignoring the role of biology in order to advance a feminist political agenda. Feminist psychologists criticize evolutionary psychologists for espousing an essentialist view of women and men, one that roots sex differences in biology without sufficient evidence and reinforces power differences and inequality between women and men (C. A. Smith & Konik, 2011). In response to this debate, some have called for a more integrative approach that avoids going too far in the direction of either nature or nurture (Eagly & Wood, 2013). Others emphasize the need for more critical thinking about sex and gender and for greater recognition of women as active agents in evolutionary processes, such as intrasexual and intersexual selection (Kruger, Fisher, & Wright, 2013).

Biosocial Constructionist Theory

Biosocial constructionist theory integrates the roles of distal (distant) biological factors and proximal (close) social and cultural influences in explaining sex differences and similarities in behavior and traits (W. Wood & Eagly, 2012). According to this theory, the key to understanding sex differences and similarities is the division of labor in societies (see Figure 3.4). All societies have to solve the problem of dividing labor, and most do this by assigning women and men different jobs. Why? Let’s start with basic physical differences. Men are, on average, larger and stronger than women, which tends to make them better suited for some types of physically demanding and dangerous occupations. In some ecologies, this might mean chopping down trees; in others, it might mean mining, deep-sea fishing, or big-game hunting. Women’s reproductive activities (pregnancy and nursing) make it less efficient for them to do jobs that require them to be away from home for long periods of time and more efficient for them to perform domestic activities that keep them close to nursing infants and young children.

In other words, this theory argues that most societies can function more efficiently when men do certain types of jobs and women do others. Two important implications follow from this gendered division of labor. First, men and women must acquire different skills, which children begin to learn early in life. Adults and peers socialize young girls to become caretakers and homemakers by providing rewards and encouragement for things like playing with dolls, taking care of younger siblings, or playing house. Adults and peers also teach young boys to become physically active risk-takers by rewarding them for bravery, rough-and-tumble play, and activities that foster physical skill development. Second, people form expectations about the qualities and abilities of men and women as a result of their socialized skills and gendered social roles. Because of their roles as primary caretakers of children, women are expected to be—and perceived as—more warm, emotional, and nurturing than men. Because of their roles as providers, men are expected to be—and perceived as—more independent, assertive, and risk-taking than women. According to biosocial constructionist theory, this is how gender stereotypes are formed, a topic we will examine in detail in Chapter 5 (“The Contents and Origins of Gender Stereotypes”). So, gender role beliefs are learned, reinforced, and perpetuated through socialization and the practice of skills. These beliefs tend to become internalized as well, meaning that people often regulate their own behaviors and interests to be consistent with gender role stereotypes.


Figure 3.4 The Biosocial Constructionist Model of Sex Differences

Source: W. Wood and Eagly (2012).

Of course, as societies change, the types of occupations that women and men do change as well. In contemporary Western, industrialized societies, most people do not work in the fields or hunt big game all day. Advances in technology have led to decreases in some physically demanding jobs and to increases in more cognitively oriented jobs, with a trend toward women and men doing more similar jobs. In some societies, the introduction of birth control has allowed women to delay or forgo having children, just as breast pumps and baby formula now allow men to assume more active roles in child-rearing, compared with generations past. According to the biosocial constructionist theory, these changes should lead to different (and more similar) skills and expectations for women and men and, consequently, to changes in gender stereotypes. Thus, this theory emphasizes not only biological sex differences that lead to gendered labor divisions, but also more immediate environmental influences that shape gender roles and stereotypes.

Girls are encouraged to learn caretaking skills early in life by, for example, taking care of dolls.

Source: ©

Note that this leads to a divergence between evolutionary theory and the biosocial constructionist perspective. Evolutionary theorists tend to view the human brain as a fossil that represents adaptations to conditions that occurred tens of thousands of years ago, which means that recent changes to occupational or social roles should not change our fundamental psychology much. Moreover, evolutionary psychologists propose that sex differences in tendencies such as mating preferences and sexual jealousy are genetically coded at the species level and therefore unlikely to change over the short term, even though the brain is flexibly adaptive to local contexts and environments (Buss & Schmitt, 2019). In contrast, biosocial constructionist theory assumes that while some biological sex differences are coded in genes, sex differences in psychological factors, such as personality traits, mating preferences, and jealousy, are not genetically heritable. Therefore, this theory predicts that changes in social roles can create fairly rapid changes in the psychology of men and women, and these can lead rather rapidly to changes in cultural gender stereotypes. Thus, as noted earlier, the divergence between these theories boils down to one of emphasis: Evolutionary psychology primarily emphasizes biology while acknowledging the important role of environments, and biosocial constructionists primarily emphasize the environment while acknowledging the important role of biology. However, both approaches have a weakness in common in that they conceptualize sex and gender as binary categories, which, as we discussed earlier, oversimplifies reality.


Consider the major changes to the workforce that occurred in the United States and similar nations as women entered the workforce in record numbers beginning in the second half of the 20th century. How might these changes shape the psychology of women and men according to the biosocial constructionist theory? What about according to evolutionary theory?

So, what should a comprehensive theory of sex differences and similarities look like? First, it should be interactionist, recognizing the influences of both nature and nurture. Second, it should address how a range of physical and biological differences (e.g., in size, strength, and reproductive capabilities) among the sexes may produce different behaviors and traits. In doing so, however, it should move beyond the sex and gender binaries and consider how sex differences might vary based on other important categories of identity such as race, class, age, and ability. It should also consider the factors that motivate not just heterosexual, reproductive mating but also same-sex sexual activity, which is common in both human and nonhuman animal species. Finally, a comprehensive theory should acknowledge that the environment can alter physiology and biology in subtle ways that can have significant consequences (think back to the discussion of epigenetics). As we have emphasized throughout this chapter, biological and social factors are inextricably intertwined and flexibly responsive to one another: Biology shapes our social and cultural experiences, and sociocultural factors shape our biology. We encourage you to keep these ideas in mind not only as you read this book but also as you observe how sex and gender operate in your daily life. Remember—it’s not “nature versus nurture” but “nature and nurture.”


· 3.1 Explain how nature and nurture interactively contribute to the development of sex and gender.

Determining the relative contributions of nature (biological factors) and nurture (social and environmental factors) in shaping sex and gender is challenging. Scholars today generally recognize that both nature and nurture play complex, interconnected roles in creating and shaping sex and gender. In gene-by-environment interactions, a genetic effect on behavior only emerges under certain environmental circumstances. Epigenetics is the study of the biological mechanisms that guide whether or not certain genes get activated. Life experiences can influence epigenetic environments and thus genetic expression. Thus, although biological and sociocultural factors are conceptually independent, they interact in ways that make them difficult to disentangle.

· 3.2 Explain how chromosomes, genes, and hormones shape sex differentiation in both typical and atypical (intersex) cases.

Typical sexual differentiation begins at conception, when a sperm carrying either an X (female) or Y (male) chromosome fertilizes an egg. For the first 6 weeks of life, male and female embryos remain largely undifferentiated, having the same internal and external structures. Female sex is the biological default, but a gene called SRY on the Y chromosome instructs the body to produce androgenizing hormones that cause male gonads (testes) to develop. In the absence of the SRY gene, female gonads (ovaries) develop. The external genitalia remain undifferentiated until about the 12th week of gestation, when the penis and scrotum or the clitoris and labia typically develop.

In intersex children, the biological components of sex (chromosomes, hormones, genitals, and internal and external sex organs) do not align consistently as either male or female. Differences from the typical XX or XY chromosome pair include XO (Turner’s syndrome), XXX (triple X syndrome), XXY (Klinefelter syndrome), and XYY (Jacob’s syndrome). These intersex conditions suggest that the presence of a Y chromosome strongly predicts having a male appearance and gender identity, while the absence of a Y chromosome strongly predicts having a female appearance and gender identity. But hormones can override chromosomes in shaping gender identity. Genetic females (XX) who overmanufacture androgens, a condition called congenital adrenal hyperplasia (CAH), sometimes have male-typed external genitalia (but female internal reproductive organs) and adopt male gender identities. Conversely, genetic males (XY) who cannot respond to androgens, a condition called complete androgen insensitivity syndrome (CAIS), often have an outward female appearance and adopt female gender identities.

· 3.3 Analyze the biological and sociocultural factors that shape sex assignment and gender identity.

Although people often equate sex with nature and gender with nurture, even “biological” sex is a social construction. That is, cultures define and give meaning to the categories of sex. Many cultures acknowledge only two sexes, male and female, even though biological sex extends beyond this binary to include other variations. In strongly binary cultures, intersex or transgender individuals may be shunned, stigmatized, or pressured to conform to conventional notions of sex. Parents often elect for their intersex newborns to undergo surgery to bring their physiology in line with their optimal sex, as defined by doctors. Transgender individuals sometimes experience gender dysphoria, defined as clinically significant levels of distress arising from the discrepancy between one’s assigned sex at birth and one’s psychological gender identity. Some—though not all—transgender individuals undergo gender confirmation procedures, including hormone treatments or surgery, to bring congruence between their physiology and their psychological gender identity. A lack of financial resources, low educational attainment, and lack of health insurance can prevent some transgender people from accessing needed hormones or surgeries.

· 3.4 Evaluate evidence for sex differences in the brain and the prevalence of neurosexism.

There are small but reliable sex differences in the structure of the brain. Some brain regions, such as in the hippocampus and the amygdala, tend to be larger or denser in male brains, whereas other brain regions, such as the frontal pole cortex, tend to be larger or denser in female brains. Researchers debate the meaning and importance of these differences. It is important to understand that structural sex differences in the brain are not necessarily innate. Adults may create different environments for newborn girls and boys, which can produce or enlarge structural sex differences in the brain because the brain is malleable. Neurosexism occurs when people interpret the findings of neuroscience research in ways that reinforce gender stereotypes without valid supporting evidence.

· 3.5 Examine the roles of nature and nurture in theories of the origins of sex differences.

Theories that explain the origins of sex differences and similarities increasingly incorporate both nature and nurture. Evolutionary theories assume that much of human thought and behavior reflects adaptive psychological mechanisms that helped our ancestors survive and reproduce. Biosocial constructionist theory argues that physical sex differences lead to a division of labor that, in turn, leads to the socialization and acquisition of different skills and behaviors in boys/men and girls/women. Differing roles then create different expectations and self-perceptions of male and female behavior. While influential, both of these theories neglect to account fully for the experiences of intersex, nonbinary, and sexual minority individuals.

Test Your Knowledge: True or False?

· 3.1. Sex (whether a person is male, female, or intersex) is shaped entirely by biological factors. (False: Sex is shaped by complex interactions between nature and nurture.) [p. 74]

· 3.2. Genetically female (XX) individuals who get exposed to high levels of androgens in utero can have male-typed genitalia at birth and are sometimes raised as boys. (True: Genetically female individuals with congenital adrenal hyperplasia [CAH] experience high levels of androgens in utero and are sometimes raised male, especially in countries that lack access to diagnosis and treatment) [p. 95]

· 3.3. All female individuals have an XX pair of chromosomes, and all male individuals have an XY pair. (False: While most girls have XX pairs and most boys have XY pairs, some female-identifying and male-identifying individuals have different combinations of chromosomes.) [p. 88]

· 3.4. From twin studies, we know that gender identity is determined primarily by socialization (nurture). (False: Twin studies indicate that gender identity is determined by a combination of genetic and environmental factors.) [p. 103]

· 3.5. In the past decade, neuroscientists identified structural sex differences in the brain and linked them definitively to specific psychological sex differences. (False: Neuroscientists do not fully understand the connections between brain structure and function, and hypotheses about human sex differences in brain functions remain speculative.) [p. 106]

Descriptions of Images and Figures

Back to Figure

The biological components for women are listed below:

· Chromosomes: XX (No SRY gene)

· Gonadal differentiation:

o Gonads: Ovaries

o Hormones: Estrogen, progesterone

o External genitalia: Vulva

o Female phenotype.

The biological components for men are listed below:

· Chromosomes: XY (SRY gene)

· Gonadal differentiation:

o Gonads: Testes

o Hormones: Testosterone

o External genitalia: Penis, scrotum

o Male phenotype.

Back to Figure

The autosome pairs are arranged in four rows. The visual representation of each pair of chromosomes in each row are listed as follows:

· First row:

o 1: Two long chromosomes in the shape of X

o 2: Two long chromosomes that is bent toward the right

o 3: A long chromosome and a medium chromosome arranged in the shape of X

o 4: Two medium chromosomes with one in the form of inverted S and other in the form of inverted L.

o 5: A medium chromosome with curves and another medium chromosome in the form of L.

· Second row:

o 6: A medium chromosome with curves and a long chromosome in the shape of C

o 7: A long straight chromosome and a medium bent chromosome

o 8: Two medium chromosomes bent toward the left in the form of C

o 9: Two medium chromosomes with their heads pointing toward the left

o 10: Two medium chromosomes with their heads pointing toward the right

o 11: A medium chromosome in the form of question mark and a long chromosome that is bent outward at the bottom

o 12: Two medium chromosomes arranged in the form of X.

· Third row:

o 13: Two medium-small chromosomes in the shape of L facing opposite to each other

o 14: A medium-small chromosome that is bent toward the left at the bottom and another medium-small chromosome in the form of L

o 15: Two straight medium-small chromosomes

o 16: Two medium-small chromosomes in the form of C facing in opposite directions

o 17: A medium-small chromosome in the form of inverted L and a medium-small chromosome slightly bent toward the right

o 18: Two straight medium-small chromosomes.

· Fourth row:

o 19: Two straight medium-small chromosomes

o 20: Two medium-small chromosomes bent outward

o 21: Two small chromosomes

o 22: Two small fading chromosomes

The last pair of chromosomes in the fourth row are labeled X Y with one medium chromosome in the form of S along with another small chromosome.

Back to Figure

The three stages of genitalia development with the parts are mentioned below:

First stage: Embryo, 6 weeks:

· An oval opening in the middle labeled urethral fold.

· On both right and left side of urethral opening are two oval glands labeled labioscrotal swelling.

· A round opening below the urethral fold is labeled anus.

· A big round gland above the urethral fold is labeled as genital tubercle.

Second stage: Fetus, second trimester:

· Female:

o An oval opening in the middle labeled urogenital slit.

o Around the urogenital slit are three oval layers.

o A big round gland at the top of the urogenital slit is labeled glans.

o A small opening below the urogenital slit is labeled anus.

· Male:

o An oval opening in the middle labeled urethral slit.

o Around the urethral slit are three oval layers.

o A big round gland at the top of the urethral slit is labeled glans.

o A small opening below the urethral slit is labeled anus.

Third stage: Birth:

· Female:

o A small oval opening in the middle is labeled vagina.

o Around the vagina are two oval layers with increasing diameter. The top of the first oval layer is labeled opening of urethra.

o The second layer is labeled inner labia and a small round gland at the top is labeled glans of the clitoris.

o The gland outside the second oval layer is labeled as outer labia.

· Male:

o A U-shaped outgrowth labeled scrotum.

o Above the scrotum is a long, cylindrical gland extending outward. The base is labeled as the shaft of the penis and the top is labeled as glans of the penis.

Back to Figure

The flow between each components of the model are listed as follows:

· From two factors: local culture, ecology, economy and women’s reproductive activities, men’s size and strength to division of labor.

· From division of labor to: Social construction of gender, gender role benefits

· From social construction of gender, gender role benefits, an arrow labeled socialization flows back to division of labor.

· Three components diverge from social construction of gender, gender role benefits:

o Hormonal regulation

o Self-regulation to gender identities

o Social regulation.

o The three components converge and flows toward sex-differentiated affect, cognition, and behavior.