The Psychology of Women and Gender: Half the Human Experience + - Nicole M. Else-Quest, Janet Shibley Hyde 2018

Biology and Gender

“An extraordinarily important part of the brain necessary for spiritual life, the frontal convolutions and the temporal lobes are less well developed in women and this difference is inborn. . . . If we wish a woman to fulfill her task of motherhood fully, she cannot possess a masculine brain. If the feminine abilities were developed to the same degree as those of the male, her maternal organs would suffer, and we should have before us a repulsive and useless hybrid.”

Moebius (1907), Concerning the Physiological Intellectual Feebleness of Women

The human brain is a spectacular organ, containing over 100 billion neurons. It is constantly rearranging its more than 100 trillion connections between neurons in response to learning and exposure to new stimuli. Spectacular though the human brain is, scientists have wondered for more than 100 years whether men’s brains weren’t more spectacular than women’s. Women¹ have also been thought to be the victims of their “raging hormones.” In this chapter, we examine what is known about genes, hormones, and the brain in women compared with men and in transgender individuals.

1. This chapter focuses on biology, and we use women throughout as shorthand to refer to people with all or some of these biological characteristics: XX chromosomes, a uterus, vagina, clitoris, and so on. Here, women is not used to refer to people’s gender identity. Thus, this chapter focuses on the biology of cisgender women, a subset of transgender men who have female organs, nonbinary individuals who were assigned a female gender at birth, and some intersex individuals.

Genes

Typically, a human has a set of 46 chromosomes in each cell of the body. Because chromosomes occur in pairs, there are 23 pairs, classified as 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. Typically, women have the sex chromosome pair XX and men have the sex chromosome pair XY. Thus there are no genetic differences between men and women except for the sex chromosomes and the genes on them.

Traits that are controlled by genes on the sex chromosomes are called sex-linked traits. For such traits, a woman will have a pair of genes controlling a particular sex-linked trait, but a man will have only one gene for that trait, because he has only one X chromosome. The Y chromosome is small, containing fewer than 50 genes, compared with the X chromosome, which contains between 1,000 and 2,000 genes (Wizemann & Pardue, 2001).

Sex-linked trait: A trait controlled by a gene on the X chromosome (and occasionally on the Y chromosome).

Photo 10.1 X-chromosome inactivation: A tortoiseshell cat such as this one is always female (carries XX chromosomes). The pattern of coat colors results from having one X chromosome carrying the gene for black coat color and the other X chromosome with the gene for orange coat color. Different X chromosomes were inactivated in different parts of the coat, causing patches of different colors. The patches of white coat are due to a different gene.

©iStockphoto.com/adogslifephoto.

Rapid advances in genetic research, including the Human Genome Project, have given us much better information about what’s happening with the X and Y chromosomes. The Y chromosome contains a few especially interesting genes. One is the SRY (Sex-determining Region Y chromosome) gene, which during the prenatal period directs the fetus’s gonads to differentiate in a male direction, forming testes, which then produce testosterone. It also contains a few genes related to male fertility (Lahn & Page, 1997).

The X chromosome, with its large number of genes, influences many aspects of the functioning of cells, growth, and development. It contains several genes responsible for differentiation of the ovaries during fetal development. But there is a difference between men and women in “gene dosage” from the X chromosome. Women have twice as many genes because they have two X chromosomes. This extra gene dosage is compensated for by a process called X-chromosome inactivation, in which one of the X chromosomes in female fetuses is inactivated or silenced in almost all cells, so only one X chromosome functions (Okamoto et al., 2004; Percec et al., 2002). A gene on the X chromosome, called “Xist,” causes the inactivation (McCarthy & Arnold, 2011). The same X chromosome is not inactivated in every cell—in some cells the X chromosome from the mother is silenced, and in other cells it is the X chromosome from the father that is silenced.

X-chromosome inactivation: In female fetuses, the process in which one of the two X chromosomes is inactivated or silenced in nearly every cell, so only one X chromosome functions.

Epigenetics

One of the hottest topics in genetics today is epigenetics, which refers to changes in gene expression caused by factors others than DNA (Bird, 2007; Salk & Hyde, 2012). (If you’re interested in the details, one mechanism involves methylation—attachment of a methyl group—to cytosine in the DNA sequence.) What is meant by gene expression? All of us carry genes that are expressed at some times and not others—perhaps depending on how old we are or what our levels of sex hormones are. For example, men carry genes for beard growth, but those genes were not expressed when they were 5 years old. They aren’t expressed until puberty, with its surge in testosterone levels. The bottom line with epigenetics is that environmental experiences can modify whether certain genes are expressed in an individual.

Epigenetics: Changes in gene expression caused by factors other than DNA.

It is tough to conduct epigenetic research with humans, so researchers have used animal models instead. As an example, mother rats lick their pups a lot; it stimulates the pups and helps them grow. Some rat mothers, though, engage in more licking than other mothers do. Female pups born to high-licking mothers are themselves high lickers when they become mothers, whereas female pups who don’t get much licking grow up to be low lickers (Champagne, 2008). Genetics cannot account for the effect, though, because if a pup born to a low-licking mother is given to a high-licking mother to raise, that pup is a high licker in adulthood. The explanation lies in epigenetics. The licking or care that pups receive leads to long-term effects on gene expression, which then show up when the females become mothers themselves (Champagne, 2008, 2010, 2013).

Epigenetics represents a real breakthrough, not just for geneticists, but for gender researchers as well. Epigenetic research shows that genes (DNA) are not destiny—that even the effects of genes are modified by the environment that the individual experiences over the lifespan (Salk & Hyde, 2012).

Are There Genes for Being Transgender?

Scientists have searched for genes that are associated with transgender and gender dysphoria. The research has focused specifically on transsexuals, who are a subset of trans persons; transsexuals have a clear gender identity as male or female, which does not match their gender assigned at birth, and they want to undergo a social transition and medical treatments so that their body aligns with their gender identity (see Chapter 1). The term transsexual is not used much anymore. Instead, the terms used are trans woman (who has a birth-assigned male gender and female gender identity) and trans man (who has a birth-assigned female gender and male gender identity).

According to early research, the genes seem to be different for trans women compared with trans men (Bentz et al., 2008; Hare et al., 2008; Henningsson et al., 2005). In one study, trans women were more likely than cisgender men to have a mutation in the androgen receptor gene (Hare et al., 2008). However, not all the trans women had this mutation. Later studies that have tried to replicate the results for specific genes have often failed to find the same results, though (Zucker et al., 2016). Therefore, at least right now, there is no solid evidence of a particular gene or genes that create a tendency to becoming transgender. If there are genes for being transgender, we don’t yet know what they are.

Focus 10.1 Feminist Biology

Photo 10.2 Dr. Marlene Zuk, a feminist biologist at the University of Minnesota

By Michal Klajban (Hikingisgood.com)—Own work, CC BY-SA 4.0.

To some people, “feminist biology” sounds like an oxymoron. Biology is a science. It’s objective. Feminism is political. The two can’t mix.

In fact, feminist biology has been around for a while and is rapidly gaining momentum. Feminist biology involves two goals: (1) to identify gender bias in traditional biology and (2) to create new biological research that corrects these biases. In many ways, it is like feminist psychology. Here we present three feminist biologists and their research.

Dr. Marlene Zuk is a professor at the University of Minnesota specializing in animal behavior and evolution. In her book Sexual Selections, Zuk (2002) documents how, in animal research, males are most often the subjects; we saw the same pattern in human research in Chapter 1. That is, there is gender bias in the choice of subjects, even with animal research. Some of that research involves early-stage drug studies that will lead to new drugs for humans, and if the drugs work differently in females, it would be important to know it. Even at the level of cells, bias can occur. For example, much research and theory has focused on males and what their sperm do to achieve fertilization of eggs, but in fact females control much in mating. They control the frequency of mating, and they choose to mate with some males and not with others. And here is an example that’s actually funny. The concept of dominance hierarchies and the “alpha male” that is so popular actually originated in research with chickens and the pecking order of hens, who can be quite ruthless! Yet the translation to “alpha males” implies that females have no dominance hierarchies or tendencies to dominate.

Dr. Sari van Anders is a professor of psychology and women’s studies at the University of Michigan. She specializes in social neuroendocrinology in research with humans. That is, she studies the interplay between hormones and social behavior. In one study of hers, the experimental condition involved the masculine behavior of wielding power in a competitive situation, compared with a control condition that involved watching a travel documentary (van Anders et al., 2015). In the power condition, the participant acted out firing a subordinate in the workplace. They were directed to do it in a masculine way, for example, taking up space, infrequent smiling, and interrupting (see Chapter 5 for a discussion of these gendered behaviors). Participants provided a saliva sample both before and after the experience, and testosterone was assayed from the saliva samples. The results indicated that the experience of wielding power increased testosterone levels in women compared with the control condition. The same effect did not occur in men, probably because their testosterone levels were in a much higher range already. The bigger point here is that, although people (including biologists) usually think of hormones as influencing behavior, the reverse process also occurs. That is, behavior—in particular, gendered behavior (wielding power)—can have an effect on hormones.

Dr. Caroline VanSickle (2014) is a biological anthropologist who studies female fossils. It turns out that most of the people who have studied fossils were focusing on male skeletons. She was also part of the team that recently discovered a new species of prehumans in South Africa, Homo naledi. The feminist archaeologists and geologists have a website: www.trowelblazers.com.

All three of these researchers exemplify the goals of feminist biology, identifying sex bias in traditional research and then creating innovative new research on women and gender.

Basic Physiological Processes

Men and women differ in a few basic physiological processes, including metabolism and drug absorption (Hornstein & Schwerin, 2013; Wizemann & Pardue, 2001). After puberty, men have more muscle mass and, on average, a lower percentage of body fat than women. Muscle tissue metabolizes faster than fat tissue, so men have larger energy requirements—they need more food. Stated another way, women add fat if they consume the same food as that eaten by a man who does not gain fat—even if the two have the same body size. Biology can be very unfair.

Other differences between men and women in metabolism can create differences in risk for metabolic diseases and certain blood cancers, such as multiple myeloma (Petrosino et al., 2014).

Quite a bit of work has been done on differences between women and men in the immune system. In general, girls’ and women’s bodies mount a stronger immune response to infections than boys’ and men’s bodies do (World Health Organization, 2011). That said, women are more vulnerable to a few diseases than men are; one example is influenza (the flu; Klein et al., 2012). Also, being in the biological state of pregnancy has an impact on immune functioning. For example, during the H1N1 pandemic of 2009, women in the third trimester of pregnancy were especially vulnerable to infection (World Health Organization, 2011). Sex hormones appear to play a role in both phenomena, that is, why women usually have a stronger immune response and why they can be more vulnerable to infection during pregnancy (Robinson & Klein, 2012). Physiological processes such as these are important as we consider women and health issues (see Chapter 11).

Although these gender differences in basic physiological processes are important to health, there is little evidence that they have behavioral or psychological effects. Yet there are exceptions. For example, meta-analyses show that women are more sensitive to pain than men are, an effect that is found across many species (Berkley & Holdcroft, 1999; Riley et al., 1998). And it is thought to be related to gender differences in levels of testosterone and estrogen (Wizemann & Pardue, 2001). This brings us to another biological factor that may influence gender differences.

Sex Hormones

Hormones are powerful chemical substances manufactured by the various endocrine glands of the body. Endocrine glands secrete hormones into the bloodstream so that they have effects throughout the body, including effects on target organs far from the endocrine gland that secreted them. Among the endocrine glands are the gonads (ovaries and testes) and the pituitary, thyroid, and adrenal glands.

Testosterone is one of a group of “male” hormones called androgens, which are manufactured by the testes. The “female” sex hormones are estrogen and progesterone, which are manufactured by the ovaries. If these hormones influence behavior, then they could create gender differences.

Testosterone: A sex hormone manufactured by the testes and, in lesser amounts, by the ovaries; one of the androgens.

Androgens: A group of “male” sex hormones, including testosterone, produced more abundantly in men than in women.

Estrogen: A sex hormone produced by the ovaries; also produced by the testes.

Progesterone: A sex hormone produced by the ovaries; also produced by the testes

But actually it is a mistake to call testosterone the “male” sex hormone and estrogen and progesterone “female” hormones. Testosterone, for example, is found in women as well as men. The difference is in amount, not presence or absence. In women, testosterone is manufactured by the adrenal gland and the ovaries, and the level in women’s blood is about one-tenth or less than that in men’s (Janowsky et al., 1998). Estrogen and progesterone are also found in men’s blood.

The differences in levels of sex hormones may affect behavior at two major stages of development: prenatally (the time between conception and birth) and during and after puberty (adulthood). Endocrinologists refer to the effects that occur prenatally or very early in development as organizing effects because they cause a relatively permanent effect in the organization of some structure, whether in the reproductive organs or the nervous system. Hormone effects in adulthood are called activating effects because they activate or deactivate certain behaviors. To understand the prenatal effects, we need to examine the process of prenatal gender differentiation.

Prenatal: Before birth.

Prenatal Gender Differentiation

Male—female differences exist at the moment of conception. If the fertilized egg contains two X chromosomes, then the genetic sex is female; if it contains one X and one Y chromosome, the genetic sex is male. The single cell then divides repeatedly, becoming an embryo and then a fetus. Interestingly, during the first 6 weeks of human prenatal development, the only differences between male and female fetuses are in genetic sex. That is, anatomically and physiologically, male and female fetuses develop identically during this period. Beginning approximately during the sixth week of pregnancy, and continuing through about the sixth month, the process of prenatal gender differentiation occurs (Figure 10.1). First, the sex chromosomes direct the differentiation of the gonads. Here’s how that happens. As we saw earlier in the chapter, a Y chromosome contains the SRY gene, and it directs the synthesis of a substance called TDF (testis-determining factor). It causes the neutral gonads to turn into testes. If there is no Y chromosome and no SRY gene, there is no TDF and the neutral gonads turn into ovaries. The gonads then begin secreting sex hormones. That means that the internal environment becomes different for female fetuses and male fetuses because of the differences in levels of hormones that are present.

The sex hormones then influence the course of fetal differentiation. The testes produce testosterone. If testosterone is present, a penis forms. If testosterone is not present, a clitoris and vagina differentiate. Research indicates that the presence of estrogen is also critical for the development of female sexual organs (Fausto-Sterling, 1992). In addition to influencing gender differentiation of the sex organs, the sex hormones influence the rapidly developing brain (McCarthy & Arnold, 2011). The structure most affected seems to be the hypothalamus. The importance of this differentiation will be discussed later in the chapter.

Prenatal Sex Hormone Effects

Male fetuses and female fetuses, then, live and develop in different hormonal environments. Does this have any effect on later behavior?

Most of the evidence in this area is based on experiments done with animals. It may be that the effects on humans would not be the same. But let us consider the animal experiments and then see what is known about similar processes in humans (for reviews, see Hines, 2004, 2011).

Prenatal sex hormone exposure seems to affect mainly two behaviors in animals: sexual behavior and aggressive behavior. The organizing effects of sex hormones on sexual behavior have been well documented. In a classic experiment, testosterone was administered to pregnant female guinea pigs (Phoenix et al., 1959). The female offspring that had been exposed to testosterone prenatally were, in adulthood, incapable of displaying female sexual behavior (in particular, lordosis, which is a sexual posture involving arching the back and raising the hindquarters so that the male can insert the penis). It is thought that this occurred because the testosterone “organized” the brain tissue (particularly the hypothalamus) in a male direction. These female offspring were also born with masculinized genitals, so their reproductive systems had also been organized in the male direction. But the important point here is that the prenatal doses of testosterone had masculinized their sexual behavior. Similar results have been obtained in experiments with many other species as well (Hines & Collaer, 1993).

Figure 10.1 The sequences of typical prenatal differentiation in female and male humans.

Source: Created by the authors.

In adulthood, these hormonally masculinized females displayed mounting behavior, a sexual behavior typical of males. When they were given testosterone in adulthood, they showed about as much mounting behavior as males did. The testosterone administered in adulthood activated male patterns of sexual behavior.

The analogous experiment with males would be castration at birth (removing the testes, the source of testosterone) followed by administration of “female” sex hormones in adulthood. When this was done with rats, female sexual behavior resulted. These male rats responded to mating attempts from normal males the way females usually do, with lordosis (Harris & Levine, 1965). Apparently the brain tissue had been organized in a female direction during an early critical period when testosterone was absent, and the female behavior patterns were activated in adulthood by administration of ovarian hormones.

Similar effects have been demonstrated for aggressive behavior (Beatty, 1992). Early exposure to testosterone increases the fighting behavior of female mice (Edwards, 1969). Female rhesus monkeys given early exposure to testosterone show a higher incidence of rough-and-tumble play (Young et al., 1964). Thus early exposure to testosterone also organizes aggressive behavior in a “masculine” direction.

What relevance do these studies have for humans? (For a review, see Collaer & Hines, 1995.) It would be unethical, of course, to do experiments like the ones described above on human participants. Nonetheless, a number of “natural” experiments and “accidental” experiments of this sort occur. The natural experiments are the result of a few genetic conditions that cause atypical hormone functioning prenatally. The accidental experiments have occurred when pregnant women were given drugs containing hormones. (These drugs are no longer administered during pregnancy.) We will consider one of the genetic conditions as an example.

Congenital adrenal hyperplasia (CAH) is a rare recessive genetic condition that causes the fetus’s adrenal glands to produce unusually large amounts of androgens beginning about 3 months after conception. CAH is most interesting in genetically female individuals, for whom the testosterone exposure is particularly atypical. Researchers have studied the behavior of CAH girls (Collaer & Hines, 1995; Hines, 2011). CAH girls are significantly more likely, compared with a control group of non-CAH sisters, to choose male-stereotyped toys for play and to prefer active, rough play. These outcomes with humans, then, look much like the experiments with animals, although the effects with humans seem to be smaller and more subtle.

Congenital adrenal hyperplasia (CAH): A rare genetic condition that causes the fetus’s adrenals to produce unusually large amounts of androgens. In XX individuals, the result may be a girl born with masculinized genitals so that she has an intersex condition.

Some cautions must be sounded about the research with humans. First, CAH girls are born with masculinized or ambiguous genitals—the kind of pattern that is called intersex. Some of them were given surgeries to correct the “problem,” particularly in previous decades. We have no idea how traumatic that might have been, nor whether that trauma would have an effect on behavior. Second, the girls’ parents know about the genetic condition. Might parents of CAH girls treat them differently than they would typical daughters?

Hormone Effects in Adulthood

The effects of sex hormones in adulthood that are of interest to us fall into two categories. First, sex hormones in women fluctuate over the menstrual cycle. This raises the question of whether these hormone fluctuations cause fluctuations in mood or other psychological characteristics. (See Chapter 11 for a detailed discussion of this topic.) Second, levels of sex hormones differ in men and women. For example, as noted earlier, women have about one-tenth the level of testosterone in the blood that men do. Could it be that these different levels of hormones activate different behaviors in men and women?

As noted above, studies done with animals indicate that sex hormones in adulthood have effects on both aggressive behavior and sexual behavior. Are there similar effects in humans? Testosterone has well-documented effects on libido, or sexual desire, in humans (Bancroft & Graham, 2011; Everitt & Bancroft, 1991). For example, men deprived of their main source of testosterone by castration show a dramatic decrease in sexual behavior in some, but not all, cases. Testosterone therefore has an activating effect in maintaining sexual desire in adult men.

Research indicates that androgens, not estrogen, are related to sexual desire in women (Bancroft & Graham, 2011). If all sources of androgens in women (the adrenals and ovaries) are removed, women lose sexual desire. Women who have undergone oophorectomy (surgical removal of the ovaries, typically because of cancer) report marked decreases in sexual desire. If they are treated with testosterone, their sexual desire increases (Shifren et al., 2000). Women who seek sex therapy for low sexual desire have, on average, lower androgen levels than age-matched controls (Guay & Jacobson, 2002). Interestingly, these androgen effects in women were overlooked for decades. Perhaps researchers had trouble believing that “male” sex hormones existed in women, and it would have been a huge stretch to imagine that such hormones actually had effects.

Focus 10.2 Endocrine Disrupters

A preschool girl begins growing pubic hair (Sanghavi, 2006). Frogs are born hermaphroditic, with mixed male and female organs (Hayes et al., 2002). The pesticide residues in fruits and vegetables are linked to lower sperm counts in men (Chiu et al., 2015). These cases and many others have appeared in the news in the 21st century. Are they unrelated bizarre occurrences or is there a common link that explains them?

Scientists believe that underlying such troubling cases are endocrine disrupters, or endocrine-disrupting chemicals (EDCs), which are chemicals found in the environment that affect the endocrine system as well as other aspects of biological functioning and behavior of animals, including humans. Evidence of the effects of endocrine disrupters comes both from studies of animals in the wild and from carefully controlled laboratory experiments. For example, a carefully controlled study showed that pregnant women with high exposure to phthalates (found in plastics) are more likely to give birth to baby boys with undescended testes or with hypospadias, a rare condition in which the urethral opening is not at the tip of the penis but somewhere else (Sathyanarayana et al., 2016).

Endocrine disrupters: Chemicals in the environment that affect the endocrine system as well as other aspects of biological functioning and behavior in animals, including humans.

What chemicals are the culprits? Some are pesticides and herbicides such as atrazine and DDT, used by farmers and others to kill unwanted insects and weeds. Bisphenol A (BPA) is used in making plastics such as baby bottles. PCBs, which were banned in the United States in 1976, were used in making products such as paints, plastic, and printing ink. Some of these chemicals have a half-life of over 1,000 years and therefore are still abundant in the environment even though they were banned many years ago.

These chemicals exert their effects on sexual biology and behavior by affecting the endocrine system and, specifically, the sex hormone system. Many have multiple effects. Atrazine, for example, affects both estrogen and testosterone and inhibits their binding to estrogen receptors and androgen receptors. Atrazine also depresses the LH surge that triggers ovulation, described in the next chapter in the discussion of the menstrual cycle. The insecticide DDT affects estrogen, progesterone, and testosterone by mimicking estrogen and binding to estrogen receptors as well as by altering the metabolism of both progesterone and testosterone. PCBs are both anti-estrogens and anti-androgens. These chemicals are in the food we eat and the water and milk we drink.

Even though the age of menarche (first menstruation) has changed little in the past several decades, the very early stages of pubertal development—specifically, the first development of breast tissue—is occurring nearly a year earlier than it used to. This trend is thought to be explained by EDCs in the environment. It seems likely that some chemicals are having an estrogen-like effect, and many EDCs have such effects (Mouritsen et al., 2010). Scientists are concerned that the effects of environmental contaminants may be particularly severe on children because they eat more, drink more, and breathe more than adults, relative to body weight (Trentacosta et al., 2016).

Scientists see these cases as examples of the proverbial canary in the coal mine—that is, they are small signs that something terribly dangerous is happening. The European Union is beginning to take steps to regulate these chemicals, but we have seen little action on the issue in the United States.

One innovative study examined the behavior of both trans women and trans men before and after they began hormone therapy as part of their gender-affirming treatments (Van Goozen et al., 1995; see Chapter 11 for more on health issues for transgender persons). When androgens were administered to the trans men, their aggression proneness and sexual arousability increased. When anti-androgen drugs were given to the trans women, their aggression proneness and sexual arousability decreased. The results are consistent with the broader point that sex hormones have activating effects on aggressive and sexual behaviors in humans.

To summarize, sex hormone levels probably do have some effects on behaviors in adult humans, particularly aggressive and sexual behaviors. It is also likely that these effects are not as strong as they are in animals and that they are more complex and interact more with environmental factors.

Better Hormone Models

The traditional model in psychology has maintained that “hormones influence behavior”—in other words, that the influence goes in one direction only. Feminists have criticized this model. Recall from Chapter 1 that feminist scientists urge researchers to consider bidirectional models, in which A influences B, but B also influences A. As it turns out, hormone researchers have been working on exactly these sorts of effects. For example, if women engage in resistance exercise, it raises their testosterone levels (Nindl et al., 2001). Testosterone levels rise in both men and women following an interpersonal competitive victory (Schultheiss et al., 2005). (For another example, see the research of Van Anders in Focus 10.2.) That is, behavior and experience influence hormones! We shouldn’t settle for simple, biologically deterministic models that assume that hormones determine behavior or that biological factors such as hormones are fixed and unchanging. Nor should we ignore hormones completely. In short, we need more complex models that will help us understand how hormones and behavior influence each other.

The Brain

In this section we will consider various hypotheses that have been proposed about differences between male and female brains and what effects those differences might have on behavior.

Brain Size

In the late 1800s, scientists discovered that men had somewhat larger brains than women. In the culture of the time, they concluded that this brain difference was the cause of the well-known lesser intelligence of women. The hypothesis was later discredited when other scientists found that men’s larger brain size was almost entirely accounted for by their greater body size. Men have larger kidneys and livers than women do, too, but that doesn’t mean that it gives them an advantage. The organs are just proportional to body size.

Amazingly, this same brain-size hypothesis resurfaced in the 1990s. Two scientists separately found that men’s brains were larger in volume and weighed more than women’s, and they argued that this brain difference had an impact on gender differences in intelligence (Ankney, 1992; Rushton, 1992). Interestingly, the same scientists also claimed that Caucasian Americans had larger brains than African Americans and that Asian Americans had larger brains than either group (Rushton, 1992)—so the argument had racial aspects as well, but here we will focus on the argument about gender.

We now have a meta-analysis on brain size. It indicates that, on average, men’s total brain volume is 11.5% larger than women’s (Marwha et al., 2017). That is roughly the same as the overall size difference between men and women. And there is no evidence that, among humans, brain size is correlated with intelligence.

The Hypothalamus

Gender differences do exist in the hypothalamus, a tiny but powerful region of the brain on its lower side (McCarthy & Arnold, 2011). These differences are the result of differentiation of brain tissue in the course of fetal development, much as is the case for the reproductive organs (Figure 10.1). Additional differentiation occurs in the days immediately after birth. Recall that the sequence of typical development consists of the sex chromosomes directing the differentiation of gonadal tissue into ovaries or testes. The gonads then secrete sex hormones, which cause further reproductive system differentiation. The fetal sex hormones, as well as several genes, also cause gender differentiation of the hypothalamus (McCarthy & Arnold, 2011). Basically, then, hypothalamus differentiation in the fetus is a process much like reproductive system differentiation.

Hypothalamus: A part of the brain that is important in regulating certain body functions, including sex hormone production.

One of the most important organizing effects of prenatal sex hormones is the determination of the estrogen sensitivity of certain cells in the hypothalamus, which contain estrogen receptors (Choi et al., 2001; McEwen & Milner, 2017, argue that there are also subtle organizing effects in other regions beyond the hypothalamus). If testosterone is present during fetal development, certain specialized receptor cells in the hypothalamus become insensitive to estrogen; if estrogen is present, these cells are highly sensitive to levels of estrogen in the bloodstream. This is important because of the hypothalamus-pituitary-gonad feedback loop (see Chapter 11). In this process, gonadal hormone output is regulated by the pituitary, which is in turn regulated by the hypothalamus. The hypothalamus responds to the level of gonadal hormones in the bloodstream. Hypothalamic cells in men are relatively insensitive to estrogen levels, whereas hypothalamic cells in women are highly sensitive to them. We also know that estrogen (and progesterone as well) lowers the threshold of central nervous system excitability in adults. Therefore, the estrogen sensitivity effect amounts to a greater increase in central nervous system excitability in response to estrogen in women than it does in men. The estrogen sensitivity effect is a result of the organizing effect of hormones. Hormones administered in adulthood activate male and female nervous systems differentially depending on early determination (organizing effects) of estrogen sensitivity.

What are the consequences of these gender differences in the hypothalamus? One consequence is the determination of a cyclic or acyclic pattern of pituitary release of hormones beginning with puberty. The hypothalamus directs pituitary hormone secretion. A hypothalamus that has undergone differentiation in the female direction will direct the pituitary to release hormones cyclically, creating a menstrual cycle, whereas a hypothalamus differentiated in the male direction directs a relatively steady, acyclic production of pituitary hormones.

The gender differences in the hypothalamus may have some consequences for behavior, too (Ngun et al., 2010). As discussed earlier, the organization of the hypothalamus in a male or a female direction may have some influence on both sexual and aggressive behavior.

Other Brain Regions

Researchers have claimed that other regions of the brain show gender differences. Here we consider the evidence.

The amygdala is a structure in the central part of the brain that is highly involved in processing emotions. There has been much speculation about whether men have larger amygdalae than women and whether that might account for the gender imbalance in some disorders such as depression (more women than men) and autism (more men than women; Marwha et al., 2017).

A meta-analysis found that amygdala volume was about 10% larger in men than women (Marwha et al., 2017). However, that is roughly the same discrepancy as in overall brain volume. If studies correct for total brain volume, gender differences in amygdala volume are nonsignificant.

The hippocampus, located close to the amygdala, is important in memory. Research shows that the volume of the hippocampus is reduced in people with depression (Tan et al., 2016), although what isn’t clear is whether the reduced volume is the result of the depression or whether it existed before the depression and predisposed the person to it. Because more women than men become depressed, researchers have wondered whether women have smaller hippocampi than men. Again, we have a helpful meta-analysis to settle the question. It showed that, when corrected for total brain volume, there is no gender difference in hippocampal volume (Tan et al., 2016).

Some researchers have claimed that there are differences between men and women in the corpus callosum (CC), a region in the central part of the brain containing fibers that connect the right hemisphere and the left hemisphere. The original report, based on research with nine male and five female humans, documented a larger CC—actually, one larger subsection of the CC, the splenium—in women than in men (Delacoste-Utamsing & Holloway, 1982). But these findings were disputed (Fausto-Sterling, 2000), and later studies found inconsistent results. When a meta-analysis was conducted, it revealed that men have a slightly larger CC overall (d = 0.21), but again probably because men have a somewhat larger brain volume. Moreover, there were no gender differences in the size or shape of the splenium (Bishop & Wahlsten, 1997). Once again, gender similarities are the rule, even when it comes to brain anatomy.

To make matters more complicated and interesting, certain regions of the corpus callosum increase in size in women through their 50s, whereas for men the size peaks in their 20s to 30s and then declines (Cowell et al., 1992). These findings defy the notion that brain anatomy is fixed and unchanging. They also challenge simple characterizations of gender differences. For the CC, the gender difference might go in one direction or the other, depending on the age of the sample.

Right Hemisphere, Left Hemisphere

The brain is divided into two halves, a right hemisphere and a left hemisphere. These two hemispheres carry out somewhat different functions. In particular, in right-handed persons, the left hemisphere is specialized for language and verbal tasks and the right hemisphere for spatial tasks. The term lateralization refers to the extent to which a particular function, such as verbal processing, is handled by one hemisphere rather than both. For example, if verbal processing in one person is handled entirely in the left hemisphere, we would say that that person is highly lateralized or completely lateralized for verbal tasks. If another person processes verbal material using both hemispheres, we would say that that person is bilateral for verbal functioning.

Lateralization: The extent to which one hemisphere of the brain organizes a particular mental process or behavior.

Based on the old belief that there are gender differences in both verbal ability and spatial ability (see Chapter 8), various theories have been proposed using gender differences in brain lateralization to account for the supposed differences in abilities (for a detailed review, see Halpern, 2000).

Psychologists typically use two types of tasks to measure brain lateralization. One is the dichotic listening task, in which the researcher presents different stimuli to each ear through headphones. As it turns out, people have ear—or hearing—dominance on the same side as hand dominance. If you are right-handed, you are also right-eared! That is, your right ear is more ready, willing, and able to process stimuli than your left ear is. This ear dominance, in turn, relates to the hemispheres of the brain. Researchers in this field believe that the more dominant your right ear is (in accuracy and speed of processing stimuli), the more lateralized you are; the same would be true if you were very left-ear dominant. The other task used to measure lateralization is the split visual field, in which different stimuli are presented to different sides of your eyes, much like the different stimuli to different ears.

What does the evidence say about gender differences in brain lateralization? A meta-analysis based on 266 studies showed that—whether measured in the visual or auditory mode and whether verbal or nonverbal tasks are used—gender differences in lateralization are close to zero, d = 0.06 (Voyer, 1996). Clearly, then, statements such as “women are left-brained, men are right-brained” are far from the truth.

Another serious criticism of the brain lateralization hypotheses about gender differences is that they were designed to explain gender differences in verbal abilities and spatial abilities, yet the results of meta-analyses indicate currently there are no gender differences in verbal ability, and there are gender differences in only one type of spatial ability (see Chapter 8). Moreover, a meta-analysis of studies of gender differences in lateralization for language indicated no gender difference (Sommer et al., 2008).

Brain lateralization is an active area of research, and there are often flashy newspaper or magazine articles on a scientist who has discovered the cause of gender differences in abilities based on right-hemisphere/left-hemisphere differences. It is therefore worthwhile for you to know the kinds of ideas that have been proposed and the meta-analytic results showing no gender differences in brain lateralization (Voyer, 1996).

Neural Plasticity

You won’t hear a modern neuroscientist say something like “Gender differences in the brain are hardwired” or “The brain is hardwired.” The reason is that a major theme in neuroscience today is neural plasticity, which refers to the fact that the brain changes in response to experience (Eliot, 2009). The brain simply is not hardwired (Jordan-Young, 2010). New connections between neurons are constantly being made to register learning, and other, unnecessary connections are pruned away.

Neural plasticity: Changes in the brain in response to experience.

Focus 10.3 Single-Sex Schooling and the Brain

Single-sex public schools are the latest fad in the United States. To be clear from the outset, we are talking only about public schools, not private schools such as Catholic schools, which are free to make whatever choices they wish. The question we address here is this: Should taxpayer dollars be used to support single-sex schools because they yield better student outcomes than coed schools do?

One argument by the advocates for single-sex schooling concerns boys’ brains and girls’ brains. According to the website of the National Association for Single-Sex Public Education (NASSPE; 2017), single-sex schools break down gender stereotypes and are geared to the “facts” that “the brains of girls and boys differ in important ways. These differences are genetically programmed and are present at birth.” Yet as we have seen in this chapter, differences between girls’ brains and boys’ brains are at most small, and what neuroscientists know about neural plasticity implies that any differences could be due to differences in experience, not anything that was genetically programmed.

In their book The Boys and Girls Learn Differently Action Guide for Teachers, Michael Gurian, a major advocate for treating boys and girls differently in the classroom, and Arlette Ballew (2003) claim, “Boys use the right hemisphere of the brain more, girls the left” (p. 11). Yet as discussed in this chapter, a meta-analysis of studies of gender differences in lateralization found no gender difference. So a bogus scientific claim is used to make the case that boys and girls learn differently and must be segregated into different classrooms and even different schools.

Neuroscientist Lise Eliot (2009) reviewed all the available evidence on gender differences in the brain and concluded, “What I found, after an exhaustive search, was surprisingly little solid evidence of sex differences in children’s brains” (p. 5). She then went on to emphasize neural plasticity.

A second claim from advocates of single-sex education is that it is needed to accommodate girls’ and boys’ different learning styles. Boys are “thrilled” and “aroused” by energetic teachers who talk loudly, whereas girls are intimidated to the point of nausea; thus boys should be taught through loud confrontation, whereas girls must be treated delicately (Sax, 2006). The idea of learning styles has captured the popular imagination. “I’m a visual learner” or “I’m an auditory learner,” people say. In fact, though, a blue-ribbon panel commissioned by the Association for Psychological Science concluded that there is no evidence that people with one preferred learning style actually benefit more from one instructional method than from another (Pashler et al., 2009). So the argument for the importance of learning styles in education, much less gender differences in learning styles, fails the test of science.

The third argument by proponents of single-sex schools is that they break down stereotypes and allow kids to excel in counter-stereotypic areas. Developmental psychologists, however, have much evidence to indicate that, when social groupings—for example, boys versus girls or Blacks versus Whites—are made salient to children, stereotyping and prejudice toward the other group increase (Bigler & Liben, 2007). Researchers can even get some of these effects by putting some of the kids in a classroom in red T-shirts and others in blue T-shirts and having the reds and blues line up separately. The reds start preferring other reds and feeling negative about the blues. Single-sex education does exactly that—it makes gender very salient by saying that girls and boys are so different that they cannot be educated in the same classroom. This sets the stage for increases, not decreases, in stereotyping.

Does single-sex schooling actually produce better outcomes for children? The background is that many studies on this question have weak designs. They may compare a private single-sex school against a nearby public coed school, but the children in the single-sex school come from wealthier families and their parents have more education. To use the language of Chapter 1, it is a quasi-experimental design. If the kids in the single-sex school perform better on a standardized math test, for example, it is impossible to know whether it is because of the single-sex schooling or whether the kids started out with a lot of advantages.

A large meta-analysis, with data from 1.6 million children, separated the studies into those that were uncontrolled and used weak designs like the one described above and controlled studies that used strong designs involving random assignment of children to single-sex versus coed (true experimental designs) or designs that controlled for preexisting differences between children (Pahlke et al., 2014). The results indicated a few advantages for single-sex schooling in the uncontrolled studies, but in the high-quality, controlled studies, there were no advantages for single-sex schooling on outcomes such as math performance, science performance, and self-concept. In short, single-sex schooling does not produce better outcomes.

In 2011, NASSPE changed its name to the National Association for Choice in Education. That shifted the argument from whether single-sex schooling is actually better than coed schooling to whether public schools should provide parents with a choice of single-sex schooling for their children. That would be extremely costly for schools to do, of course. How has the change in the name of the organization co-opted the feminist principle of choice?

The implication for gender—brain research is profound. If a researcher uses brain scanning methods (such as fMRI) with a sample of college students and finds that region X “lights up” more for men doing math problems than for women doing math problems, there is no way of telling whether it is because there are innate, hardwired, unchangeable male—female differences in region X or because men have had more experiences that use region X (see Figure 10.2). As an example, 10-year-old Ahmad and his father spend about a half hour every evening out in the backyard tossing a baseball back and forth. They both enjoy it. Katie, the 10-year-old who lives next door, does not toss a baseball with her father at all. He never suggested it. Neural circuits are forming in Ahmad’s brain that are not forming in Katie’s, and it isn’t because they were born that way; it’s because they are having different experiences.

Figure 10.2 Neural plasticity: If a researcher uses fMRI and finds that a region “lights up” more for men doing math problems than for women, we can’t infer that those differences are hardwired. They might be due to differential experiences that caused men to use that region more than women.

Source: Adapted from Scherf, Elbich, & Motta-Mena (2017).

Feminist Criticism

Books such as Louann Brizendine’s (2007) The Female Brain become best sellers. Brizendine claims that there are hardwired differences between male and female brains that create all sorts of psychological effects such as men’s purported inability to understand emotions and women’s purported innate skill at doing so. These arguments, of course, fly in the face of modern neuroscience and the concept of plasticity, but these books sell like hotcakes. Why?

Cordelia Fine (2008) has coined the term neurosexism for what is going on. Somehow, neuroscience research using measures such as fMRI seems more real and authoritative than other research (Beck, 2010). It has become the new way to dignify old-fashioned sexism and stereotypes, for example, that women are emotional and men are unemotional and very rational. The average reader is not a neuroscientist, hasn’t heard of neural plasticity, doesn’t know the limitations of fMRI research, and doesn’t know how ridiculous these claims are, so they accept the ideas gladly and find their stereotypes confirmed.

Neurosexism: Claims that there are fixed differences between male and female brains and that these differences explain women’s deficits in performance or why they should occupy certain roles and not others.

The Brain Mosaic

Feminist neuroscientist Daphna Joel, based on research with thousands of human brains, has discovered an entirely different way to think about male and female brains. She calls the human brain a gender mosaic (Joel et al., 2015). Her research uses MRI scans of multiple brain regions, assessing the volume of each. Each region is then classified as female-leaning if, on average, it is larger in women, and male-leaning if it is larger in men. Regions that don’t show a male—female difference are termed intermediate. Her discovery is that most people have brains that are a mosaic of female-leaning, male-leaning, and intermediate regions (see Figure 10.3). Very few women have brains with all female-leaning regions, and very few men have brains with all male-leaning regions. This research contradicts beliefs that there is a “male brain” and a “female brain.” Most brains are gender mosaics or, to use a term usually used for genital structures, most brains are intersex. As Daphna Joel says, there is not a male brain and a female brain; there are human brains.

Figure 10.3 A brain mosaic.

Source: Image courtesy of Ann Fink.

Transgender and the Brain

Scientists have wondered whether there might be brain differences between transgender individuals and cisgender individuals that would, at least in part, account for the psychological differences. Like the genetic research described earlier in the chapter, this research has focused specifically on trans men and trans women, who are a subset of trans persons.

The background for the research is that it has been based on the assumption that there are clear differences between human male and female brains and it has tried, for example, to see whether trans women have brains that are more male-like or female-like. The problem with these assumptions, as we have seen, is that differences between men’s and women’s brains are not all that clear or pronounced. But let’s continue with this strain of research.

Research that looks at brain volume in untreated (have received no hormone or surgical treatments) trans women shows that their brain volume does not differ from that of cisgender men (for a review of this and the points that follow, see Guillamon et al., 2016). That is, their brain volume matches their gender assigned at birth. Similar research on trans men is in short supply, so we can’t reach strong conclusions.

When considering specific brain regions, the idea behind the research is that, for some regions of cisgender brains, men have the larger region, denoted M > F (“masculine” regions), and for other regions, women on average have the larger region, denoted F > M (“feminine” regions). The major review on the question concluded that trans women have brains that are a complex mixture of masculine, feminine, and neutral regions (Guillamon et al., 2016; see also Smith et al., 2015). But, of course, that is exactly what Daphna Joel concluded about the brains of cisgender men and women, and the findings of gender mosaic brains. As of now, it is not clear that there are major differences between the brains of trans women and cisgender men, and there is little research on trans men. This pattern is true in many other areas of transgender research—trans women have been studied much more than trans men. Why do you think this has occurred?

What happens, then, when transgender folks receive hormone treatments to make their transition? Trans women receive estrogen plus anti-androgens, and trans men receive testosterone. Longitudinal studies of trans women find that, pretreatment, their brain volume matches their natal gender, male. After 4 months or more of hormone treatment, brain volume decreased and was more in the female range (Guillamon et al., 2016).

This research is very new and is generally based on small samples. It certainly does not warrant a conclusion such as “The difference between transgender women and cisgender men is in brain region X.” Moreover, the research we have seen in this chapter on the similarities between cisgender women’s and cisgender men’s brains should make us skeptical about whether there will be clear brain markers in transgender persons.

Experience the Research: Biology and Gender Differences in the Media

Search through back issues of Time, the New York Times, U.S. News & World Report, or news websites such as Huffington Post to find at least two articles that report on gender differences. Do the articles report on a psychological gender difference or a biological one? If it is a psychological gender difference, what explanation does the author of the article offer? Does the author imply that it is biologically caused or environmentally caused, or is there a balanced discussion of both possibilities? If the article is about a biological gender difference, what is it? Is the information consistent with what you have learned in this chapter?

Chapter Summary

Sex-linked traits are controlled by genes on the X chromosome and, occasionally, on the Y chromosome, which is smaller and contains relatively few genes. The SRY gene is on the Y chromosome. The field of genetics is much less deterministic than it was previously because of the discovery of epigenetics—changes in gene expression caused by factors other than DNA.

To date, despite research attempts, genetic differences between transgender and cisgender individuals have not been identified.

Men and women differ in a few basic physiological processes including, especially, metabolism and drug absorption, and these differences have implications for health. There are also differences between women and men in the immune system.

Testosterone (produced at higher levels in male bodies) and estrogen and progesterone (produced at higher levels in female bodies) may have organizing effects (due to prenatal exposure) or activating effects (due to exposure in adulthood). The process of prenatal gender differentiation begins with XX or XY chromosomes. The Y chromosome contains the SRY gene, which causes the production of TDF, which directs the gonads to become testes. In the absence of a Y chromosome, ovaries result. The gonads then secrete sex hormones, which cause additional differentiation of reproductive structures.

Congenital adrenal hyperplasia is a condition in which genetic female fetuses are exposed to high doses of androgens prenatally, creating genital structures that are intersex.

In adulthood, sex hormones can have activating effects on aggressive behavior and sexual behavior, although the effects are probably weak in humans compared with animals. Feminist scientists encourage more complex hormone models in which hormones influence behavior but behavior and experience also influence hormone levels.

Gender differences in brain size are proportional to gender differences in body size. The same is true of specific regions, such as the amygdala and the hippocampus. The hypothalamus does differ between men and women, particularly in estrogen sensitivity due to estrogen receptors. Despite hypotheses about gender differences in brain lateralization, research shows no gender difference.

Neural plasticity refers to the fact that the brain changes in response to experience. It is not “hardwired.”

New research shows that the brain is a gender mosaic; each person has a mixture of regions that are male-leaning, female-leaning, and intermediate. This research defies the notion of a “male brain” or “female brain.”

Neuroscientists have attempted to identify differences between the brains of transgender and cisgender individuals but, at this point, they have not succeeded.

Suggestions for Further Reading

Eliot, Lise. (2009). Pink brain, blue brain: How small differences grow into troublesome gaps—and what we can do about it. Boston, MA: Houghton Mifflin. Eliot, a respected neuroscientist, reviews the evidence on male-female brain differences and other possible biological influences on behavioral gender differences.

Fine, Cordelia. (2017). Testosterone rex: Myths of sex, science, and society. New York, NY: Norton. Fine is a wonderful writer, and this book provides a feminist critique of many biological explanations for gendered behavior.

Zuk, Marlene. (2002). Sexual selections: What we can and can’t learn about sex from animals. Berkeley: University of California Press. This book is a great example of feminist biology by animal behaviorist Zuk. With an enjoyable writing style, she considers what biology and feminism have to offer each other, and gives detailed examples of how scientists’ observations of animal behavior have been biased by gender stereotypes.