Biological Theories - Theories of Gender

Gender, Nature, and Nurture - Richard A. Lippa 2014

Biological Theories
Theories of Gender

Biological theories of gender suggest that there are some innate differences between males and females, and that we may, to some extent, be born masculine or feminine. The biological basis of sex differences is obvious for physical traits. Women produce ova and men produce sperm. Women menstruate and have cyclic hormonal cycles that men do not. Women give birth and lactate; men do not. Women's bodies produce more estrogens (female hormones) and men's bodies produce more androgens (male hormones). On average, women have bigger hips and breasts and more body fat than men do; men have broader shoulders and more muscle mass than women do. Women have less body hair and men relatively more. Even the diseases that men and women suffer from differ, to some extent, a fact that is becoming increasing important to medical researchers.

Although few would disagree that the two sexes are physically different and that these differences are largely caused by biological factors, scholars disagree continually over the related questions. Are the two sexes psychologically different? Do biological factors lead to sex differences in human behavior? At Level 1, biological theories of gender focus on evolutionary processes and how they mold men's and women' genes, hormones, and nervous systems—and ultimately their behaviors.

Evolutionary Theory

The basic assumptions of Darwin's (1959) original theory of evolution are simple:

1. The traits of all living species show variation.

2. Traits can be passed from generation to generation (principle of inheritance or heredity).

3. Natural selection is the filter that determines which traits are passed from generation to generation.

The principle of natural selection—perhaps the core assumption of Darwin's theory—proposes that it is the organism's environment that selects which traits are passed from one generation to the next. How? In essence, the environment selects those traits that work; that is, those traits that help organisms to survive and reproduce in that environment. Survival and reproduction are not independent, of course, for an animal must survive—at least to a certain age—in order to reproduce.

Natural selection is not a conscious, purposeful process, although its products often give the uncanny appearance of having been designed (Dawkins, 1986). Rather, natural selection is a blind algorithm—an unthinking, deterministic process—that occurs when variable organisms vie for existence and struggle to reproduce in changing environments. Some organisms live, but many die, before maturity. Some organisms reproduce, but many do not. Life is a competition in which organisms struggle to survive and reproduce, and any trait that gives the slightest advantage in this struggle will be bred into the species over many generations.

Traits that foster survival and reproduction in a given environment are said to be adaptive. Adaptations are organized systems of physical or behavioral traits that have evolved because they serve some function that helps the organism to survive or reproduce. For example, eyes are adaptations; they are organized, evolved structures that help animals to survive (e.g., to run away from attacking predators) and to reproduce (e.g., to detect and approach attractive, available mates). Similarly, the reflex to duck when a fast-moving object streaks toward your head is an adaptation, which likely helped many of your ancestors survive and thus to live, reproduce, and ultimately produce you as one of their descendents. You would not be here today if many of your ancestors had not ducked at the right moment.

Classic Darwinian theory focused on individual survival and reproduction, and it described how organisms adapt to their environments and gradually develop new traits, even to the point of evolving into new species. Natural selection is the unthinking process that decides which traits pass from generation to generation. Modern evolutionary theory has refined some of Darwin's ideas. Recent views of evolution, for example, focus more on genetic survival as the central principle of natural selection (Dawkins, 1989). Natural selection, according to this view, is a process that maximizes the transmission of genes to future generations. Genes are successful to the extent that they increase and spread in future populations. Conversely, they are unsuccessful when they decrease in numbers. The ultimate failure of a gene (or of a set of genes) occurs when it ceases to exist altogether, such as when a species goes extinct.

The selfish gene view of evolution holds that plants and animals are, in a sense, gene machines designed to carry and protect their genes for a while and then pass them on to new gene machines (i.e., offspring) to be carried into the future. This selfish gene view may seem a bit disconcerting at first glance, for you probably view yourself as having your own goals and plans and not as a temporary physical container for a set of genes clamoring to be injected into some new bodies and passed on to future generations. You might think that the gene-centered versus individual-centered approach to natural selection is just a matter of semantics. It is not. Genetic survival is not the same as individual survival. For example, who is more successful according to the gene-centered view of natural selection: an 18-year-old boy who fathers 10 children and dies in a motorcycle crash before reaching 19, or a successful and rich businessman who lives to age 99 and has many lovers without fathering any children?

There is another important way in which genetic survival is not identical to individual reproduction. The way animals typically pass their genes on to future generations is through reproduction. However, this is not the only way. The theory of kin selection, also known as inclusive fitness, proposes that animals may also ensure that their genes live on by helping those who share their genes (i.e., blood relatives) to survive and reproduce (Hamilton, 1964). Altruism toward kin can evolve because it fosters the transmission of our own genes (i.e., the ones we share by descent with our kin) to future generations. Thus being altruistic to kin is genetically selfish in the sense that we foster the propagation of our own genes whenever we help our kin to flourish and reproduce.

Evolutionary theories argue that, over the history of our species, men and women have been subject to somewhat different evolutionary pressures. Edward O. Wilson (1975,1978), the father of modern sociobiology, proposed that because hominid women were responsible for bearing, nursing, and caring for children, they evolved to be more nurturing. Men were responsible for hunting and fighting; therefore, they evolved more aggressiveness and better visual-spatial ability.

The evolution of sex differences extends well beyond the human species. One way to think of the difference between females and males—at least at a basic biological level—is that females produce relatively few and large germ cells (eggs), which often come supplied with nutrients and are internally fertilized. Fertilized eggs may sometimes be sheltered and protected by females (as birds' eggs often are) or carried within the female's body (as mammals' embryos are). In contrast, males produce many tiny gametes (called sperm)—minimalist, mobile packages of genetic code—that compete to fertilize as many eggs as possible. Contributing sperm does not require much investment in time, nutrients, or energy on the part of males. In contrast, producing eggs and (for humans) gestating, breastfeeding, and rearing offspring requires a huge amount of time, nutrition, energy, and wear and tear on one's body (Trivers, 1972).

There is another fundamental difference between female and male reproduction. Compared to men, women are much more limited—both theoretically and practically—in the number of offspring they can produce. At birth, woman have millions of ova (eggs), but only a small number of these will ripen each month, once menstruation and ovulation starts. Most others will decay or be reabsorbed by the body. Because of the extended periods of time required for gestation and lactation and because of the bodily demands of pregnancy and childrearing, women can have only so many offspring over the course of their lifetimes. In contrast, men produce millions of sperm every day and, theoretically, at least, they can have dozens of offspring. Evolutionary theorists suggest that all of these differences between female and male reproduction have led men and women to evolve different reproductive strategies (Buss, 1999).

Parental investment in offspring is particularly high in human mothers, who invest more in their offspring than do the mothers of virtually any other species (Hrdy, 1999). Human mothers not only carry their offspring internally for 9 months and breastfeed them for many months but also carry around their helpless infants for many months more. Then they must rear their children into well-socialized adults who learn to speak language fluently and to understand cultural rules, rituals, and technologies. This process may take as long as two decades. Although men invest in childrearing too, women traditionally invest much more, both physically (gestation, childbirth, lactation) and in terms of time and energy (childrearing, child care, and instruction). The wear-and-tear of childbearing is captured by the German proverb, Ein Kind, ein Zahn ("one child, one tooth"). In terms of the calcium a mother loses through bearing and breastfeeding a baby, this may not be far off the mark (Hrdy, 1999).

According to evolutionary arguments, then, women must guarantee that at least some of the relatively few, high-investment offspring they bear will survive. In contrast, men (who may father an indefinite number of offspring and don't necessarily invest much in some individual offspring) are more likely to "sow their wild oats." Women have evolved more of a quality of offspring strategy, whereas men have evolved more of a quantity of offspring strategy. As a result, men have evolved to be more sexually aggressive, competitive, and promiscuous, whereas women have evolved to be more sexually selective and desirous of committed relationships, which provide them and their children with protection and stable resources.

It is important to note that when evolutionary theorists talk about men and women's evolved mating strategies, they are not necessarily describing conscious strategies. Rather, they are referring to dispositions, sometimes unconscious, that have evolved over many generations even millennia. Men do not necessarily walk around in everyday life thinking, "How can I best, maximize my transmission of genes to future generations? Oh, I know—I can impregnate as many women as possible!" They don't need to have such conscious thoughts if evolution has produced, on average, higher male sex drives and less committed male attitudes toward sexual intimacy.

Modern technological advances may sometimes short-circuit the original evolutionary purpose of a behavior. Although some men may be promiscuous, for example, the use of contraception cancels the evolutionary advantage of their behavior. Nonetheless, the dispositions persist even though their fitness has changed in modern environments. The human preference for fatty and sweet foods provides a similar example. It may have been adaptive in our prehistoric past to show such preferences, when fatty and sweet foods were rare and people required many calories to fuel their energetic hunter-gatherer lives; but in today's food-rich and sedentary world, human preferences for sugar and fat bring diabetes and cardiac arrests rather than increased survival.

Darwin (1871) distinguished between two kinds of natural selection, and these have implications for the evolution of sex differences. The first kind of natural selection produces traits adapted to animals' natural environments. Examples of such traits are the long necks of giraffes (which allow giraffes to eat succulent leaves on high branches), the thick fur of polar bears (which provides warmth in frigid environments), and the fibrous, prickly, skins of cacti (which conserve water in arid settings and protect against animals that might want to eat juicy cactus flesh). Darwin described a second, more specialized form of natural selection, which he termed sexual selection. Sexual selection occurs when traits evolve because they help animals attract mates, drive off same-sex rivals, and reproduce.

In a rough sense, the first kind of natural selection favors traits that are adaptive in natural environments. The second kind—sexual selection—favors traits that help animals to compete for mates with same-sex members of their own species and to attract mates from opposite-sex members of their own species. The environment in the first kind of natural selection is almost everything: food supplies, climate, radiation, predators, and so on. The environment in sexual selection, however, consists of members of your own species, the same-sex members with whom you must compete and opposite-sex members whom you must entice as mates.

The gaudy tail feathers of peacocks provide a textbook example of sexual selection (Hamilton & Zuk, 1982; Petrie, Halliday, & Sanders, 1991; Zahavi & Zahavi, 1997). Peacock tails are costly, requiring a lot of food and energy to grow. Furthermore, they are cumbersome, making peacocks less nimble and more vulnerable to predators. Why then did they evolve? The simple answer is, because they are alluring to peahens. Peacock tails are not simply a frivolous display, however. Rather, they provide an honest signal of fitness. Well formed, beautiful tail feathers tell a peahen that the peacock displaying them has good genes, good health, and good nutrition—in short, that he would make a good mate. Such a signal cannot be easily faked. Peacocks with bad genes, infectious diseases, and poor nutrition tend to produce shabby, stunted, asymmetrical tail feathers.

Once a process of sexual selection gets started (and this may initially occur by chance), runaway sexual selection may commence. A positive feedback loop is created, causing the sexy trait to become more and more exaggerated. Peacocks evolve larger and larger tails until the overwhelming costs of such tails (in terms of nutrition, vulnerability to predators, interference with flight) eventually stops the process.

Can sexual selection explain human sex differences? Evolutionary theorists have recently argued that some physical traits of human males and females (men's large penis size compared with that of other primates, women's large protruding breasts, and exaggerated hip-to-waist ratios in fecund human females) may result from sexual selection (Barber, 1995). These physical traits may be comparable to peacocks' tails; they are sexy, honest signs of fitness we display to one another to show our youth, fertility, and good prospects as mates.

Even more intriguing are recent speculations that human brain size, language development, psychological astuteness, and artistic creativity may have evolved through sexual selection (G. F. Miller, 2000). Perhaps humans evolved to entrance and seduce desirable mates through the use of language, storytelling, dance, and humor. Does this theory relate to sex differences? Miller proposed that men use language and artistic creativity more as a kind of status and sexual display than women do, and this may account for greater levels of male productivity in certain kinds of artistic and creative endeavors.

In general, evolutionary theories of gender focus most of their attention on sex differences in human mate choices and sexual behavior. Men prefer youth and beauty in a mate more than women do (youth and beauty are presumed to be signals of health and high fertility), whereas women prefer status and monetary resources in a mate more than men do (because such resources are presumed to ensure that their few offspring will flourish; see Chapter 1). Females are seen as the choosier of the two sexes. Because women produce relatively few offspring in which they invest considerable bodily resources and time, they must carefully choose mates who contribute good genes and sufficient resources to their offspring. Good genes ensure that a woman's offspring will survive and grow into successful, sexy adults who in turn survive and reproduce. Good resources (food, money, status, enduring commitment from a mate) ensure that a woman will be able to protect and rear her children successfully over the long haul. Because males can produce many offspring and because they may invest little in some of them, evolutionary theorists argue that men have evolved to be more promiscuous and indiscriminate, at least in their short-term mate choices, than are women (Buss, 1999).


Sexual selection: According to evolutionary theory, males compete for sexual access to females.

Although some men produce many offspring (e.g., Indian maharajas with large harems), others may faii altogether in the mating game and end up with no offspring. Consistent with this observation, evolutionary theorists propose that there is more variability in men's mating success than in women's. One implication of this asymmetry is that fertile women become the limiting resource for male reproduction. Women can be relatively picky and try to choose the best mate, in terms of his genes and resources. Women can also trade mating privileges for other goods (gifts, food, money, nice homes, stocks and bonds) (Symons, 1979). Because women are a limited resource, men must actively court and compete with other men for desirable mates. And because of their greater chances of failure, men often take greater risks to attract or acquire a mate. (Think of the swagger and risk-taking of young males, who often put on a show of prowess for admiring young women: on the football field, in sportscars, or on the battlefield. Literally, young men are dying to impress attractive women.)

Male displays of money, power, status, and talent can be viewed—from an evolutionary perspective—as an evolved strategy for attracting mates. Evolutionary theories suggest that it is no accident that prominent athletes, rock stars, actors, and CEOs are desirable as mates. Through their creative and career successes, such men compete with other men and indirectly display their good genes and resources to the desirable women they hope to attract. The desirable women, according to evolutionary theory, are young, fertile, and likely to produce and sustain viable offspring.

It seems a reasonable prediction that natural selection would lead females—particularly mammalian females who devote substantial bodily resources to offspring over long periods of time—to develop a mothering instinct. Although girls and women, like primates in general, are often attracted to infants and desire to have and nurture children, the mothering instinct proves to be contingent on a host of situational factors. In the words of evolutionary anthropologist Sarah Hrdy (1999, p. 363), "... in the pragmatic and not-at-all nice domain of Mother Nature, mothers evolved to factor in costs (which, in the human case, can range from mother's age and physical condition to a conscious awareness of future costs) as well as to factor in benefits (for example, a social milieu that offers sons better opportunities than daughters)." Thus, while it may be true that natural selection has led women, on average, to be more nurturing to babies and children than men are, throughout the course of evolutionary history, the mothering instinct has had to be highly discriminating and sometimes even cruelly rejecting. In many preindustrial forager and hunter-gatherer societies, for example, mothers practiced infanticide, at least in some situations: if food supplies were scarce and caring for the new baby was life-threatening to the mother, if a new baby threatened the survival of an older, still-nursing sibling, or if the new baby was weak or deformed. However, once a human mother bonded with her new infant over the first days of life, natural selection seems to have ensured that mother love is often strong.

Evolutionary theories of gender do not focus exclusively on sex, mating, and reproduction. Other traits, such as dominance and physical aggression, may also have evolutionary origins. Dominant males have more power and resources, and therefore they are more attractive to women, more likely to mate, and more likely to pass their genes on to future generations. Aggression (or the threat of aggression) is part of male male competition. Male-on-male homicides are more common than any other kind (see Chapters 1 and 4). Furthermore, evolutionary theory proposes that certain kinds of aggression—for example, that directed by jealous males at mates suspected of infidelity—serve an evolutionary purpose; they protect a male's reproductive assets and ensure that his mate's offspring are in fact his (Daly, Wilson, & Weghorst, 1982).

ASthough evolutionary theories have had much to say about sex differences in mating strategies, nurturing, and other social behaviors, they have generally had much less to say about individual differences in masculinity and femininity. In some species, males are known to take different forms (termed morphs) that specialize in different kinds of mating strategies. For example, in the bluegill sunfish, large dominant males take on a distinctive coloration and acquire a harem over which they dominate and maintain breeding rights. Smaller, nondominant males, in contrast, may maintain a coloration more like females, which allows them to raid dominant males' harems and breed with at least some of the females (Gross, 1982). Similarly, many male orangutans remain so-called Peter Pan morphs when there is a dominant male in their troop; that is, they appear to be slight, scrawny adolescents, sometimes for as long as 20 years. As Peter Pans they engage in sneak copulations with females behind the back of the dominant male. When the dominant male dies or is routed, however, some Peter Pans transform quickly into bulky, more hirsute mature males and take over the harem (Galdikas, 1985; Maggioncalda, Sapolsky, & Czekala, 1999). One way evolutionary theories attempt to explain individual differences among men and among women is to argue that individual males and females differ so that they can adapt to specialized niches in the mating game (E. M. Miller, 2000).

Another evolutionary view of individual differences is that they simply represent random noise in evolutionary processes (Markow, 1994; Moller & Swaddle, 1998). The development of individual males and females can be jiggled by innumerable biological and environmental factors: outside temperature, infectious agents, immunological reactions, maternal stress and its associated hormones, chemical exposure, and so on. Such factors may perturb the development of individual males and females and produce individual differences in masculinity and femininity, variations in how male-typical or female-typical any particular male or female turns out to be.

A final evolutionary view, and a complicating factor in the evolution of males and females, is that because males and females share most of their genes, genetic traits that greatly increase the fitness of one sex may sometimes show up in the other sex. As an obvious example, think of nipples, which have a more obvious function in women than in men. Despite the fact that nipples foster women's but not men's reproductive success, they exist in both sexes. Another example is orgasm. Some theorists have debated whether female orgasms serve a purpose, in evolutionary terms, or whether they are vestiges of male orgasms (Baker & Bellis, 1995; Slob & van der Werff ten Bosch, 1991). Whenever evolution produces differing traits in males and females, it must generate complex mechanisms that turn on genes in one sex but turn off corresponding" genes in the other sex. Often, these mechanisms involve the action of sex hormones at various critical stages of development. Variations in the timing and strength of these hormonal events could lead to individual differences in masculinity-femininity.

The Genetics of Sex

Ultimately, evolution influences our genes and our bodies. In terms of Fig. 3.1, causes at Level 1 (e.g., biological evolution) influence causes at Levels 2 and 3 (e.g., genes, hormones, physiology). Although Darwin assumed that traits could be passed from one generation to the next, he understood nothing of modern genetics. Gregor Mendel's classic experiments on the genetic transmission of traits In pea plants took place during Darwin's lifetime. However, Darwin never read Mendel's paper, which was published in an obscure agricultural journal. Other biologists also ignored Mendel's work, at least until the beginning of the 20th century.

Mendel's seminal discoveries showed that there are discrete packets of heredity, what we now call genes. In the mid-20th century, the molecular basis of genetics was revealed. The exact chemical structure of DNA, the molecule of heredity, was deciphered. We now know that genes—segments of DNA—work by coding for the manufacture of various proteins, which are the building blocks of life. Some popular writers describe DNA as the blueprint of life. A more appropriate metaphor is that DNA provides the recipe for life (Dennett, 1995). DNA instructs cells how to manufacture proteins needed to run the cell and to build additional cells,. The chemicals (proteins, hormones, enzymes) produced under the guidance of DNA form the very stuff of which cells are made, and they guide cell growth, division, and differentiation into various tissues and organs.

Once proteins are produced under the direction of DNA, they feed back and further influence the action of DNA. For example, hormones (chemical messengers carried by blood from one part of the body to another) are manufactured according to the instructions of DNA. Once they come into existence, sex hormones may turn off some segments of DNA and turn on others segments within cells and thereby trigger protein synthesis, change neurotransmitter levels in the nervous system, and guide tissue growth (the word hormone derives from Greek roots that mean "to set in motion" or "to stimulate").

The outside environment can also influence the action of DNA. A peculiar but fascinating example is provided by certain reptiles, whose eggs hatch into females when it is very hot or cold outside, but into males when temperatures are more moderate (Crews, 1994; Crews et al., 1994). In some reptiles, only females are produced by low incubation temperatures, and in still other reptiles, only males are produced by low incubation temperatures. Therefore, the genes that lead the reptiles to become male or female are turned on or turned off by environmental factors.

Think again of the metaphor of a recipe. A recipe for a cake tells you which ingredients go into a cake and in what order to add the ingredients, A recipe, however, does not provide a precise blueprint for a cake, and there definitely is no little model cake stored inside a recipe. Sometimes, recipes require that the cook following a precise sequence of actions: "Wait until the sauce has cooled a bit before adding the beaten egg; otherwise, it will curdle." Recipes can be very sensitive to outside environments. "It's best not to make this pastry if it is too hot and humid outside," or, "If you bake this cake at high altitudes, you must alter the amount of baking soda." The outcomes of recipes can be jiggled by noise-like events affecting the cooking process: slamming the oven door, using jumbo rather than large eggs, not realizing that your oven thermostat is off a bit, and so on.

The analogy between DNA and a recipe is intended to sensitize you to two important points:

1. Heredity is not destiny, a least in any fixed, precise, and deterministic sense.

2. Complex, multistage recipes—and the DNA instructions for building living organisms are as complex and multistage as recipes get—produce lots of noise-like variations in their outcomes.

Some of these may be due to variations in the timing of events and variations in environments while the ingredients are assembled. Other variations may be due to variations in the recipes themselves. Genetic recipes vary for two main reasons. First, some genes come in more than one variety (or allele), and therefore the recipes for human beings all differ to some degree. (There are lots of different recipes for apple pies too.) Second, recipes can vary because mistakes occur when the recipe is passed from cook to cook. In genetic recipes, such mistakes are called mutations.

Biologists now know that the recipe of life—DNA—is arranged in 23 paired packages of genetic material called chromosomes, which consist of many genes strung together, along with junk DNA (sections of DNA that do not code for useful information or that coded for useful information in the evolutionary past but not today; recent research suggests that some presumed-to-be junk DNA may not be so junky after all, but that it regulates the action of other genes). One pair of chromosomes is critical for determining sex, and these chromosomes are called the sex chromosomes. In humans, but not always in other animals, there are two kinds of sex chromosomes: X and Y. Most females are born with two X chromosomes (XX), and most males are born with an X and a Y chromosome (XY). It is the presence or absence of the Y chromosome that leads some embryos to develop into males and others into females. Because females have two X chromosomes, mothers always pass X chromosomes on to their offspring, whether they are sons or daughters. However, fathers pass on their X chromosome only to their daughters and their Y chromosome only to their sons.

There is a problem with having two X chromosomes, as virtually all mammalian females do: The "double dose" may generate too many of the protein products produced by genes on the X chromosomes. The solution? In females, one of the X chromosomes is inactivated early in development in each cell of the body (Bainbridge, 2003). Indeed, one of the standard genetic tests for determining whether an individual is male or female looks for Barr bodies in cell nuclei, which in fact are inactivated X chromosomes that are separated from the active chromosomes. If you have Barr bodies, you are female; if not, you are male. X inactivation occurs early in female embryological development, with the result that some female cell lineages have one X chromosome inactivated, but others have the other X inactivated.

Thus mammalian females are mosaics; their bodies consists of two populations of cells, each with somewhat differing genotypes (sets of DNA instructions), A well-known example is provided by calico cats, whose multicolored fur patches represent different cell lineages, with different X chromosomes inactivated in each patch. Calico cats are always females. The mosaic nature of human females can have important consequences. One reason women suffer more than men do from autoimmune disorders such as rheumatoid arthritis, lupus, and multiple sclerosis may be that, because of their mosaic nature, females' bodies mistakenly identify some of their tissues as foreign (Bainbridge, 2003)..

The Y chromosome is much smaller than the X chromosome and carries much less genetic material. This helps explain why males suffer more from certain hereditary disorders (e.g., color blindness and hemophilia) than females do. Such conditions are caused by recessive genes on the X chromosome. Genes typically come in matched pairs, one on each of two matched chromosomes. Some genes come in alternate forms or alleles. An allele is dominant when its effects win out over those of its matched partner, and an allele is recessive when its effects lose out against its paired gene. Thus, for example, genes for brown eyes are dominant over genes from blues. If you carried two genes, one for blue eyes and one for brown eyes, you would be brown-eyed.

Because the Y chromosome is small and does not carry many of the genes that are found on the X chromosome, a male must make do with the genes that are on his one X chromosome, which is always inherited from his mother. Thus if a male inherits a bad recessive gene from his mother, such as for hemophilia, it will express itself because there is no dominant matched gene on a second X chromosome to override its effects. If there is a deleterious mutation of a gene on the X chromosome, females tend to be more buffered against its effects than males because they have another X chromosome, which probably carries a normal version of the mutated gene. As noted before, a mutation is a change in the chemical structure of a gene, usually caused by copying errors when a cell divides or by environmental factors such as radiation or chemicals that alter DNA. Most mutations are deleterious, that is, they are maladaptive, often to the point of being lethal. The problems created by mutated genes on the X chromosome may help explain why more male than female fetuses are spontaneously aborted and why more males than females suffer from a variety of developmental problems such as childhood autism, attention deficit disorder, and speech disorders (Beal, 1994).

Hormones and Male Versus Female Development

Although small, the Y chromosome carries one very important gene called Sry. This gene determines the individual's sex. This sex-determining gene triggers the production of a substance called H-Y antigen which signals the fetal sex glands (gonads) of males to develop into testes. Once testes come into existence, they produce testosterone (a male sex hormone), which is carried by the blood stream and affects physical development. In the absence of this gene (in XX females), fetal, sex glands develop into ovaries. It is the sex-determining gene that begins a cascade of events that leads XY embryos to develop into males.

It has been argued by some that the default sex of a human fetus is female. In other words, unless acted upon by the cascade of androgens (male hormones) triggered by the sex-determining factors on the Y chromosome, the fetus will develop as a female. It takes a departure from this female norm for male development to occur. Androgens (male hormones) seem to be more important in causing male development than estrogens (female hormones) are in causing female development, although research on this topic is not settled (Collaer & Hines, 1995; Hines. 2004). Nonetheless, it appears that some minimum levels of sex hormones (typically estrogens) are necessary for normal female development. The effects of androgens and estrogens may also vary depending on the stage of fetal development.

In early male development (before puberty), there are two periods during which male hormones increase: (a) early in fetal development, starting at around the seventh week and peaking in the middle trimester (middle third) of pregnancy, and (b) for about half a year after birth (Wilson, 1999). The first androgen surge is better understood than the second. Biological theorists argue that androgens during the second trimester of pregnancy are critical for the development of both male internal and external genitals and a male-typical nervous system. Exposure to testosterone during this period of fetal development may even influence gender identity (see Chapter 4). Research—both in animals and in humans—shows that fetal hormone levels guide the development of male or female reproductive organs and external genitals. Research further suggests that prenatal androgens may guide the development of parts of the nervous system and influence gender-related behaviors such as sexual orientation, aggressiveness, rough-and-tumble play, maternal/paternal behavior, and certain kinds of cognitive abilities (e.g., visual-spatial abilities).

A distinction is often made between the organizing influence of sex hormones, which is thought mostly to take place prenatally in humans, and the activating effects of sex hormones, which may take place throughout life but especially after puberty (Collaer & Hines, 1995; Cooke, Hegstrom, Villeneuve, & Breedlove, 1998; Hines, 2004). According to this distinction, prenatal sex hormones affect the organization of the central nervous system (e.g., the growth of nerve cells and nerve connections, the size of brain structures and other parts of the nervous system), whereas sex hormones after puberty activate neural systems and behavioral patterns that have been laid down earlier.

To give an example, prenatal hormones may influence, early in life, sexual orientation. However, hormonal surges at puberty may activate orientations set earlier and motivate adult sexual behaviors consistent with these orientations. Although some have challenged the distinction between organizational and activational effects, it remains useful as a way of thinking about the possible effects of hormones. This distinction should sensitize you to the fact that the effects of prenatal hormones may differ from the effects of hormone levels in adulthood. Prenatal exposure to testosterone may masculinize behavior and increase the odds that an individual will be sexually attracted to women, for example. In adulthood, however, high testosterone levels may not affect sexual orientation, but they may affect sex drive, increasing interest in sex, whatever the sexual orientation.

No one doubts that sex hormones affect how we physically develop into males or females. During the first trimester of pregnancy, the human fetus has both male and female internal structures: Wolffian ducts, which are destined to become the vas deferens and seminal vesicles in males (i.e., the internal plumbing of the male reproductive system), and Müllerian ducts, which are destined to develop info the fallopian tubes and uterus in females (i.e., internal structures of the female reproductive system). In males, the sex-determining gene leads the testes to produce testosterone and a related male hormone called dihydrotestosterone. These hormones, respectively, trigger the development of the Wolffian ducts and the external male genitals (penis and scrotum). Another hormone—the Müllerian inhibiting factor—causes males' Müllerian tubes to disappear. In the absence of the sex-determining gene and the male hormone production triggered by this gene, female gonads develop into ovaries, and female external genitals develop into the clitoris, labia, and vaginal opening. The penis and clitoris are homologous structures; that is, the same fetal bud of tissue is destined to grow into one or the other, depending on prenatal hormones.

Structural Differences Between Male and Female Nervous Systems

The notion that prenatal sex hormones have organizational effects implies that hormones may lead to structural differences in male and female nervous systems. Do the brains and nervous systems of men and women actually differ? This is a highly controversial and contentious research topic. Although the debate continues (see additional evidence in Chapter 4), recent research suggests that there are some significant on-average differences between parts of male and female brains. It is important to emphasize, however, that showing a sex difference in brain structure does not tell us why the difference exists (Breedlove, 1994). Brain structures are molded by environmental influences as well as by genes and hormones. Furthermore, the fact that men and women's brains differ in some ways should not obscure the fact that men and women's brains are much more similar than they are different.

On average, men have larger brains than women, but conversely, women have more densely packed neurons (nerve cells) In parts of their brain (Janowsky, 1989). Whatever the difference in brain size, most experts Slave concluded that men and women do not differ much in their average general intelligence (see Chapter 1). However, men and women do show on-average differences on certain specific mental abilities (e.g., mental rotation and verbal fluency), and these differences may be related to brain differences.

Some researchers have suggested that men have more lateralized brains than women do (Annett, 1985; Hellige, 1993). Lateralization refers to differences between the right and left hemispheres (or halves) of the brain. Lateralization in human brains is linked to language and visual-spatial abilities. For most people, the brain areas responsible for producing and understanding language are located more on the left side of the brain, whereas the brain areas responsible for certain kinds of visual-spatial, geometric problem solving, and pattern recognition tasks are found more on the right side of the brain. Men's brains seem to be more lateralized than women's in two senses: (a) the respective compartmentalization of language and visual-spatial processing in the left and right hemispheres seems to be more extreme in men than in women, and (b) certain size asymmetries between areas of the left and right hemispheres are more extreme in men than in women (Fitch, Miller, & Tallal, 1997; Geschwind & Levitsky, 1968).

The greater lateralization of men's brains suggests that men's right hemispheres may be more exclusively devoted to visual-spatial tasks and men's left hemispheres to linguistic tasks, whereas women may have more diffuse areas devoted to both kinds of tasks (e.g., parts of both the right and left hemispheres seem to be devoted to language tasks in women). One piece of evidence supporting this hypothesis is the finding that left hemisphere injuries disrupt men's language abilities more than women's. (Hampson, 2002; McGlone, 1977). Brains may not only show different degrees of lateralization but they may also be functionally organized somewhat differently within each hemisphere (Pugh et at, 1996). For example, the language functions in women's left hemisphere seem to be more anterior (forward), whereas those of men seem to be spread out more diffusely over the entire left hemisphere (Kimura, 1987, 1999).

One theory holds that the greater lateralization of male brains is due to the early effects of testosterone (Geschwind & Galaburda, 1987). Research suggests that the left hemisphere is slower to develop than the right hemisphere. If it is slower to develop, the left hemisphere would be more vulnerable to factors that could interfere with its development. Testosterone is one such factor; it has the effect of slowing the growth of neurons. The net result is that males—who have high levels of testosterone—may experience less development of the left hemisphere than women do. Recall that in most people, the left hemisphere is more responsible for language abilities. In contrast, females—who have low levels of testosterone—may experience a greater relative development of the left hemisphere. One prediction of Geschwind and Galaburda's theory is that left-handedness should be more common in men than in women. Left-handedness reflects a more dominant right hemisphere. Because the right hemisphere controls the muscles of the left side of the body, increased development of the right hemisphere produces more left-handedness. A number of studies have supported this prediction (for a review, see Halpern, 2000), although the reasons for males' slightly greater rates of left-handedness are still debated (Lippa, 2003b).

The two hemispheres of the brain are joined by a great connecting cable called the corpus callosum, a huge crescent-shaped band of nerve fibers. Studies suggest that the corpus callosum is proportionally larger in women than in men (Allen & Gorski, 1992; Bishop & Wahlsten, 1997; Holloway, 1998; Holloway. Anderson, Defendini, & Harper, 1993). If the proportionally larger size of women's corpus callosum is supported by additional research, it may suggest that the two sides of the brain have more fluent communication in women than In men. This might help explain research findings that show that women are more verbally fluent than men (see Chapter 1).

Another brain region that has received considerable scrutiny is the hypothalamus, a little structure attached to the pituitary gland, buried deep in the brain. This little structure is responsible for many essential motives such as hunger, thirst, aggression, and sex. Some regions of the preoptic area of the hypothalamus show sex differences; for example, they are larger in men than in women. Animal research suggests that corresponding areas of the hypothalamus in animals are related to sexual behaviors, such as sexual mounting in male rats and assuming the sexually receiving posture (termed lordosis) in female rats. Some research has suggested that the size of certain preoptic structures in the hypothalamus may be related to sexual orientation in men (LeVay, 1991; see Chapter 4). Gay men seem to have more female-like preoptic areas than do heterosexual men. Another study found that a different region of the hypothalamus (called the bed nucleus of the stria terminalis) showed a size difference between male-to-female transsexuals and non-transsexual men; the trannsexuals' bed nuclei were more similar in size to women's than to men's (Zhou, Hofinan. Gooren, & Swaab, 1995).

In short, studies have suggested that there are sex differences in some parts of the human brain. In a recent review of this rapidly changing research area, Marc Breedlove and Elizabeth Hampson (2002) listed the following as likely anatomical and physiological sex differences in the human brain:

· Right-versus-left hemisphere weight differences are smaller for females than for males.

· The planum temporale (a region of the brain's temporal lobe that is involved in language) is relatively larger in females' right hemispheres than in males'.

· Gray matter volume and brain cell densities are higher in women's language-related areas of the cortex than in men's.

· Certain regions of the occipital lobe (a region to the back of the brain, which is involved in vision processing) show stronger right-versus-left hemisphere size differences in men than in women.

· In early gestation, the right prefrontal cortex is relatively more developed in males and the left prefrontal cortex is relatively more developed in females.

As brain studies continue and as their methods become ever more sophisticated, it seems likely that additional sex differences in brain structure and function will be identified. The more difficult task, however, will be to demonstrate how such brain differences come to be and how they are related to behavioral sex differences.

Are Hormones Everything?

The central dogma of biological theories of sexual development over the past half century has been that sex chromosomes (XX and XY) have an impact on development only through their effects on gonadal development (whether the individual develops ovaries or testes), and ultimately, through their effects on the production of sex hormones (androgens, estrogens, Müllerian inhibiting factor, and so on). However, recent research evidence questions the assumption that hormones are everything (Arnold, 2003). Apparently, there are some physical sex differences in animals (and probably in humans too) that are caused directly by genes, without the mediation of sex hormones.

What's the evidence for this surprising conclusion? Some studies of rats show that sex differences occur in embryonic brain cells before sex differences in plasma testosterone occur, and recent research has identified genes that differentially express themselves in the developing brains of male and female infant mice, well before gonadal hormones influence brain development (Reisert & Pilgrim, 1991). One clever recent study transplanted quail forebrains from embryos of one sex to embryos of the other sex before embryonic gonads had differentiated into ovaries or testes (Gahr, 2003). According to the hormones are everything dogma, ail brain tissue (whether composed of chromosomally male or female cells) should have developed as dictated by gonadal hormones. Thus, quails with ovaries should have developed female brains and behaviors, whereas quails with testes should have developed male brains and behaviors. But this is not what happened. When male embryos had female forebrains grafted onto them, for example, they developed into adults that did not show male mounting or crowing behaviors. This suggests that the female forebrain tissue "knew" it was female, even though it was exposed to a male hormonal environment.

Additional evidence comes from a fascinating, stranger-than-fietion zebra finch, which was half male and half female (Agate et at, 2003). This bird had male chromosomes, a testis, and colorful male plumage on the right side of its body, but female chromosomes, an ovary, and drab female plumage on the left side of its body. According to the hormones are everything dogma, the two sides of this hermaphroditic finch's brain should have been equally masculinized because the finch's bloodstream carried gonadal hormones equally to both sides of its body. In fact, however, histological examinations showed that the bird's brain was more masculinized on the right side than on the left side. Once again, the chromosomally female tissue seemed to "know" it was female, even though it was exposed to male hormones (which, paradoxically, in birds are estrogens).

Although research on direct genetic effects on sexual development—effects that are independent of mediating gonadal hormones—is still in progress, it nonetheless suggests that the hormones are everything dogma will likely need to be revised in coming years.

Recapitulating Biological Theories

Four interrelated biological perspectives help explain sex differences and variations in masculinity and femininity: evolutionary theory, genetic theory, research and theory on the effects of sex hormones, and research and theory on differences in the nervous systems of men and women. Biological theories of gender argue that men and women have evolved to differ on certain behavioral traits (e.g., mating strategies, aggressiveness).

How does evolution produce these sex differences? Biological theories propose that males and females differ in their sex chromosomes, they follow different paths of fetal development, and they experience different levels of sex hormones at critical stages of development. This ultimately leads to different brain structures and patterns of brain functioning in the two sexes. Similarly, individual differences in masculinity and femininity may depend 011 variations in exposure to prenatal sex hormones, variations in the density and location of hormone receptors in various tissues, and variations in the ways in which male and female fetuses develop. Individual differences in masculinity and femininity may be due in part to genetic variations among people, in part to variations in levels of sex hormones during critical stages of development, and in part to the noise-like variations that inevitably occur when complex DNA recipes produce living bodies.