Sex differences in the brain, sex-typed behavior and gender identity, and sex differences in cognitive ability should be studied at all points in the life span. Hormones play a role in behavioral and cognitive sex differences but are not solely responsible for those differences.
In addition, sex differences in perception of pain have important clinical implications. Research is needed on the natural variations between and within the sexes in behavior, cognition, and perception, with expanded investigation of sex differences in brain structure and function. The purpose of this chapter is not to review all the evidence about the nature and determinants of sex differences in behavior or any other characteristic but to describe how basic genetic and physiological differences between males and females might produce phenotypic differences throughout the life span.
The fetal environment, particularly hormones present during development, affects aspects of later behavioral and cognitive sex differences.
Sex differences in behavior are important in their own right, but also suggest ways in which prenatal influences can contribute to sex differences in nonbehavioral traits, including those associated with health and illness. The information presented in this section should not be interpreted to mean that all behavioral sex differences are caused by hormones during prenatal development but, rather, should serve as an illustration of the potential role of prenatal hormones in producing phenotypic sex differences.
No single factor produces sex differences in any one behavioral or cognitive trait, let alone in all of them. Until recently, it has been popular to focus on cultural or experiential causes of these differences. Thus, for example, sex differences in the occurrence of depression have been considered to reflect women's greater social orientation which is itself assumed to be cultural or stresses associated with women's multiple social roles as also mentioned in Chapter 3.
In the past 10 years, however, there has been increasing appreciation of the fact that genetic and physiological differences between males and females might also influence behavioral sex differences. Although some might argue that the pendulum has swung too much in favor of genes and physiology Fausto-Sterling, , there is considerable interest in examining the joint effects of genes, physiology, and experiences.
In particular, there is recognition that the environment is not independent of the individual Scarr and McCartney, Individuals actively construct their environments and are responded to by others in their environments. The effects of imposed environments are not the same for everyone. Thus, questions about sex differences concern not just differences between individual males and females but also differences between male and female cultures Maccoby, Psychosexual Differentiation Studies with nonhuman vertebrate species suggest that the sexual role adopted at maturity is determined by the hormonal environment in early life.
As for other aspects of sex differentiation, there appears to be a predisposition for individuals to develop female sexual postures. The development of male patterns of sexual behavior in nonhuman species is influenced to a large extent by exposure to androgens—in particular, testosterone—during the prenatal and perinatal periods.
This organizing capacity of testosterone administered at a critical stage of development has been localized to specific areas of the brain. Sexually dimorphic organizations of target cell nuclei detected during behavior-related events in other species are the result of local aromatization conversion of testosterone to estradiol in the central nervous systems of these species.
In humans, masculinization of the central nervous system does not appear to result from aromatized estradiol but appears to result from forms of testosterone Grumbach and Auchus, Sex Differences in the Central Nervous System and Brain Sex differences in the central nervous system extend beyond functions and structures traditionally associated with reproduction. These differences might be better understood if they were studied in the context of new and exciting conceptualizations of how the brain works, which encompass notions of lifelong plasticity, ensemble processing and distributed networks, and the brain's role as an endocrine organ.
The classic examples of sex differences in the brain involve neuroanatomical differences that are developmentally programmed. In several species, sex differences in the patterns of synaptic innervation are observed in the preoptic area and are influenced by the perinatal hormone environment but not by hormonal conditions in the adult animal Gorski et al.
These early studies reveal the effects of castration of males and the administration of testosterone to females early in development and established the idea that differences in the wiring of the brain are programmed at birth. There are now many documented sex differences in a wide range of species, including primates Forger, In canaries and zebra finches, for example, differences in singing behavior between males and females have been correlated with differences in the sizes of three vocal control areas in the brain Nottebohm and Arnold, , but, importantly, the young male bird must hear the adult male song to initiate its own repertoire.
There are also sex differences in the human brain, including the higher cognitive centers. These differences have been observed in adults, and the nature and origins of these differences are subjects of active investigation. Recent studies suggest sex differences in brain structure size as the brain develops in children Giedd et al. It is important to remember that these differences are not absolute and that it is currently not possible, nor may it ever be, to look at a brain or a brain image and know the sex of its owner.
The principles that have emerged from studies with nonhuman species have generally been confirmed in humans, although differences in details exist. For example, androgens act as masculinizing agents in all species, but they appear to do so through different metabolites. Another important principle that has emerged from studies with animals and that has been confirmed in humans is that the central nervous system remains plastic throughout the life span.
Finally, former notions that discrete brain regions have specific and static functions have been modified by work on ensemble neuronal activity Laubach et al.
Areas that have not been traditionally thought to be sexually dimorphic may be involved in sexually dimorphic behavior. Some examples are 1 dopamine functions within the striatum and nucleus accumbens Becker, ; 2 the responsiveness of neurons in the gracile nucleus to stimulation of skin and pelvic organs Bradshaw and Berkley, neuronal responsiveness and activity in the two regions vary with the estrous cycle and hormonal manipulation in a manner that correlates with lordosis and other reproductive behaviors; and 3 modulation of functions in the hippocampus, inferior olive, and cerebellum Smith et al.
The Brain as an Endocrine Organ A great deal of evidence indicates that the brain functions as an endocrine hormone-secreting organ. Throughout life, there are profound sex differences in the brain's responsiveness to sex hormones, some of which are established early in development and which have implications for later behavior, including cognitive function.
The brain is also involved in the regulation of other hormones that show sex differences and that are involved in both reproductive and nonreproductive behaviors. For example, aggression in male mice is considerably more intense than that in female mice, and this difference is known to be influenced by testosterone. Recent studies suggest that the story may be more complex. Nitric oxide, a compound that participates in cellto-cell signaling, may be involved.
The neural form of nitric oxide is measured by changes in nitric oxide synthase nNOS and plays an important role in the expression of aggressive behavior in males Nelson, These studies suggest that nitric oxide from neurons has important but opposite effects in the mediation of aggression in male and female mice Nelson and Chiavegatto, In the rat brain, the ventromedial hypothalamus is important in the regulation of reproductive behavior such as lordosis.
The estrogen-inducible progesterone receptors in the ventromedial nucleus appear to play a role Parsons et al. Estrogens have also been shown to induce receptors for oxytocin in the hypothalamus, and blockage of oxytocin receptors interferes with the expression of lordosis behavior. Estrogens also cause the formation of new synaptic connections between ventromedial hypothalamic neurons in the hypothalamus.
Rats display a characteristic set of motor behaviors following activation of serotonin receptors or elevation of synaptic serotonin levels after treatment with L-tryptophan.
Fischette and colleagues have shown that androgens, via androgen receptors, modulate the reduced sensitivity of male rats to the tryptophan drug challenge. Sex-Typed Behavior and Gender Identity Discussions about the determinants of human sex-typed behavior, especially gender identity, have recently become highly visible because of scientific and popular accounts of a prominent case Colapinto, ; Diamond and Sigmundson, The case challenged the established belief that individuals are born with the potential to develop male or female gender identity and that the specific gender identity can be determined exclusively by sex of rearing Hampson and Hampson ; Money and Ehrhardt, ; Money et al.
For detailed reviews and discussions, see Bradley et al. The case involved a boy 46,XY karyotype with male-typical development whose penis was ablated after a mishandled circumcision and whose gender was subsequently reassigned and reared as a female. Contrary to early reports, the child never adjusted to the female assignment, despite having no knowledge of his early history. Sex reassignment was requested, and the individual is now reported to live successfully and happily as a man.
Because this individual is a normal genetic male who was exposed to male-typical hormones in prenatal and early neonatal life, this case lends credence to the view that gender identity is determined by early hormones that act on the developing brain and argues against the view that rearing sex is the main determinant of gender identity Diamond and Sigmundson, ; Grumbach and Conte, The conclusion, however, must be considered in light of other details of this case and other cases.
The individual described above Diamond and Sigmundson, was reared unequivocally as a boy at least until age 7 months, when the accident occurred, and perhaps longer, because the final decision about female reassignment was not made until his second year and surgery was not completed until age 21 months. Furthermore, the outcome for another individual with an ablated penis was very different: As an adult, she shows no evidence of gender dysphoria, although she has a male-typical occupation and a bisexual orientation Bradley et al.
Ongoing studies with boys with cloacal exstrophy malformed or absent penis with normal testes who are reared as girls should help to provide systematic evidence about the determinants and malleability of gender identity.
These boys are usually reassigned as girls because of concerns about adjustment problems associated with inadequate male genitalia. Preliminary reports from an ongoing systematic study Reiner, indicate that more than half of these female sex-assigned XY children identify as boys, consistent with their male-typical prenatal androgen exposure, and not with their female-typical rearing.
Interestingly, however, some of these children continued to accept their female assigned sex, so it will be important to determine what differentiates children with male identity from those with female identity, despite their common 46,XY chromosome constitutions. This is clearly an area deserving of further investigation.
Other Sex Differences in Human Behavior Although identification as male or female is the most obvious psychological sex difference, it is far from the only one. A variety of important human behaviors covering a range of domains are more common or occur at higher levels in one sex than in the other.
The behaviors that have received the most attention include aspects of normal social behavior and cognition, such as childhood play behavior and related activities and interests, personality such as aggression and interest in babies , nonverbal communication, sexuality, and cognitive abilities Hall and Carter, ; Halpern, ; Maccoby, ; Ruble and Martin, Activities related to these behaviors are performed at different frequencies by males and females in most cultures studied Daly and Wilson, Again, the goal of this chapter is not to provide an exhaustive review of behavioral sex differences but to illustrate some of the differences and to indicate how they might be influenced in part by sex hormones.
There are also sex differences in health-related behaviors, such as frequency of visits to health professionals and use of complementary medicine, but these have not been well studied. There are also sex differences in the incidence and course of some mental disorders and substance abuse National Institutes of Health, Office of Research on Women's Health, b. These differences in mental health may also produce differences in physical health.
Cognitive Function A large body of research has now converged to indicate that there are sex differences in specific areas of cognitive function. Although there has been some controversy over the proverbial question of which sex is the smarter one, a reasonable conclusion reached by many scientists is that there are no meaningful differences in intelligence between males and females Halpern, In some cases, the sex differences are most marked at the extreme ends of a particular ability, for example, among those who are the most skilled Figure 4—1 Hampson, in press; Hampson and Kimura, Although there may be slight but significant differences between the mean scores for males and females on some tests, they are invariably smaller than the differences between the highest- and lowest-scoring males or females on the same tests.
As a consequence of the differences in the means, the number of individuals scoring above a given point will differ for the two sexes; for example, the mean for more Cognitive abilities can be subdivided and considered in any number of ways. Maccoby and Jacklin prepared a useful classification in which they delineated three general cognitive domains demonstrating sex differences: Although for ease of presentation the report refers to these three main groups of cognitive abilities, these encompass heterogeneous areas of function, with each one representing several different functions.
Furthermore, the specific cognitive processes of interest may be assessed quite differently, often leading to conflicting results. Despite these caveats, it should be noted that a reasonable consensus has emerged relating sex differences to specific patterns of cognitive function: The following sections summarize data that support this general statement. The term applies to all components of language usage: Sex differences have been demonstrated for some but not all of these verbal abilities; however, when there is a difference, it invariably favors females.
Two aspects of language showing perhaps the most consistent sex differences are verbal fluency and speech production, both of which share the need to have the ability to quickly access and to produce speech sounds and words.
Verbal fluency Hampson and Kimura, ; Hines, ; Hyde and Linn, is tested by having a subject name as many words as rapidly as possible according to either a phonological or sound-based cue words that begin with a particular letter or rhyming with a specific sound or by having the subject name words that belong to a certain category such as food or plants.
Reliable sex differences have also been reported for spelling, another verbal ability closely related to reading; however, reports of sex differences in other areas of verbal ability such as vocabulary or reading comprehension have been inconsistent and are not considered reliable Hampson and Kimura, Sex differences have also been noted in tests of memory, particularly in tests of working memory the ability to hold in memory information intended for temporary use.
This is a particularly important ability because it affects many aspects of a person's everyday life, for example, remembering a phone number given by the information operator, where the keys were just put down, or a message on the answering machine.
Females have an advantage over males in remembering both verbal and nonverbal information. Females' superiority in verbal memory has received much attention, although their skill in remembering visual details, for example, spatial locations, has often been overlooked. As summarized below, males outperform females in visuospatial abilities when the task requires the manipulation of the spatial information; females, however, remember visual information better Halpern, ; Hampson and Kimura,