Important structural and functional differences in the brains of human females and males are in part a result of prenatal sex — differentiation processes (Becker et al., 2008; Hines, 2004; McCarthy et al., 2011). Many areas of the developing prenatal brain are significantly affected by circulating hormones (both testosterone and estrogen), which contribute to the development of these sex differences (Hines, 2004; Zuloaga et al., 2008).
At the broadest level, there is a significant sex difference in overall brain size. By age 6, when human brains reach full adult size, male brains are approximately 15% larger than female brains (Gibbons,
1991). Researchers believe that this size difference results from the influence of androgens, which stimulate faster growth in boys’ brains (Wilson, 2003). Other specific human brain sex differences involve at least three major areas: the hypothalamus (hy-poh-THAL — uh-mus), the left and right cerebral hemispheres, and the corpus callosum (I Figure 5.4).
A number of studies link marked differences between the male and female hypothalamus to the presence or absence of circulating testosterone during prenatal differentiation (McEwen, 2001; Reiner, 1997a, 1997b). In the absence of circulating testosterone, the female hypothalamus develops specialized receptor cells that are sensitive to estrogen in the bloodstream. In fetal males the presence of testosterone prevents these cells from developing sensitivity to estrogen. This prenatal differentiation is critical for events that take place later. During puberty the estrogen-sensitive female hypothalamus directs the pituitary gland to release hormones in cyclic fashion, initiating the menstrual cycle. In males the estrogen-insensitive hypothalamus directs a relatively steady production of sex hormones.
Research has uncovered several intriguing findings pertaining to sex differences in one tiny hypothalamic region called the bed nucleus of the stria terminalis (BST) (Chung et al., 2002; Gu et al., 2003). The BST contains androgen and estrogen receptors and appears to exert a significant influence on human sex differences and human sexual functioning. One central area of the BST is much larger in men than in women (Zhou et al., 1995), and a posterior region of the BST is more than twice as large in men as in women
I Figure 5.4 parts of the brain: (a) cross section of the human brain showing the cerebral cortex, corpus callosum, hypothalamus,
and pituitary gland; (b) top view showing the left and right cerebral hemispheres. Only the cerebral cortex covering of the two hemispheres is visible.
Gender Issues
(Allen & Gorski, 1990). Researchers have also reported sex differences in an anterior region of the hypothalamus, called the preoptic area (POA). One specific site in the POA is significantly larger in adult men than in adult women (Swaab et al., 1995). Evidence from these and other studies has led some theorists to hypothesize that sex differences in both human sexual behavior and gender-based behavior in children and adults result, in part, from a generalized sex-hormone-induced masculinization or feminization of the brain during prenatal development (Cohen-Kettenis, 2005; Mathews et al., 2009).
Other key differences between male and female brains have been demonstrated in the function and structure of the cerebral hemispheres and the corpus callosum. The cerebrum, consisting of two cerebral hemispheres and the interconnection between them, is the largest part of the human brain. The two hemispheres, although not precisely identical, are almost mirror images of each other (see Figure 5.4b). Both cerebral hemispheres are covered by an outer layer, called the cerebral cortex, which is a major brain structure responsible for higher mental processes, such as memory, perception, and thinking. Without a cortex we would cease to exist as unique, functioning individuals.
As Figure 5.4b illustrates, the two hemispheres are approximately symmetric, with areas on the left side roughly matched by areas on the right side. A variety of functions, such as speech, hearing, vision, and body movement, are localized in various regions of the cortical hemispheres. Furthermore, each hemisphere tends to be specialized for certain functions. For example, in most people verbal abilities, such as the expression and understanding of speech, are governed more by the left hemisphere than by the right. In contrast, the right hemisphere seems to be more specialized for spatial orientation, including the ability to recognize objects and shapes and to perceive relationships between them.
The term lateralization of function is used to describe the degree to which a particular function is controlled by one rather than both hemispheres. If, for example, a person’s ability to deal with spatial tasks is controlled exclusively by the right hemisphere, we could say that this ability in this person is highly lateralized. In contrast, if both hemispheres contribute equally to this function, the person would be considered bilateral for spatial ability.
Even though each cerebral hemisphere tends to be specialized to handle different functions, the hemispheres are not entirely separate systems. Rather, our brain functions mostly as an integrated whole. The two hemispheres constantly communicate with each other through a broad band of millions of connecting nerve fibers, called the corpus callosum (see Figure 5.4a) (Smith et al., 2005). In most people a complex function such as language is controlled primarily by regions in the left hemisphere, but interaction and communication with the right hemisphere also play a role. Furthermore, if a hemisphere primarily responsible for a particular function is damaged, the remaining intact hemisphere might take over the function.
Keeping in mind this general overview of brain lateralization, we note that research has revealed some important differences between male and female brains in the structure of the cerebrum. First, studies of the fetal brains of both humans and rats have found that the cerebral cortex in the right hemisphere tends to be thicker in male brains than in female brains (De Lacoste et al., 1990; Diamond, 1991). Perhaps of even greater significance is the finding of differences between male and female brains in the overall size of the corpus callosum. Several studies have demonstrated that this structure is significantly thicker in women’s brains than in men’s brains (Smith et al., 2005). This greater thickness of the corpus callosum allows for more intercommunication between the two hemispheres, which could account for why female brains are less lateralized for function and male brains have larger asymmetries in function (Savic & Lindstrom, 2008).
Research has clearly demonstrated differences between male and female brains in the degree of hemispheric specialization for a variety of cognitive tasks. One recent study found significant sex-linked differences in neural activity among men and women as they judged the aesthetic quality of artistic and natural visual stimuli. Brain activity was bilateral or
symmetrical in the hemispheres of women exposed to stimuli they described as beautiful, whereas in men aesthetically pleasing stimuli instigated neural activity lateralized in their right hemispheres (Cela-Conde et al., 2009). Other research has demonstrated sex differences in the degree of hemispheric specialization for verbal and spatial cognitive skills. Women tend to use both brain hemispheres when performing verbal and spatial tasks, whereas men are more likely to exhibit patterns of hemispheric asymmetry by using only one hemisphere for each of these functions (Savic & Lindstrom, 2008; Wisniewski et al., 2005). The stronger communication network between the two halves of a female’s brain might explain why women typically exhibit less impairment of brain function than men do after comparable neurological damage to one hemisphere (Majewska, 1996).
Researchers and theorists are debating whether these structural differences between male and female cerebrums can explain differences between the sexes in cognitive functioning. Females often score higher than males on tests of verbal skills, whereas the reverse is often true for mathematics and spatial tests (Halpern & LaMay, 2000; Hetzner, 2010; Nowak et al., 2011). Some researchers suggest that differences between male and female hemispheric and corpus callosum structures indicate a possible biological basis for such differences between the sexes in cognition (Geer & Manguno-Mire, 1997; Leibenluft, 1996). However, many theorists argue that reported differences between males and females in cognitive skills are largely due to psychosocial factors (Hyde, 2007; Kurtz — Costes et al., 2008). This viewpoint is supported by substantial evidence that such differences have declined sharply or disappeared in recent years. Several major national studies have reported few differences in the science and mathematics skills of male and female children and adolescents over the last three decades (Kurtz-Costes et al., 2008). A recent National Science Foundation study found that girls had achieved parity with boys on standardized math tests in every grade from 2 through 11 (Hyde, 2006). Another national study found that girls perform as well as boys on state math tests (Hetzner, 2010). Nevertheless, females are markedly less likely than males to enter math-intensive professional occupations (e. g., engineering, computer sciences, and physics), a discrepancy that may have more to do with factors related to parental child-rearing practices than to differences in cognitive skills (Barnett & Rivers, 2012; Ceci & Williams, 2011; Zakaib, 2011).
Finally, to put the question of sex differences in proper perspective, we acknowledge the informed observation of eminent psychologist Carol Tavris (2005), who stated that "the similarities between the sexes in behavior and aptitude are far greater than the differences" (p. 12).