Couldn’t I have left out that correction about estradiol? Possibly, but it conveys all by itself the fact that surprising mechanisms are in brains and bodies because we are evolved animals, not designed from scratch by a team of engineers. It also conveys a small irony. Ultimately, it is a female hormone that plays a big role in masculinizing the male brain.
Where do these testosterone-sensitive changes happen? Mostly in small regions of the hypothalamus, an evolutionarily ancient structure. Some hypothalamic nuclei regulate thirst and drinking behavior, others regulate hunger and eating behavior. As we saw in Chapter 4, some hypothalamic regions are important for parental care in mammals. Yet other regions regulate sexual behavior. These hypothalamic regions are important in the male’s sexual behavior—his interest in females and his capacity to mount and penetrate the female.
The size of these cell groups in the hypothalamus is not by itself the cause of male versus female differences in sexual behavior and sexual feelings. What really makes the difference is the pattern of connections among the neurons within each group and the connections between one group and other networks in the brain. The size is just one structural difference that we can currently detect. Because the size correlates with stereotypical male sexual behavior, that tells us what to study next to begin to uncover the broader causal details.
In a slightly different part of the hypothalamus—an area shown in rats to be important for ovulation—the neurochemical dopamine inhibits cell death. In the female brain, this area expands—not prenatally, however, but during puberty. In males, the cells in this area are not only fewer in number but also different biochemically, making and releasing homegrown opioids. If by chance a female had opioid neurons in this area, they would inhibit ovulation. The cells in this area project into the nearby pituitary gland, and this is the crucial communication channel between brain and ovaries. When these neurons are active, the pituitary releases hormones that stimulate the ovaries to produce estrogen. The female cycle of release of eggs from the ovaries will then begin.
Within the male (XY) and the female (XX) populations, there is individual variation in the number of neurons in the sex-sensitive hypothalamic nuclei; that is, some men will have about the same number of neurons as some women. Averages are only averages, not invariant principles.
Another significant difference between male and female brains may help explain why females are somewhat more fearful and cautious than males. This holds not just for humans, but for other mammals as well. And I mean, of course, on average. So some particular female hyena may be less fearful than some particular male hyena. In female brains, the hypothalamus (the ventral medial area) is more densely connected to another subcortical structure, the amygdala. The amygdala is important for generating the fear response and for learning what is to be feared. From an evolutionary point of view, the female mammal’s role in pregnancy, birth, and child rearing means that she is vastly more vulnerable than the males. She needs to be a little more cautious. To be sure, the behavioral significance of the amygdala difference is not yet thoroughly understood, and many environmental factors likely go into a particular person’s profile of risk aversion and fear. Nevertheless, the finding may well lead us to a fuller understanding of these average differences in behavior.
Intricate also are the interactions between a whole orchestra of potent neurochemicals that can affect mood, personality, and temperament. They can affect risk taking, aggression, trust; whether you are shy or outgoing, easygoing or prickly. Levels vary across individuals and also within a person over time. They include such neurochemicals as serotonin, vasopressin, oxytocin, stress hormones, and somatostatin. What else is in the orchestra? Well, there are interactions involving other parts of the body, including the pituitary gland, thyroid gland, and adrenal glands. And then there is the immune system, which interacts with all these things, and the brain.