To take an example, although repeated studies of aggression and testosterone in prison inmates have produced a confused picture, one intriguing discovery stands out: Among male prison inmates, in one very good study, the higher the adult testosterone level, the earlier the age of the first arrest. That is, the men who had the highest levels had been arrested youngest, in early adolescence. In another study, the level of testosterone in male juvenile delinquents was correlated with their level of observed aggressive behavior. This finding brings us to one of the most central facts about the gonadal hormones: They rise very dramatically at adolescence. From very low levels during early and middle childhood, testosterone (especially but not exclusively in males) and estradiol and progesterone (both especially but not exclusively in females) all rise to adult levels over the course of a few years, and the female monthly cycle is instituted. Few studies have measured hormones and behavior in the same individuals, but it is likely that adolescent behavior — and its gender differentiation — is influenced by these massive hormonal changes. Gender differences in fat, muscle mass, and the pitch of the voice, all of which contribute to gender-specific behavior, are determined in large part by the teenage boy’s rise in testosterone. One could conceivably leave the picture here, stress the similarity between the sexes in neurobehavioral plan, and suggest that evolution made a single beast with a single twist: an infusion of different hormones, coming from the gonads, just at the moment of reproductive maturity, just when we would expect the genders to begin to be really different. The difficulty with this neat picture is that we have overwhelming evidence that the sexes differ in their behavior long before puberty, when previously we had thought that there were not enough circulating sex steroids to make the difference. There is increasing evidence that the accounting may lie deep in the brain. In 1973 it was shown for the first time that male and female brains differed structurally. In the most forward portion of the hypothalamus, male and female rats differed in the density of synaptic connections among local neurons. Furthermore, castration of males just after birth would leave them with the female brain pattern, and injection of testosterone into females — likewise just after birth — would give them the Men are more violent than women, and women are more nurturant, at least toward infants and children, than men. male pattern. To say that this study by Geoffrey Raisman and Pauline Field “rocked the neuroscience community” seems an extreme statement, yet I believe it to be accurate. There are several reasons. For one thing, it was the-first demonstration that the brains of the sexes differ, in any animal. For another, the difference was in a region where it should have been — a region concerned with the brain’s regulation of the very gonadal hormones we have been looking at. But most impressive of all, to those who knew the field, was the demonstration that sex hormones, circulating at birth, could change the brain. One of the most interesting experiments of the kind produced “pseudo-hermaphrodite” monkeys by administering male gonadal hormones to female fetuses before birth. As they grew, these females showed neither the characteristic low female level of aggressive play nor the characteristic high male level but something roughly in between. For these reasons, investigators had, before 1973, already begun to talk about a change in the brain by male sex hormones around the time of birth; to put it crudely, a masculinization of the brain. But the involvement of the brain was only speculative until the report of Raisman and Field, which then gave the phrase its first genuine meaning. That, as it now appears, was only the beginning of the story. A few years later, Dominique Toran-Allerand did a tissue-culture experiment — with brain slices in petri dishes — in which she watched the process in action. She made thin slices of the hypothalamus of newborn mice — of both sexes — and kept them alive long enough to treat them with gonadal steroid hormones, including testosterone. Her brief paper, published in Brain Research, shows the stunning results in photomicrographs. The cells in the slices treated with testosterone show more and faster growing neural processes than with the testosterone-free solution. In effect, she was able to watch as testosterone changed the newborn brain. Her work did not imply that the faster, more florid growth made the testosterone-treated hypothalamus better — only different. For these and a variety of other reasons, the community of scientists working in this field concluded that the basic plan of the mammalian organism is female and stays that way unless told to be otherwise by masculine hormones. That this was not a necessary arrangement was shown by the sexual differentiation of birds, in which the opposite seems to be true; the basic plan is male, and the female course of development is the result of female hormones. But the mammal story was becoming clear: The genetic signal for masculinity, from Y chromosome, did its work on a female structural plan, through masculine hormones. It is only natural to doubt whether such generalizations are applicable to that most puzzling of all mammals, the one that does research on its own nature. My own doubts in the matter — formidable at the time — were largely dispelled by the investigations of Anke Ehrhardt and her colleagues, first at the Johns Hopkins School of Medicine, later at the Columbia College of Physicians and Surgeons. Ehrhardt has spent years studying the condition and clinical treatment of certain unfortunate “experiments in nature” — anomalies of sexual and psychosexual development. In one such set of anomalies, known as the adrenogenital syndrome, a genetic defect produces abnormally large quantities of the sex steroid testosterone. For girls with the syndrome, masculine levels of the hormone are floating around in the blood throughout gestation, until the time of birth. Shortly after birth the condition can be corrected, so that it is presumably only in the prenatal period that the hormone can have its effects. At age 10 these girls are psychologically different from their sisters and from unrelated controls. They are described by themselves and by their mothers as doing less doll play, being more “tomboyish,” and expressing less desire to marry and have children when they grow up. Whatever value judgment we choose to place on these phenomena — I am inclined, for the moment, to place none — they seem to be real. They have been repeated by different investigators with Clinton St. Quarterly 2
RkJQdWJsaXNoZXIy NTc4NTAz