Fine begins Part 2 of Delusions of Gender with an overview of brain development, in particular the “fetal fork”: the moment around the sixth week of gestation when a fetus’s gonads become testes or ovaries. In a male fetus, the testes produce a surge of testosterone, which “is essential for bringing about male genitalia” (101). Neuroendocrinologists have hypothesized that this surge might also organize the developing brain, and they have found behavioral evidence for this idea in the behavior of songbirds and rats: Female finches exposed to a male hormonal environment began to sing (a male behavior), and male rats castrated at birth have aggression levels comparable to those of female rats (103).

However, Fine cautions, it’s dangerous to generalize to human brains from the brains of these less complex animals—and even in rodent brains, sex differences seemed to be influenced not just by hormones but by mother rats’ different treatment of male and female babies. “Even our simple hormone-to-brain-stem storyline,” Fine writes, “has a social subplot” (105). While scientists are able to observe cause and effect in the hormonal structuring of the brain, translating physical sex difference into behavioral conclusions has proven extremely difficult (104). The very societal prejudices examined in Part 1 might incline us to draw unwarranted lines between fetal testosterone and male-coded behavior: