Studies of nonhuman female primates also provide clues that these sex differences are innate and require the right hormone-priming actions. When researchers block estrogen in young female primates during infantile puberty, the females don't develop their usual interest in infants. Moreover, when researchers inject female primate fetuses with testosterone, the injected females end up liking more rough-and-tumble play than do average females. This is also true in humans. Though we have not performed experiments to block estrogen in little girls, or injected testosterone into human fetuses, we can see this brain effect of testosterone at work in the rare enzyme deficiency called congenital adrenal hyperplasia (CAH), which occurs in about one out of every ten thousand infants.
Emma did not want to play with dolls. She liked trucks and jungle gyms and sets to build things with. If you asked her at two and a half years old if she was a boy or a girl, she'd tell you she was a boy and she'd punch you. She'd get a running start, and "the little linebacker," as her mother called her, would knock over anyone who came into the room. She played catch with stuffed animals, though she threw them so hard it was tough to hang on to them. She was rough, and the girls at preschool didn't want to play with her. She was also a little behind the other girls in language development. Yet Emma liked dresses and loved when her aunt styled her hair. Her mother, Lynn, an avid cyclist, athlete, and science teacher, wondered, when she brought Emma in to see me, if her being a jock had influenced her daughter's behavior.
Most of the time, a girl like Emma would be the one in ten who is simply a tomboy. In this case, Emma had CAH.
Congenital adrenal hyperplasia causes fetuses to produce large amounts of testosterone, the sex and aggression hormone, from their adrenal glands starting at about eight weeks after conception - the very moment their brains begin to take shape into the male or female design. If we look at genetic females whose brains are exposed to surges of testosterone during this period, we see that these girls' behavior and presumably brain structures are more similar to those of males than to those of females. I say "presumably" because a toddler's brain isn't so easy to study. Can you imagine a two-year-old sitting still for a couple of hours in an MRI scanner without being sedated? But we can deduce a lot from behavior.
The study of congenital adrenal hyperplasia provides evidence that testosterone erodes the normally robust brain structures in girls. At one year old, CAH girls make measurably less eye contact than other girls the same age. As these testosterone-exposed girls get older, they are far more inclined to scuffling, roughhousing, and fantasy play about monsters or action heroes than to pretending to take care of their dolls or dressing up in princess costumes. They also do better than other girls on spatial tests, scoring similarly to boys, while they do less well on tests that tap verbal behavior, empathy, nurturing, and intimacy - traits that are typically female. The implications are that the male and female brains' wiring for social connection is significantly affected not just by genes but by the amount of testosterone that gets into the fetal brain. Lynn was relieved to have a scientific reason for some of her daughter's behaviors, since no one had taken the time to explain to her what happens in the CAH brain.