Does a male fetus drive a truck while his sister hugs a doll? The origin of sex differences.

During my senior year of college, Harvard’s then-president Larry Summers briefly became the center of a storm of controversy for claiming that some sort of “intrinsic aptitude” in math skills could explain why women are underrepresented in science. Although this was one of only three possible explanations he suggested, he was lambasted by some for daring to hold up stories of his daughter personifying toy trucks as “daddy” and “baby” as evidence that built-in differences in the way males and females behave do in fact exist.

Even seven years later, the outrage persists as surveys showing discrimination in the sciences are held up as proof that Larry Summers was wrong, and sexist. Scientists have observed neurological differences in how preschool girls and boys solve spatial rotation problems, but you may just catch it online if you dare to suggest these are anything other than the very first signs that we give little girls too many dolls and aren’t-you-adorable’s from the moment they are born. The fact that my niece loves Monster High (and a real-life doll, if she can get one) and my youngest nephew loves trucks, planes, and most recently school buses would be held up by this crowd as a result of parents who refused to fight the stereotypes (perhaps by refusing to let anyone know their gender), nothing more.

Girls like dolls, boys like trucks: Nature, or nurture?

Girls like dolls, boys like trucks: Nature, or nurture?

I am already on the record declaring that the next major revolution in psychology will be the understanding of how microscopic organisms like toxoplasmosis and streptococcus can alter our behavior, so it should come as no surprise that I believe hormones might influence the brain the same way they influence the rest of our bodies. Don’t get me wrong, I also know for a fact that there are strong socialization pressures at work  – that adults rate male babies as more “pretty” and girls babies as more “strong” when they’re told the wrong gender (although that might just be changing), that even 21st century mothers will talk more math to boys than girls without realizing it, and that children pick up on these  cultural stereotypes at very young ages.

But when I set out to teach my students about the differences between boys and girls – from a broader behavioral perspective, that is – I find that they are well aware of the powers of socialization, from Disney princesses  and teacher bias.  The only recognition the biology might play a role is usually the story of David Reimer, who due to a botched circumcision was raised as a girl – with rather disastrous results. That story sets a compelling argument for biology rather than society determining gender identity…and then we turn to congenital adrenal hyperplasia to consider other potentially “intrinsic” sex differences.

Congenital adrenal hyperplasia, or CAH, is of particular interest to psychologists because girls with the syndrome are exposed to vast quantities of male hormones in the womb – and then when they are born, quickly identified and treated and raised by parents who, the thinking goes, might go out of their way to make sure that their little girl acts like a “normal” girl. Here we have girls, then, whose developing brains were bathed in testosterone like little boys’, but who were socialized as little girls.

If the sex differences in math, toy preferences, and career paths are all socialization and discrimination, then girls with CAH should behave no differently than girls without the extra exposure to male hormones. Instead, girls with CAH spend far more time playing with “male” toys like building blocks, are less interested in cheerleading and more in engines,  and even tend to draw pictures more similar to those of little boys (fewer colors, more trucks). In other words, my nephew is pushing around a yellow school bus instead of a pink one because I thought a pink one would be a little silly…but in a small but important way his obsession with trains, planes and things that go was set long before I or any adult was handing him toys.

This is not to say that biology is destiny. The extent of the role of prenatal hormones in wiring the brain is far from settled, although for every claim that fetal testosterone is not the all-driving force CAH suggests there are fascinating points about sex differences non-human primates (such as the way male and female chimpanzees play with sticks) despite their luck in being spared princesses and Power Rangers. But it is to say that we can’t ignore the genetic and hormonal origins of sex differences, if we want to counteract them.

My youngest nephew (the one with the truck, and being loved as a living baby doll) is a third-generation late talker. There’s enough of a family history to suggest that there is some biological reason for this. But this does not mean his mother shrugged and wrote off the possibility of her son ever being a famous orator or writer. She got him into preschool, into speech therapy, into some very cool educational games on the iPad, and she works constantly to find a way to make language interesting to a little boy who would otherwise be content to drag her by her finger to show her what he wants. We could do the same for children of either sex – math for girls, empathy for boys, pick your stereotype – if we could recognize that the seed of these stereotypical differences is a bit of biological fact, and give them a little extra training to help their brains process what comes a little less naturally.

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