Use Your Brain

New Study Finds Strong Evidence of Male-Female Brain Differences

Perhaps you’ve heard about a precocious study making the rounds about the differences between male and female brains. Perhaps you’ve noticed it reported under headlines like, “Stereotypically ‘male and female brains’ aren’t real, say scientists” or “Brains aren’t actually ‘male’ or ‘female,’ new study suggests” or “Scans prove there’s no such thing as a ‘male’ or ‘female’ brain” or “Male brain vs. female brain? Research says they’re unisex” or “There Is No Difference Between Male and Female Brains, Study Finds” or “There’s no such thing as a ‘male brain’ or ‘female brain,’ and scientists have the scans to prove it” or “Men are from Mars….and so are women! Scans reveal there is NO overall difference between the brains of the sexes” or “A welcome blow to the myth of distinct male and female brains.”

I could go on, but I won’t. Given all these headlines, it might surprise you that the study all these journalists are reporting on actually finds strong evidence of differences between male and female brains. The study is “Sex beyond the genitalia: The human brain mosaic” by Daphna Joel et al. (and that’s a big et al!). It’s sadly behind a paywall, but here’s its abstract in full:

Whereas a categorical difference in the genitals has always been acknowledged, the question of how far these categories extend into human biology is still not resolved. Documented sex/gender differences in the brain are often taken as support of a sexually dimorphic view of human brains (“female brain” or “male brain”). However, such a distinction would be possible only if sex/gender differences in brain features were highly dimorphic (i.e., little overlap between the forms of these features in males and females) and internally consistent (i.e., a brain has only “male” or only “female” features). Here, analysis of MRIs of more than 1,400 human brains from four datasets reveals extensive overlap between the distributions of females and males for all gray matter, white matter, and connections assessed. Moreover, analyses of internal consistency reveal that brains with features that are consistently at one end of the “maleness-femaleness” continuum are rare. Rather, most brains are comprised of unique “mosaics” of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males. Our findings are robust across sample, age, type of MRI, and method of analysis. These findings are corroborated by a similar analysis of personality traits, attitudes, interests, and behaviors of more than 5,500 individuals, which reveals that internal consistency is extremely rare. Our study demonstrates that, although there are sex/gender differences in the brain, human brains do not belong to one of two distinct categories: male brain/female brain.

To paraphrase that last sentence, while male and female brains are different, they’re not different in the strong sense of having some exclusive traits. All those headlines saying that “Scans reveal there is NO overall difference between the brains of the sexes” are just wrong. The scans revealed that there were differences, but that these differences weren’t shared by 100% of one gender and 0% of the other. In ordinary life, 80-20 or 60-40 is considered a difference.

Let’s be honest about why this study is of so much interest to all these journalists and the people who read them: We want to know about mental differences between men and women that manifest in different behaviour in everyday life. We also want to know if these differences are the result of nature or nurture. So when the study says that the “findings are corroborated by a similar analysis of personality traits, attitudes, interests, and behaviors of more than 5,500 individuals, which reveals that internal consistency is extremely rare,” what they mean is that they tested the thing we’re really interested in. And by golly, they found that some women enjoy football and some men enjoy romantic comedies. They didn’t find that men and women were equally likely to enjoy football or rom coms, a claim even science journalists were wise enough not to make.

The claim I take issue with is summarized in this sentence:

Documented sex/gender differences in the brain are often taken as support of a sexually dimorphic view of human brains (“female brain” or “male brain”). However, such a distinction would be possible only if sex/gender differences in brain features were highly dimorphic (i.e., little overlap between the forms of these features in males and females) and internally consistent (i.e., a brain has only “male” or only “female” features).

This really seems like a straw man, or at the very least a weak man. Who precisely is making this distinction? The “significance” blurb gives us a hint:

Sex/gender differences in the brain are of high social interest because their presence is typically assumed to prove that humans belong to two distinct categories not only in terms of their genitalia, and thus justify differential treatment of males and females.

Again, the passive voice prevents us from finding out who these mysterious people are that are making this false assumption. However, because this is of “social interest” rather than scientific interest tells us that it’s regular people, rather than neuroscientists, who the authors think are using neurological evidence to justify differential treatment of men and women.

When people talk about “male brains” and “female brains,” are they really assuming a stark distinction? I don’t think so. If I say “men are taller than women,” I don’t mean that literally every man is literally taller than every woman. I mean that if we looked at the distribution of heights for men and for women, we would see that the male distribution has a higher mean and median than the female distribution.

What this study proves is that the male and female neurological differences that they measured were more like height than like some other sexually distinct characteristics such as “having a penis” or “having a uterus.” Drawn from different distributions, but not categorically different with near-100% accuracy.

I think both the researchers and the journalists are responsible for the misleading nature of the reporting on this study. The researchers did a good piece of science that incrementally added to our knowledge about gender and the brain. They didn’t overturn the apple cart by radically departing from some well-established theory, they merely added to what we already knew. However, to promote their paper and get worldwide media attention and invitations to TED talks for life, they set up a straw man theory that they could easily knock down; setting it up in such a way as to play to journalists’ obvious biases. The journalists, and their editors, jumped at the opportunity to confirm their biases while sparking controversy and driving clicks. They wrote their titles and opening paragraphs in such a way as to make it seem that the idea of male-female brain differences had been debunked, while hiding a few sentences about the study’s actual findings further down the page.

This pattern is all too familiar, as documented in the Slate Star Codex article on “Reporter Degrees of Freedom.” The message we should all be taking is that science journalists, and especially the editors who write their headlines, cannot be trusted. This is especially the case when their stories have political implications that flatter their biases.