Gendered genes, gonads and genitals line up quite strongly. That is, if you have a female version of one, the odds are very high that you will have female versions of the others. Similarly for male versions. Rebecca Jordan–Young has argued in a number of venues that behavioral traits do not line up anything like as neatly. Rather, bits of behavior and parts of character traits seem mixed up in comparison. We may think that being nurturing and compliant go together in women and aren’t present in men, but in fact there are compliant and nurturing men, non-nuturing and compliant men, and so on. Similarly for women.
This picture should lead us to suspect that the brain, in which our bodily movements originate, should manifest the same diversity. And if female genes, gonads and genitals aren’t matched with a fairly uniform set of female character traits, we should wonder whether there is much like “a female brain” or a male one.
The topic of the gendered brain is widely discussed, but a new view is opening up, and it is much what one would expect, given the information above. Using MRI imaging, researchers have looked at regions of the brain in which there are zones more reactive in men and others more reactive in women. But the percentage of individuals possessing only the male zones or only the female zones is extremely small. The women possessing only the womanly features – nurturing, compliant, more artsy than scientific, and so on and on – form a tiny group. Ditto for men.
As a somewhat dense, but really exciting article in the Guardian puts it:
[What we expect is] Not a “male brain”, or a “female brain”, but a shifting “mosaic” of features, some more common in females compared to males, some more common in males compared to females, and some common in both.
This is exactly what the new study found for the first time, with colleagues from Tel Aviv University, the Max Planck Institute, and the University of Zurich. They tested this prediction by analyzing magnetic resonance images, which directly capture structural properties of the brain, from more than 1,400 human brains from four large data-sets. They identified in each data set the regions showing the largest differences between women and men. Next, they defined a “male-end” (males more prevalent than females) zone and a “female-end” (females more prevalent than males) zone for each of these regions, based on the range of scores of the most extreme third of men and women, respectively. They found that between 23% and 53% of individuals (depending on the sample) had brains with both “male-end” and “female-end” features. In contrast, the percentage of people with only “female-end” or only “male-end” brain features was small, ranging from zero to 8%.
Cordelia Fine is one of the authors of the article above; the other is Daphna Joel, a scientist on the study reported. Cordelia Fine should be familiar to our readers as a splendid researcher on issues about sexism in neuroscience.
Addition: I’ve just seen the somewhat dismissive NY Times report
Overall, the results show “human brains do not belong to one of two distinct categories,” male and female, the researchers concluded.
Larry Cahill, a neuroscientist at the University of California, Irvine, who didn’t participate in the new study, said he agreed that brains contain varying mixtures of male and female anatomical traits. But that doesn’t rule out differences in how the brains of the two sexes work, he said.
There’s “a mountain of evidence proving the importance of sex influences at all levels of mammalian brain function,” he said.
That work shows how much sex must matter, “even when we are not clear exactly how,” he said in an email.
Though we are not told about it, Cahill most certainly has a dog in this fight. He concedes that we don’t have male and female brains, but he wants to emphasize what he’ll see are some important differences. He may well have in mind, among other studies, “Sex differences in the structural connectome of the human brain,” from PNAS, vol 111, no 2. The study claims to show the familiar idea that “male brains are optimized for intrahemispheric and female brains for inter hemispheric connections.” It would take at least one book to sort out the history of this claim, but let me note that the research in the study is hotly contested.
Cordelia Fine, writing in Slate, summarizes challenges to the PNAS study, and notes that a subset of the study’s authors have published contradictory research. She suggests the original paper may be the most neurosexist report of the year.
I’m not claiming to settle the issue, but rather to make it clear that agreement that there aren’t in general male and female brains is significant, given how fraught the research in this field is.