FOR most of recorded history, men and women tended to have different societal roles, interests and occupations. Perhaps it was natural to assume these stemmed from innate differences in their brains, as well as the more obvious ones in their bodies. That idea has long been contentious, but now, with ideas about gender changing faster than ever, the question of whether there are differences between the brains of men and women has taken a keener edge. It remains a divisive issue, even among neuroscientists. Nevertheless, they are finally cutting through the historical discrimination and gender politics to get at the truth.
Early measurements of skull capacity showed that the brains of men are, on average, somewhat bigger and heavier than those of women. Some commentators posited that this 鈥渕issing 5 ounces鈥� was the key to men鈥檚 supposedly superior abilities. In fact, the simple explanation is that bigger bodies require more brain tissue to run them 鈥� a relationship seen across animal species.
Advertisement
Things got more complicated with the advent of brain-scanning technology in the 1990s, which suggested sex differences in the size of specific brain regions and structures. These findings were often turned into compelling tales about why, for instance, women are more empathic on average, or why men are more likely to be engineers. However, studies from the early decades of brain-scanning research should be taken with a pinch of salt, says at Rosalind Franklin University in Illinois. 鈥淲hen you control for brain size, all of the claims about volume differences of individual structures between men and women either disappear or become extremely small.鈥�
In fact, brain-scanning research has recently had something of an existential crisis. A major analysis from 2022 found that, to be trustworthy, studies need to scan a few thousand people, whereas sex difference research tends to look at just a few dozen. Participant numbers have also been very low in the few studies that have so far explored the brains of transgender people. To cap it all, in a 2021 , Eliot鈥檚 team found telltale signs of publication bias: scientists who found sex differences have been more likely to publish their work than those who found none.
So should we give up on answering this question? Maybe not. In the past few years, brain-scanning studies have improved. Shortly after Eliot鈥檚 damning review, the most yet emerged from a massive research project called UK Biobank, which has scanned the brains of 40,000 people in the UK. It did reveal sex differences in the size of certain brain structures, even when controlling for total brain size: about a third of regions were larger in men and about a third were larger in women. But these differences were small 鈥� generally just a few per cent. 鈥淎nd there wasn鈥檛 one region that really stood out that might lead to a hypothesis,鈥� says at The University of Texas at Austin, who was involved in the study.
What鈥檚 more, these small differences might not be innate but could instead be caused by the brain responding to life experiences, a phenomenon known as neuroplasticity. We know this happens in other contexts. For instance, if someone becomes blind in adulthood, some of the parts of their brain that deal with vision switch over to processing sounds. So, even if men tend to have a slightly larger part of the brain responsible for, say, spatial skills, that could be because they are more likely to take jobs that involve those activities. In other words, any brain differences between sexes could be a consequence of different gender roles, not their cause.
Neuroscientists are only at the start of their journey of understanding sex differences in the brain. 鈥淭here are small differences, but we don鈥檛 know if they鈥檙e meaningful or not,鈥� says Williams. 鈥淭his is where research should be heading next 鈥� trying to understand: 鈥榃hat do these differences mean?'鈥�
