Two Myths and Three Facts About the Differences in Men and Women's Brains
Gender brain differences are real, but we should interpret them with caution.
Posted Jul 20, 2012
Myth #1: Women’s brains are more balanced.
This is a variation of the popular idea found in many books and websites that men depend more than women on one hemisphere or the other for particular functions (especially language), and related to this, that women have a chunkier corpus callosum—the bridge of neurons that connects the two brain hemispheres.
One source of the myth is a theory proposed by the late US neurologist Norman Geschwind and his collaborators in the 1980s, that higher testosterone levels in the womb mean the left hemisphere of male babies develops more slowly than females, and that it ends up more cramped. But the Geschwind claim is not true: John Gilmore and his team scanned the brains of 74 newborns and found no evidence for smaller left hemispheres in male babies compared with females.
Also debunking the idea of greater lateralisation in male brains, a meta-analysis by Iris Sommer and her colleagues of 14 studies, involving 377 men and 442 women, found no evidence of differences in language lateralisation between the sexes. On the thickness of the corpus callosum, Mikkel Wallentin reviewed the evidence in a 2009 paper, including post-mortem and brain imaging studies. “The alleged sex-related corpus callosum size difference is a myth,” he wrote.
Fact #1: Men’s brains are bigger.
Men do have bigger brains than women, even taking into account their larger bodies. This has been documented time and again. To take just one example, Sandra Witelson and her colleagues weighed the brains of 58 women and 42 men post-mortem and found the women’s brains were 1,248 grams on average, compared with 1,378 grams for the men.
Note that there’s an overlap between the sexes, so some women will have larger brains than some men. A Danish study of 94 brains published in 1998 estimated that the larger male brain volume translated into an average 16 percent more neurons in the neocortex of men versus women.
Fact #2: There are sex differences in the size of individual brain structures.
The hippocampus, a structure involved in memory, is usually larger in women; the amygdala, a structure involved in emotional processing, is larger in men. It’s also true that the cortical mantle (made up of grey matter) is thicker in women, and that women tend to have a higher ratio of grey to white matter (white matter being the kind of brain cells that are insulated). However, it’s important to note that these differences may have more to do with brain size than with sex—in other words, it could be that smaller brains tend to have a higher ratio of grey matter, and it just happens that women tend to have smaller brains.
Myth #2: Sex-related brain differences explain behavioural differences between the sexes.
According to John Gray, author of Why Mars and Venus Collide, men are prone to forgetting to buy the milk because of their more localised brain activity (as quoted by Cordelia Fine).
It’s tempting to see the brain differences between the sexes, mythical or otherwise, and think that they explain behavioural differences; such as men’s milk amnesia, their superiority on mental rotation tasks or women’s advantage with emotional processing. In fact, in many cases we simply don’t know the implications of the sex-related brain differences. It’s even possible that brain differences are responsible for behavioural similarities between the sexes. This is known as the “compensation theory” and it could explain why men and women’s performance on various tasks is similar even whilst they show different patterns of brain activity. Bearing this in mind, readers should treat with extreme scepticism those evangelists who draw on supposed sex-related brain differences to support their claims about the need for gendered educational practices.
It’s also important to remember that behavioural differences between the sexes are rarely as fixed as is often made out in the media. Cultural expectations and pressures play a big part. For instance, telling women that their sex is inferior at mental rotation tends to provoke poor performance; giving them empowering information, by contrast, tends to nullify any sex differences. Related to this, in countries that subscribe less strongly to gender-stereotyped beliefs about ability, women tend to perform better at science. These kinds of findings remind us that over-simplifying and over-generalising findings about gender differences risks setting up vicious self-fulfilling prophesies, so that men and women come to resemble unfounded stereotypes.
Fact #3: Sex-related brain differences matter.
While we should be cautious about how we interpret sex-related brain differences (Cordelia Fine reminds us that “the male brain is like nothing in the world so much as a female brain”), it’s important not to take political correctness too far and deny that differences do exist. The neuroscientist Larry Cahill makes this point in his 2006 paper “Why sex matters for neuroscience”, in which he reviews many of the sex-related brain differences. Furthering our understanding of sex-related brain differences could help shed much needed light on conditions like autism and depression that tend to be found much more often in men and women, respectively.