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Everyone knows the difference between male and female brains.
One is chatty and a little nervous, but never forgets and takes good care of others. The other is calmer, albeit more impulsive, but can tune out gossip to get the job done.
These are stereotypes, of course, but they hold surprising sway over the way actual brain science is deed and interpreted. But as a neuroscientist long experienced in the fieldI recently completed a painstaking analysis of 30 years of research on human brain sex differences. And what I found, with the help of excellent collaborators, is that virtually none of these claims has proven reliable.
Nor do any of the alleged brain differences actually explain the familiar but modest differences in personality and abilities between men and women. When it comes to the brain, some animals do indeed exhibit sexual dimorphism, such as certain birds whose brains contain a song-control nucleus that is six times larger in males and is responsible for male-only courtship singing. But as we demonstrate in our exhaustive survey, nothing in human brains comes remotely close to this.
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Other highly touted brain sex differences are also a product of size, not sex. These include the ratio of gray matter to white matter and the ratio of connections between, versus within, the two hemispheres of the brain.
Both of these ratios are larger in people with smaller brains, whether male or female. Such inconsistency shows there are no universal features that discriminate male and female brains in humans — unlike those deer antlers. But the truth is, as studies have gotten bigger, the sex effects have gotten smaller.
The end of men
This collapse is a telltale of a problem known as publication bias. Small, early studies which found a ificant sex difference were likelier to get published than research finding no male-female brain difference. We must be doing something right, because our challenge to the dogma of brain sex has received pushback from both ends of the academic spectrum.
And the same is true for most behavioral measures. Fortunately, many refusedarguing the range of ability is always much greater among boys or among girls than between each gender as a group. In other words, sex is a very imprecise indicator of what kind of brain a person will have. Another way to think about it is every individual brain is a mosaic of circuits that control the many dimensions of masculinity and femininity, such as emotional expressiveness, interpersonal style, verbal and analytic reasoning, sexuality and gender identity itself.
Or, to use a computer analogy, gendered behavior comes from running different software on the same basic hardware.
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The absence of binary brain sex features also resonates with the increasing s of people who identify as nonbinary, queer, nonconforming or transgender. Whatever influence biological sex exerts directly on human brain circuitry is clearly not sufficient to explain the multidimensional behaviors we lump under the complex phenomenon of gender.
As you may have noticed, these can be transplanted between women and men with great success. Edition: Available editions United Kingdom.