Men And Women Have The Same Size Amygdalae

Man or woman, the little red bit of the brain is the same proportion of the whole brain. CLIPAREA/Shutterstock

Quite a few people have got very rich through telling us how different men's and women's brains are, citing neuroscience for support. The reality is otherwise, a review has found, with no measurable variation between men and women in one of the brain regions most frequently claimed as the site of these deviations.

The amygdala plays an important role in emotions, aggression, and sexual arousal. The last of these in particular has raised suspicions that if any part of the human brain might vary between men and women, this would be it. This certainly seems to be the case in rats. When early MRI studies reported that men had larger amygdalae than women, it was enough to see the claim reported in books and popular science articles.

Science is constantly evolving, however. Dr Lise Eliot of Rosalind Franklin University of Medicine and Science decided to look more closely at the expanded MRI data. Using 46 published studies measuring the sizes of healthy male and female amygdalae (including some in children), Eliot found the only reason men have larger amygdalae is because they have larger overall bodies.

On average, Eliot reports in NeuroImage, men have amygdalae that are 10 percent larger in volume – a slightly smaller discrepancy than the 11-12 percent gap between the volume of men's and women's brains. This larger brain size in turn reflects men's typically larger bodies.

After controlling for overall brain size, Eliot found that the right side of the amygdala is effectively identical in men and women. Intriguingly, the studies reported a 2.5 percent larger left amygdalae in men, but this was not statistically significant. Even if real, an extra 0.03 cubic centimeters, or one five-hundredth of a cubic inch, appears rather small to carry the hopes of those claiming men and women come from different planets.

There is so much invested, ideologically and financially, in the idea that men and women have fundamentally different brains that, if Eliot's results are not simply ignored, attention will probably shift to other brain regions. One region has already been eliminated, however. Eliot previously used an even larger sample to discredit the claim that women have larger hippocampi than men.

"There is no categorically 'male brain' or 'female brain,' and much more overlap than difference between genders for nearly all brain measures," Eliot said.

Size, of course, isn't everything. Men and women might have brain regions of similar volume, but use them differently. However, when assertions for such easily testable differences have been refuted, more subtle claims need to be treated with suspicion, particularly when it has been shown that these are almost entirely lacking in evidence.

Appropriately, Eliot's university is named for a scientist denied recognition because of her sex.

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