A sample of almost 8,000 MRI scans has revealed differences between men’s and women’s brains, but only in countries with high measures of gender inequality. In places with less discrimination, no statistically significant distinctions could be seen between brains, at least on this measure.
Being on the receiving end of gender discrimination is associated with poorer mental health and lower academic achievement. Inevitably, these observations are then used to justify the discrimination, with women labeled as less intelligent. A team spread across 72 research institutions teamed up to test whether there are visible physical consequences reflecting the observations.
Using 7,876 MRI scans from 139 study samples in 29 countries, the authors looked for differences between the brains of men and women aged 18-32 and compared what they found with a measure produced by combining two inequality indexes. In a new paper, they report that a history of discrimination can be seen – at least statistically – in three brain subregions.
Difficult childhoods are known to affect the development of certain brain regions. The paper specifically refers to “cortical surface area, thickness, and hippocampal volumes” as areas where the effects of poverty and discrimination while young have been observed, although there is some debate as to the linking mechanism.
The authors report that in countries that score high for gender equality, women and men appeared very similar in the 68 brain subregions studied, with women having a statistically insignificant lead in size in some brain subregions. However, the more discriminatory a country is, the more likely men were to have comparatively more cortical thickness – but only in three subregions: the right caudal anterior cingulate gyrus, right orbitofrontal gyrus, and left lateral occipital cortex.
Finding a difference in only three out of 68 regions raises suspicion of p-hacking. However, the fact that in all three cases, the effect was in the same direction – that is regions being thinner in women in countries with high gender inequality – makes it more likely the effect is real. These are also regions that have been identified in previous studies on how difficult childhoods affect the brain. They are, for example, areas known to be important for emotional resilience and comparing oneself to peers, and to be particularly affected by depression.
Although the paper is the first to observe this pattern on an international basis, a previous study looked at hippocampal volume in 10-year-old American girls across 17 states. Sizes were inversely correlated with the level of gender inequality in the state in which they lived.
If the measurements reported here are reflected in capacity, it would indicate widespread discriminatory views can become a self-fulfilling prophecy. That is, if a segment of the population, be it women or another minority, are treated as intellectually deficient growing up, their brains may come to reflect this and they may perform more poorly, at least on certain tests. Besides the harm to the individual, this almost certainly lowers average performance for the society as a whole. Only by rejecting stereotypes can the vicious cycle be broken.
The authors acknowledge their samples were heavily skewed toward upper and upper-middle income countries. However, all comparisons were between men and women within a country, rather than between countries, where economic circumstances might create differences.
The study is published Open Access in Proceedings Of The National Academy Of Sciences.