No major differences between women’s and men’s brains

While prenatal hormones probably cause most brain sex differences in nonhumans, there are some cases where the cause is directly genetic.This was dramatically shown by a zebra finch with a strange anomaly – it was male on its right side and female on its left.

A singing-related brain structure was enlarged (as in typical males) only on the right, though the two sides experienced the same hormonal environment. Thus, its brain asymmetry was not caused by hormones, but by genes directly.

Since then, direct effects of genes on brain sex differences have also been found in mice.