Do the anatomical differences between men and women—sex organs, facial hair, and the like—extend to our brains? The question has been as difficult to answer as it has been controversial. Now, the largest brain-imaging study of its kind indeed finds some sex-specific patterns, but overall more similarities than differences. The work raises new questions about how brain differences between the sexes may influence intelligence and behavior.
For decades, brain scientists have noticed that on average, male brains tend to have slightly higher total brain volume than female ones, even when corrected for males’ larger average body size. But it has proved notoriously tricky to pin down exactly which substructures within the brain are more or less voluminous. Most studies have looked at relatively small sample sizes—typically fewer than 100 brains—making large-scale conclusions impossible.
In the new study, a team of researchers led by psychologist Stuart Ritchie, a postdoctoral fellow at the University of Edinburgh, turned to data from UK Biobank, an ongoing, long-term biomedical study of people living in the United Kingdom with 500,000 enrollees. A subset of those enrolled in the study underwent brain scans using MRI. In 2750 women and 2466 men aged 44–77, Ritchie and his colleagues examined the volumes of 68 regions within the brain, as well as the thickness of the cerebral cortex, the brain’s wrinkly outer layer thought to be important in consciousness, language, memory, perception, and other functions.
Adjusting for age, on average, they found that women tended to have significantly thicker cortices than men. Thicker cortices have been associated with higher scores on a variety of cognitive and general intelligence tests. Meanwhile, men had higher brain volumes than women in every subcortical region they looked at, including the hippocampus (which plays broad roles in memory and spatial awareness), the amygdala (emotions, memory, and decision-making), striatum (learning, inhibition, and reward-processing), and thalamus (processing and relaying sensory information to other parts of the brain).