Sex differences in brain anatomy

Background Multiple reports of differences in brain size == male brains are bigger Some reports of regional differences supporting functions that show some gender differences (spatial performance, language) Some reports of differing proportions of gray and white matter

Nopolous, Flaum, O’Leary & Andreasen (2000) “ “Sexual dimorphism in the human brain: evaluation of tissue volume, tissue composition and surface anatomy using magnetic resonance imaging” Psychiatry Research, Vol 98, pp 1-13.

Problems with other work Samples often small Measurements usually on postmortem brains, often without access to important information about owner of the brain.

Their work Larger sample – 84 normal volunteers screened by advertisement in the newspaper Matched for age, health, IQ, parent’s social class Screened for medical and psychiatric conditions

Very careful structure MRI

Findings 1.Modest overall size difference (men had brains that were about 8% larger). 2.Female brain size correlated with height but male brain size didn’t. 3.Very little evidence for regional differences (one exception was right parietal lobe), but analysis was fairly gross 4.No differences in grey/white matter proportions

Conclusions The only reliable sex difference in matters of size is the overall increased size of brain in men It is possible that increased brain size alone has no more functional import than other sex differences (nose and liver, for instance).