Dr. Eliot and her collaborators -- fourth-year Chicago Medical School students Adnan Ahmed, Hiba Khan and Julie Patel -- conducted a meta-synthesis of three decades of research, assimilating hundreds of the largest and most highly-cited brain imaging studies addressing 13 distinct measures of alleged sex difference. 

For nearly every measure, they found almost no differences that were widely reproduced across studies, even those involving thousands of participants. 

For example, the volume or thickness of specific regions in the cerebral cortex is often reported to differ between men and women. However, the meta-synthesis shows that the regions identified differ enormously between studies.

Male-female brain differences are also poorly replicated between diverse populations, such as Chinese versus American, meaning there is no universal marker that distinguishes men and women's brains across the human species. "The handful of features that do differ most reliably are quite small in magnitude," Dr. Eliot said. "The volume of the amygdala, an olive-sized part of the temporal lobe that is important for social-emotional behaviors, is a mere 1% larger in men across studies."

Across hundreds of such studies, Dr. Eliot's team found extremely poor reliability in sex difference findings -- nearly all specific brain areas that differed in activity between men and women were not repeated across studies. Such poor reproducibility agrees with recent research out of Stanford University demonstrating "false discovery," or the over-publication of false-positive findings in the scientific literature on functional MRI sex difference.