Sex beyond the genitalia: The human brain mosaic.
Level 4 - case-series / case-control
Cross-sectional observational analysis of neuroimaging and behavioral datasets (by design analogy, not clinical CEBM).
PubMed 26621705 · doi:10.1073/pnas.1509654112
What was done
Researchers evaluated whether human brains fall into sexually dimorphic categories (distinct female vs. male brains) by assessing structural overlap and internal consistency along a maleness-femaleness continuum. They analyzed structural MRI data (gray matter, white matter, and connectivity) from more than 1,400 brains across four datasets. They also assessed internal consistency in personality traits, attitudes, interests, and behaviors in more than 5,500 individuals.
What was found
The authors report extensive overlap between females and males across all assessed gray matter, white matter, and connection distributions. Brains with internally consistent features at one end of the maleness-femaleness continuum were rare; most brains were unique mosaics of features common to females, males, or both. Analysis of personality, attitudes, interests, and behaviors in >5,500 individuals similarly showed that internal consistency is extremely rare. No specific numerical values, percentages, or effect sizes are reported in the abstract.
Why it matters
This paper challenges the binary model of distinct 'male' and 'female' brains, showing that human brain anatomy and behavior generally form heterogeneous individual mosaics rather than categorical dimorphisms.
Limits
The abstract provides no numerical data, effect sizes, or operational definitions for how features were classified along the continuum. The cross-sectional design cannot establish the relative contributions of genetic sex versus environmental or experiential neuroplasticity to the observed mosaics.
Cited by
- contradicts Women naturally have more developed right cerebral hemispheres, whereas men have more developed left cerebral hemispheres.