Here, analysis of MRIs of more than 1,400 human brains from four datasets reveals extensive overlap between the distributions of females and males for all gray matter, white matter, and connections assessed.
Moreover, analyses of internal consistency reveal that brains with features that are consistently at one end of the “maleness-femaleness” continuum are rare.
Notably, additional definitions of the “male-end” and “female-end” zones (50%, 20%, and 10%) similarly revealed a much higher prevalence of brains showing substantial variability compared with brains showing internal consistency (Table S2).
“male brain”), and consequently, of a sexually dimorphic view of human behavior, cognition, personality, attitudes, and other gender characteristics (3).
Joel (4, 5) has recently argued that the existence of sex/gender differences in the brain is not sufficient to conclude that human brains belong to two distinct categories.
Here we assess the degree of internal consistency in the human brain using data obtained from MRI, a method that allows the simultaneous assessment of multiple brain features in many individuals.
We used datasets obtained from several different imaging modalities and analyzed with different methods to ensure that our conclusion is not measure, analysis, or sample dependent.
Our results demonstrate that regardless of the cause of observed sex/gender differences in brain and behavior (nature or nurture), human brains cannot be categorized into two distinct classes: male brain/female brain.
Whereas a categorical difference in the genitals has always been acknowledged, the question of how far these categories extend into human biology is still not resolved.
Rather, most brains are comprised of unique “mosaics” of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males.
Our findings are robust across sample, age, type of MRI, and method of analysis.
Volumes falling in the “intermediate” zone are colored in white; volumes in the “male-end” and in the “female-end” zones are colored using continuous blue-white and pink-white scales, respectively. Each horizontal line represents the brain of one subject and each column represents a single brain region.
The number above each column corresponds to the region’s number in the AAL atlas and in Table S1. The number of regions at the “intermediate” zone is not depicted because the number of “male-end”, “intermediate”, and “female-end” features always adds up to the number of features included in the analysis (which is the highest value on the )].
Such an alignment may be predicted by the classic view of sexual differentiation of the brain, according to which masculinization and defeminization of the brain are under the sole influence of testosterone (9).