A mosaic of sex-related structural changes in the human brain following exposure to real-life stress

Whereas sex differences in the brain's response to stress have been reported in both humans and animals, it is unknown whether they 'add up' consistently within individual brains. Here, we studied this question in a unique data set of magnetic resonance imaging (MRI) scans obtained before and after exposure to extreme real-life stress in the form of combative military service in 34 (15 women) young (18-19 years old) healthy soldiers. Across two data sets, one of regional volume and one of cortical thickness, only a few regions (seven and three, respectively) showed sex/gender-specific changes (i.e., the most common structural change in women and men was different). The number of internally consistent brains (a male-typical or a female-typical response in all regions) was not different from the number expected by chance nor from that observed in regions showing a sex-similar response, and was lower than the number of mosaic brains (at least one region with a male-typical response and one with a female-typical response). Although these findings do not reveal the source of sex/gender differences in response to stress and of within-brain variability in this response, they demonstrate that these differences do not consistently add up to create a female-typical and a male-typical neural response to stress.