This is kinda interesting, so I'm sharing it even though it has little direct relevance to things we discuss here. The explanation is by a scientist who has done research on trying to understand brain differences in homosexuals. (He's gay.)
It's well known that females are more susceptible than males to several
auto-immune disorders, and this is true both in mice and humans. A newly
published study in Science (Markle JGM et al, Science 339, 1084-1088)
demonstrates that gut microbes are major players in generating this difference, and
that they do so by changing testosterone (T) levels.
Here are the main findings:
1. In mice susceptible to an autoimmune disorder (type 1 diabetes), females
develop the disorder at a much higher rate than males. This sex difference
is mediated by the sex difference in T levels.
2. Raising these mice without gut microbes lowers T levels in males but
raises T levels in females. In these microbe-free mice, the sex difference in
susceptibility to diabetes is eliminated.
3. Transfering gut microbes from adult males (but not adult females) to
immature females raises T levels in those females and protects them against
diabetes. This effect of microbial transfer is prevented by treatment with
So the study suggests that the sex-specific gut microbiome amplifies the
innate sex difference in T levels through a positive-feedback loop, at least
in these diabetes-prone mice. The authors discuss evidence suggesting that
the same thing may be true in healthy humans. I speculate that this
feedback loop might influence not only autoimmune diseases but also T-dependent
gendered psychological traits. If so, this would open the door to possible
new interpretations of many prior observations. For example, the
well-established influence of same-sex and opposite-sex older siblings on gendered
characteristics might come about because these older siblings are sources of
sex-specific gut microbes, rather than through psychological role-modeling.