In total, our results are consistent with the likelihood of considerable genetic variation in the expression of male gender nonconformity, and possibly even in its causes

Familiality of Gender Nonconformity Among Homosexual Men. J. Michael Bailey. Archives of Sexual Behavior, January 16 2020. https://link.springer.com/article/10.1007/s10508-020-01626-w

Abstract: We examined whether recalled childhood gender nonconformity and self-reported adult gender nonconformity is familial, using data from 1154 families selected for having at least two homosexual brothers. Specifically, we examined the extent to which homosexual men’s variation in gender nonconformity runs in families by examining pairs of genetic brothers who were both homosexual (N = 672–697 full sibling concordant pairs). We also examined similarity between homosexual and heterosexual brothers (N = 79–82 full sibling discordant pairs). Consistent with past studies, concordant pairs yielded modest positive correlations consistent with moderate genetic and/or familial environmental effects on gender nonconformity. Unlike results of smaller past studies, discordant pairs also yielded modest positive, though nonsignificant, correlations. Our results support the feasibility of supplementing genetic studies of male sexual orientation with analyses of gender nonconformity variation.

Keywords: Sexual orientation Homosexuality Gender nonconformity Familiality Genetics