Biology and sexual orientation INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation. A 2010 study, Garcia-Falgueras and Swaab asserted that "the fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge.
Fraternal birth order and male sexual orientation not non-biological older brothers predicted sexual orientation in men. Further, if rearing or social factors underlie the fraternal birth order effect, then the amount of time reared with older brothers, either biological or non-biological, should predict sexual orientation because rearing time determines the relative opportunity that older brothers have to affect their younger sibling's (postnatal) sociosexual development. If a prenatal factor underlies the fraternal birth order effect, then a postnatal factor such as rearing time with older siblings (be they biological or non-biological) should have no impact on the sexual orientation of younger male siblings. The study found that the