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 studies have demonstrated that the fraternal birth order effect does not operate through social or postnatal (e.g., learning or rearing or environmental) mechanisms; rather it is biological in nature and operates prenatally. It is also known that the fraternal birth order effect on male sexual orientation is unaffected by family dynamics, living with older brothers or learning. Fraternal birth order and male sexual orientation A correlation between fraternal birth order and male sexual orientation has been suggested by research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. In several studies, the observation is