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 support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth order effect is probably the result of the mother's body's “memory” for both male births as well as male gestations (i.e., every instance of being pregnant with a male fetus). So even if a male fetus was not carried to term, he still increases the likelihood of homosexuality in any subsequent male fetuses gestated by the same mother. The existence of the fraternal birth order effect on male sexual orientation has been confirmed many times. One approach for establishing the ubiquity of this effect