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 sexual orientation via the fraternal birth order effect (for example, homosexual men with no older brothers) owe their homosexuality to other causes such as certain polymorphic genes (colloquially called "gay genes") or prenatal hormone levels during fetal development. Both direct and indirect evidence have demonstrated that the mechanism by which the fraternal birth order effect operates is prenatal in timing and biological in nature, rather than postnatal in timing or psychosocial in nature. Indirect evidence was found first when it was discovered that the fraternal birth order effect interacts with birth weight. Then, Bogaert (2006) produced direct evidence that the