We don’t know why, but there’s an interesting theory. When a mother has a series of boys, and becomes pregnant with another male, her body may have an immune system response that affects fetal development, including sexual differentiation of the brain. This could be a reaction of her female body reacting to a series of male embryos or it could be an evolutionary adaptation to ensure there is a feminized boy in the family if girls fail to appear.
A later study validated the results, and added the calculation that each older brother increases the chances the next son is homosexual by 33%. At an average rate of homosexual orientation of 4%, it would take 9-10 older brothers to reach a 50-50 chance of being gay.
- Gay Men: The more older brothers, the more likely a son is gay.
- Lesbians: No sibling effect observed.
- R. Blanchard and A.F. Bogaert, Proportion of homosexual men who owe their sexual orientation to fraternal birth order: an estimate based on two national probability samples, Am. J. Hum. Biol. 16 (2004), pp. 151–157.
- R. Blanchard, Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men, J. Theor. Biol. 230 (2004), pp. 173–187.