Sexuality
The fraternal birth order effect is the name given to the observation that the more older brothers a man has, the greater the probability is that he will have a homosexual orientation. The fraternal birth order effect is the strongest known predictor of sexual orientation, with each older brother increasing a man's odds of being homosexual by approximately 33%. Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect of the number of older sisters.
In the book Homosexuality, Birth Order, and Evolution: Toward an Equilibrium Reproductive Economics of Homosexuality, Edward M. Miller suggests that the birth order effect on homosexuality may be a by-product of an evolved mechanism that shifts personality away from heterosexuality in laterborn sons. This would have the consequence of reducing the probability of these sons engaging in unproductive competition with each other. Evolution may have favored biological mechanisms prompting human parents to exert affirmative pressure toward heterosexual behavior in earlier-born children: As more children in a family survive infancy and early childhood, the continued existence of the parents' gene line becomes more assured (cf. the pressure on newly-wed European aristocrats, especially young brides, to produce "an heir and a spare"), and the benefits of encouraging heterosexuality weigh less strongly against the risk of psychological damage that a strongly heteronormative environment poses to a child predisposed toward homosexuality.
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