Review and theory of handedness, birth order, and homosexuality in men.

Research has repeatedly shown that older brothers increase the odds of homosexuality in later-born males. This phenomenon has been called the fraternal birth order effect. The most highly developed explanation of this phenomenon is the maternal immune hypothesis, which proposes that the fraternal birth order effect reflects the progressive immunisation of some mothers to male-specific antigens by each succeeding male foetus and the concomitantly increasing effects of anti-male antibodies on the sexual differentiation of the brain in each succeeding male foetus. Recent studies indicate that older brothers increase the odds of homosexuality in right-handed males but not in non-right-handed males. The present article explores how the maternal immune hypothesis might be extended or modified to account for the apparent interaction of older brothers and handedness. Two possibilities are considered: (1) non-right-handed foetuses are insensitive to the presence of maternal anti-male antibodies, and (2) mothers of non-right-handed foetuses do not produce anti-male antibodies.