Fraternal birth order and male sexual orientation

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Fraternal birth order, also known as the older brother effect, has been correlated with male sexual orientation, with a significant volume of research finding that the more older brothers a male has from the same mother, the greater the probability he will have a homosexual orientation. Ray Blanchard and Anthony Bogaert first identified the association in the 1990s and named it the fraternal birth order effect. Scientists have attributed the effect to a prenatal biological mechanism, since the association is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. The mechanism is thought to be a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. Biochemical evidence for this hypothesis was identified in 2017, finding mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons.^[1][2]

The effect becomes stronger with each additional male pregnancy, with odds of the next son being gay increasing by 38–48%. This does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the first born son, to 3% for the second, 5% for the third and so on.^[1][3] It has generally been thought that this maternal response would not apply to first born gay sons and that they may owe their orientation to other mechanisms.^[2] However, the 2017 lab study found that mothers with no sons also had antibodies to male cells, which may be caused by common early miscarriages or abortion, and thus induce the effect for a first live born son.^[2][4]