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The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men".
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The effect becomes stronger with each additional male pregnancy, with odds of the next son being gay increasing by 38–48%. 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. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. 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. 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. Ray Blanchard and Anthony Bogaert first identified the association in the 1990s and named it the fraternal birth order effect.
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Fraternal birth order 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.