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. 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.
