Fraternal birth order and male sexual orientation

The fraternal birth order effect is an observation that the more older brothers a male has from the same mother, the greater probability he will have a homosexual orientation. It was identified by Ray Blanchard and Anthony Bogaert in the 1990s, who attributed the effect to a prenatal biological mechanism, as the association is only present in men with older biological brothers, and not present among men with older step-brothers.

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