Fraternal Birth Order Effect
Fraternal birth order effect is the theorized effect that a male child's sexual orientation is influenced by the number of older male siblings they have.
Also known as the "older brother effect," it has been observed that the more older brothers a male child has, the greater the probability of his having a homosexual orientation. Research shows that this prediction holds true for approximately 15% of the male homosexual population. Conversely, there does not seem to a correlation for female birth order.