Number of older brothers and social orientation: New tests and the attraction/behavior distinction in two national probability samples.

The extent to which number of older brothers or "fraternal birth order" predicted the 2 main components that researchers have traditionally used to conceptualize sexual orientation-that is, psychological attraction and sexual behavior-was examined in 2 recent national probability samples. In both studies, fraternal birth order predicted same-sex attraction in men, with each additional older brother increasing the odds of homosexual attraction by an average of 38%. Results also indicated that the fraternal birth order/same-sex attraction relationship in men was independent of sexual behavior, including early same-sex behavior. No sibling characteristics predicted sexual orientation in women. Results suggest experience-based theories (e.g., early same-sex play) of the fraternal birth order effect in men are unlikely to be correct.