English Dictionary

fraternal birth order effect

noun

The scientific observation that a man's statistical probability of being homosexual increases with the number of older brothers he has.

The fraternal birth order effect is a phenomenon in which the more older brothers a man has, the greater the likelihood that he will be gay.

意味（1）

The scientific observation that a man's statistical probability of being homosexual increases with the number of older brothers he has.

Thus the fraternal birth order effect is specific to male sexual orientation, and does not affect female sexual orientation.