Step up to the plate, nature-versus-nurture combatants. New research suggests that male homosexuality is determined in the womb—and that fraternal birth order may be the key. A study done by Anthony Bogaert of Brock University in Ontario shows that men with multiple biological elder brothers were more likely to be homosexual (about a 30 percent higher chance per brother) than those who merely grew up around a lot of older nonbiological male siblings. In fact, only the number of biological older brothers predicted homosexuality in men, regardless of their upbringing, which suggests that the triggering factor is prenatal. Just one more thing to fight about at the family Fourth of July picnic. —Martha Harbison