How Are The Gay Younger Brothers Doing?

The fraternal birth order effect is best understood as tied to older siblings in general, with older brothers exerting a slightly stronger effect than older sisters. A Dutch study of nine million people by Ablaza, Kabatek, and Perales found that replacing an older sister with an older brother was associated with a 12.5% increase in the probability of entering a same-sex union. By contrast, a 2006 Danish study by Frisch and Hviid of about two million people found no clear birth order effect, and Vilsmeier et al.'s meta-analysis reported confidence intervals that included zero.