Scott Alexander examines recent studies on the Fraternal Birth Order Effect, finding evidence for a broader sibling birth order effect on homosexuality, though its exact nature and mechanism remain debated. Longer summary

Scott Alexander reviews recent studies on the Fraternal Birth Order Effect (FBOE), which suggests that men with more older brothers are more likely to be gay. He discusses three key studies: Frisch and Hviid's large-scale Danish study that found no clear FBOE, Vilsmeier et al.'s meta-analysis that questioned the statistical validity of previous FBOE claims, and Ablaza et al.'s extensive Dutch study that found a significant effect of older siblings (both brothers and sisters) on homosexuality. Scott concludes that while the FBOE hypothesis has been forced to evolve, the core idea that more older siblings increases the likelihood of homosexuality seems to stand, though the biological mechanism remains uncertain. Shorter summary