Natural selection on human twinning

The frequency of twin deliveries varies among human populations1. The highest twinning rates for caucasian populations have been recorded on the archipelago of Åland and Åboland, in southwest Finland2,3, whereas multiple deliveries in adjacent mainland areas are historically rarer3. Using data from the pre-industrial era (1752-1850), we compare the lifetime reproductive success of mothers who produced twins with that of mothers of singletons in these archipelago and mainland sites. When we restrict our analysis to mothers with a genetic tendency to produce twins, we find that lifetime reproductive success is maximized by having twins on the archipelago, but by having singleton offspring on the mainland. This result is consistent with the difference in twinning rate being maintained by natural selection.