What causes birth order-intelligence patterns? The admixture hypothesis, revived.

Recent evidence shows that the relation between birth order and intelligence is not the same in cross-sectional and within-family data. This simple empirical observation invalidates the conclusions from hundreds of previous birth order studies that relied on cross-sectional data. Simultaneously, the empirical foundation disappears from underneath theories like dilution and the confluence model that use explanatory processes occurring within the family. A theory proposed almost 25 years ago--the admixture hypothesis--effectively accounts for these empirical patterns. In this article, the author describes why birth order is of such intense interest to both parents and researchers (the birth order trap), discusses past birth order-intelligence patterns, shows that the admixture hypothesis accounts for those patterns, and reframes the original argument to support future productive research efforts.