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

Recent evidence shows that the relation between birth order and intelligenc e is not the same in cross-sectional and within-family data. This simple em pirical observation invalidates the conclusions from hundreds of previous b irth 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 f amily. A theory proposed almost 25 years ago-the admixture hypothesis-effec tively accounts for these empirical patterns. In this article, the author d escribes why birth order is of such intense interest to both parents and re searchers (the birth order trap), discusses past birth order-intelligence p atterns, shows that the admixture hypothesis accounts for those patterns, a nd reframes the original argument to support future productive research eff orts.