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.