Hi! While delaying age at first birth might cause women to earn more, it is also possible that women earning more causes them to delay children. (reverse causality!) (there’s also the possibility of selection bias in their cohorts of married/cohabiting women and omitted variables related to women earning more. ) The original study is very careful to call this relationship an association and not infer the direction of causality. So we don’t know for sure that it’s delaying the children which improves lifetime labor income.