It may well be the biggest puzzle in evolutionary psychology: Why do humans have fewer kids when they get richer, when their extra resources allow them to support more? Robin blogs a new explanation from Ted Bergstrom:
Because of a genetic conflict of interest between mates, evolution could have shaped preferences so that "human females would fail to bear the optimal number of children in the absence of pressure of mates and kin." ... Thus men would desire more children and women fewer children than their own genetic interest dictates. Differences in birth rates across time and between cultures would occur as one side or the other gains increased leverage in this tug-of-war. In modern economics, women have increased influence in household decisions and, together with improved contraceptive technology, have gained greater control of their own fertility.
This doesn't sound right to me. Other men are usually surprised by my pro-natalist views. When couples I know disagree about family size, it's almost invariably the case that the woman wants more than the man.