Incentives matter, some people believe, matter. Even (especially?) when it comes to reproduction. Social stigma matters, as does technological availability. Technological availability of abortion and contraceptives mean a disconnect between sex and having kids/pushing for a long term relationship. It sounds, especially after the numbers (unwed mothers) haven't really gotten better since welfare reform, like Charles Murray's thesis that money drove the unwed mother trend is not completely accurate.
It doesn't really make conservative critics wrong, just might mean that homo economicus isn't quite right when explaining humans. It's possible Murray's thesis works better with other outputs of an increased welfare state.
