I'm guessing this is the study harsh is referencing. An interesting read:
Most important, our analysis of the changes in out-of-wedlock birth suggests that a return to the old system of shotgun marriage will not be brought about by significant reductions in welfare benefits, and possibly not even by very large reductions. With ***ual activity taking place early in relationships and with little social stigma enforcing the norm of shotgun marriage, fathers no longer have strong extrinsic reasons for marriage. Cuts in welfare therefore have little effect on the number of out-of-wedlock births, while reducing dollar-for-dollar the income of the poorest segment of the population. The initial goal of the welfare program was to see that the children in unfortunate families were adequately supported. The support of poor children not the alteration of the behavior of potential mothers should remain the major policy goal of welfare in the United States. This level of support must be tempered by equity between those who collect welfare and do not work and those who do work and also are paying taxes that, at least in part, go to pay for the less fortunate. In this regard a generous Earned Income Tax Credit serves two roles. Not only does it reward those who work, but by increasing the differential between the working poor and the nonworking poor, it allows greater benefits equitably to be paid to nonworking mothers.