I found this, from Julian Sanchez's review of two books on marriage and motherhood, pretty provocative:
In the 1980s books like Charles Murray's Losing Ground argued that poor women would respond to economic incentives such as welfare benefits for single mothers by bearing more children out of wedlock. But if the model was correct, the margin at which those decisions occur may have been misidentified: For many, it seems, childbearing is the default in the absence of some potent economic incentive not to have a child—some prospect for personal fulfillment other than through motherhood.
I think that means the model wasn't correct, save in its basic point of agreement with the discipline of economics, which believes human beings respond to incentives. Murray argued that welfare benefits were causing rampant childbirth. That was wrong. So far as he had a model, it was all about the margins.
That aside, this is an interesting point, and one that's fairly intuitive if you think about it. Income has long been inversely correlated with fertility. As folks amass more lifestyle choices and economic freedom, the necessity of children -- which is to say, they take care of you in old age, can help with the home, etc -- decreases while the opportunity costs increase. It's also why fertility is lower in urban centers -- giving up your nights and weekends means more. Indeed, children become a question of increased or decreased happiness. Those most content with their current lifestyles are least likely to take a risk on new entrants; those least content are most likely to welcome the change. If you're worried about young black women having children, the answer would seem to lie not in abstinence education (they're choosing to keep the children, after all) but in economic empowerment.
