Another interesting possibility that comes from juxtaposed reading.
Edin and Kefalas, in Promises I Can Keep, report that black welfare moms are less likely to be beaten by their boyfriends than their white and Hispanic counterparts because the black women are less likely to live with the fathers of their children.
Sarah Blaffer Hrdy, in Mothers and Others, reports that men become better fathers by sharing the hormones of pregnant women and newborns. The sustained physical contact makes men's prolactin levels rise (a nurturing hormone) and their testosterone levels to fall.
Perhaps one of the reasons that poor black men are so much less likely to be daily providers for their children is that they don't live with mothers and babies enough to get the nurturing hormonal changes that help make other men into providing fathers.