this is definitely a discussion worth getting into. The same statistics happen where i am from (Kenya- E. Africa), where poorer people tend to have more kids, and a whole cornucopia of things. Its not just an american thing, in fact i think if you look at most third world countries poverty for some reason results in high teenage pregnancies, hiv/aids (hence thabo mbeki's argument that poverty is a contributing factor to the high hiv/aids cases in africa). and i am not saying poor people tend to have sex more, or such bullshit arguments that dont wash, but i think it might have to do with the environment that poverty creates (y'all know what i am talking about, nihilism, drugs, blablabla). i think it correlates to self worth, or self esteem, or lack of hope that comes with being poor., or lack of knowledge that comes out of not having access to basic needs let alone supposed luxuries like information.
n.b: just because a few people have managed to move out of the ghettos does not make it an easy step. that kind of thinking is the reason why the black bourgeoisie refuses to help and support blacks in the poor neighbourhoods (the whole if i could make it anyone can mentality that does not leave space for us to help uplift our less successful brothers. and y'all know its time we started doing something because the state of the black world is progressively getting worse).