Perhaps its because poor decision making runs in their family? People often grow up watching their parents, and therefore develop into them and take their bad habits. For instance, someone would drop out of school too soon, end up having a kid, get into drugs, etc. and have no way of paying for anything, so the kid grows up with the wrong values. Careless parents bring up careless children most times. And so its possible they're the ones having more children. More unprotected sex, more one night stands, more lovers, its a vicious cycle. Plus if you're poor, having kids probably feels right, its something thats their own, and might make them even feel important! I'm not sure though, just some thoughts.
Life's a bummer when you're a hummer. ~ Smashing Pumpkins