I've known this for years, but it's nice to see scientific evidence for it:
Willpower is a depletable resource
Essentially the article shows that if you make a hard decision now, then you're less capable of making a hard decision later. And if you're poor, every decision you make is hard. So pretty soon you're making a lot of bad decisions.
This isn't meant to excuse people for making bad decisions; it's meant to explain it. The difference is that an excuse means nothing changes, whereas an explanation implies you'll make changes so it won't happen again. We need to make some changes.
Oops. Sorry. I meant you need to make some changes. I already live in a society where the government doesn't view poverty as a genetic character flaw treatable only by death.