Believing it's most poor people's fault they're poor is the null hypothesis. We generally believe people's decisions matter. It's especially the null hypothesis for me, because this is what I grew up seeing with my own eyes, in my own family. It's even more so because the people I grew up with would have also said "it's not my fault I'm poor," even when I could clearly see them make bad decision after bad decision. Then there's the additional circumstantial evidence of seeing who invests in poor financial vehicles like lottery tickets and playing slot machines.

Yes, I'd love to know these are all exceptions to the rule-- it would make me feel much better about the politics I'd like to believe, but the other camp is going to have to provide the data to back up their alternative hypothesis.