No, the root reasons for people being poor have not changed. I'd agree there is more scope for people to 'not stay poor', but equally there are also greater fiscal pressures on all people in society now than there were, say, 800 yrs ago.
Looking for evidence would be a good way of breaking that nightmare cycle. It gets tricky with humans though, because we behave very oddly for animals which can make prolonged study a bit difficult. Also we tend to have 'motives' for doing things, rather than doing them simply as responses to stimuli, and these can be near-impossible to detect or quantify.
R_E is not speculating - he's drawn that explanation from evolutionary biology and behavourial ecology. I'm sure he could find plenty of studies and papers supporting it.