Mankiw, a libertarian economist intellectual who once worked for George (torture) Bush, points with interest to an article alleging a strong and possibly causal correlation beween "immigration, black wages, black employment rates and black incarcerations". This is something many have suspected, but evidence is hard to come by. The same, I believe, will be found to be true for other groups including white collar workers (maybe without the inarceration effect, they have more room to fall).
Mankiw is right that this will likely have a big influence on the immigration debate. It's not proof, but that's hard to come by in economics.