Wal-Mart's low prices may come with a hidden cost, according to a study in Social Science Quarterly. Looking across the 1990's, Penn State University economist Stephan Goetz and his colleagues compared poverty rates in US counties with new or existing Wal-Mart stores, to similar counties with no Wal-Mart. During this period, the national poverty rate fell.

STEPHAN GOETZ (Penn State University): What we found was that in communities with Wal-Mart stores, the drop was smaller. So in that sense, the presence of a Wal-Mart store impeded success on the poverty front.

It had a similar effect on per capita food stamp use. The study can’t prove that Wal-Mart caused these effects, but it adds to the debate over whether Wal-Marts are good for local communities.