"We estimate the effects of Wal-Mart stores on county-level employment and earnings, accounting for endogeneity of the location and timing of Wal-Mart openings that most likely biases the evidence against finding adverse effects of Wal-Mart stores. We address the endogeneity problem using a natural instrumental variable that arises from the geographic and time pattern of the opening of Wal-Mart stores, which slowly spread out from the first stores in Arkansas. In the retail sector, on average, Wal-Mart stores reduce employment by two to four percent. There is some evidence that payrolls per worker also decline, by about 3.5 percent, but this conclusion is less robust. Either way, though, retail earnings fall. Overall, there is some evidence that Wal-Mart stores increase total employment on the order of two percent, although not all of the evidence supports this conclusion. There is stronger evidence that total payrolls per person decline, by about five percent in the aggregate, implying that residents of local labor markets earn less following the opening of Wal-Mart stores. And in the South, where Wal-Mart stores are most prevalent and have been open the longest, the evidence indicates that Wal-Mart reduces retail employment, total employment, and total payrolls per person"--National Bureau of Economic Research web site.