Immigrants may often do types of work that natives are largely unwilling to do, contributing to greater economic prosperity for the economy as a whole: for instance, Mexican migrant workers taking up manual farm work in the United States has close to zero effect on native employment in that occupation, which means that the effect of Mexican workers on U.S. employment outside farm work was therefore most likely positive, since they raised overall economic productivity. Research indicates that immigrants are more likely to work in risky jobs than U.S.-born workers, partly due to differences in average characteristics, such as immigrants' lower English language ability and educational attainment. Further, some studies indicate that higher ethnic concentration in metropolitan areas is positively related to the probability of self-employment of immigrants.