labor mobility have argued that complementary immigration, which deviates from the outcome predicted by the above model, is a common phenomena. Illegal immigration in the United States provides one useful example of this critique. The above model would predict that illegal immigration in the United States would cause the wages of domestic unskilled workers to fall. Illegal immigrants would move to the United States seeking higher wages than in their home countries. The influx of foreign laborers willing to work for wages below the pre-immigration market price in the United States would cause wages for U.S. domestic unskilled workers to fall and cause U.S. domestic unskilled workers to lose their jobs to the new foreign workers. However, there is both theoretical and empirical evidence that this may not always be the case. The idea behind this critique is that immigrant unskilled labor differs in certain fundamental qualities from the domestic unskilled labor force. The central difference may be immigrants willingness to work in particular occupations that are shunned by domestic unskilled workers. The occupations that foreign unskilled workers fall into may in some cases actually be complements to the occupations of domestic unskilled workers, and, therefore, the work of the foreign unskilled workers could raise the marginal productivity of domestic laborers, rather than reduce their wages and employment rates as the traditional model predicts. A great deal of empirical research has been done to assess the impact of certain groups of foreign workers. Most of these empirical studies attempt to measure the impact of immigration by looking at a cross-section of cities or regions in a country and using variations in immigrant or foreign worker density to determine how immigrants effect a particular variable of interest. Wages of domestic and foreign workers are obviously a common variable of interest. There are problems with this approach, however. In open economies with free trade, factor price equalization is likely to occur, so even if immigrants affect native national wages, the uneven distribution of immigrants across the nation may not result in long run cross-sectional wage differences. In the short run though, wage differences could indeed be present. Another issue is that immigrants may selectively move to cities that are experiencing high growth and an increase in wages. It has been suggested, however, that this issue can be resolved if wage data is examined over a period of time. In Friedburg and Hunt's