within the enclave to be exploitative, and the third condition requires data showing employment within the enclave leads to a 'dead end' and offers no chance of upward mobility. They acknowledge that the fulfillment of these three requirements is difficult as there is little data available to accurately test them. Jennifer Lee adds to the discussion noting the particular niches and types of business immigrant groups enter. She notes that it is most common for immigrants to participate in long hours of physically demanding work in the retail industry. The retail market is a viable option due to the relatively low startup costs and knowledge of the host country's language required. Different niches have different levels of communication, for example the retail and self-service niche, (fruit and vegetable markets, take out restaurants) typically require the lowest level of customer interaction and communication. Lee notes the embeddedness of ethnic enclaves and brings the thought that such practices are good for those within the enclave but harmful to certain groups outside them. She also notes the adverse effects patterns of ethnic embeddedness can have on surrounding ethnic groups by noting the difficulty other groups face in joining the network. She argues that this type of retail niche domination can have positive consequences for co-ethnics, as Portes and Wilson believe, however can also have negative effects on surrounding ethnic groups who face exclusion due solely to their ethnic dissimilarity from the network. Ethnic enclaves in the United States Immigration to the United States has occurred in waves that demonstrate the predominance of certain sets of ethnic minorities. As immigrants tended to cluster in certain cities and states, separate waves were responsible for the establishment of ethnic enclaves in separate physical spaces. The best-known ethnic enclaves in American cities began to appear with the arrival of large numbers of Irish immigrants during the first third of the nineteenth century and continued forming throughout that century and the twentieth as successive waves of immigrants arrived in the United States. In the early 20th century, immigrants chose to live in enclave neighborhoods because of language barriers and cost-of-living benefits. Sociologists Robert Park and Edward Burgess integrated a model that studied these patterns in the 1920s. This model showcased how immigrants who arrived in the U.S. in the early 20th century were drawn to urban enclave neighborhoods as they opened up opportunities for social networking and employment.