The most important effect of urbanization of the workforce in the second half was that it led to appalling living conditions in many major cities. Many urban workers, especially unskilled workers, could only afford to live in crowded, disease-ridden tenements that were also hotbeds of crime. These neighborhoods became symptomatic of the enormous divisions between wealth and poverty that accompanied industrial expansion in the United States in the second half of the nineteenth century. In New York, they also became the targets of both progressive reformers like Jacob Riis and shrewd political bosses like William "Boss" Tweed. Another effect was a change in the ethnic composition of major cities, as many of the thousands of people who poured into major cities were immigrants from a variety of European nations.