Commercial and industrial development, immigration from Europe, and migration from rural areas led to the rapid growth of U.S. cities from 1820 on. Urbanization stimulated economic expansion but also created social upheaval. Cultural divisions intensified in urban areas where Catholics and Protestants, workers and the well-to-do, immigrants, African Americans, and native-born whites lived side by side. The emergence of a middle class of shopkeepers, professionals, and clerks might have bridged these divides, but most middle-class Americans highlighted their distinctiveness from both the wealthy few at the top and the mass of workers and the poor at the bottom.