The full impact of immigration on American society is obscured in policy and academic analyses that focus on the short-term problems of immigrant adjustment. With a longer-term perspective, which includes the socioeconomic roles of the children of immigrants, immigration appears as one of the defining characteristics of twentieth-century America. Major waves of immigration create population diversity with new languages and cultures, but over time, while immigrants and their descendants become more "American," the character of American society and culture is transformed. In the early decades of the twentieth century, immigrants and their children were the majority of the workforce in many of the largest industrial cities; in recent decades, the arrival of immigrants and their families has slowed the demographic and economic decline of some American cities. The presence of immigrants probably creates as many jobs for native-born workers as are lost through displacement. Immigrants and their children played an important role in twentieth-century American politics and were influential in the development of American popular culture during the middle decades of the twentieth century. Intermarriage between the descendants of immigrants and old-stock Americans fosters a national identity based on civic participation rather than ancestry.