Global immigration during the twentieth century was particularly rapid during the first half of the century. Due to the emergence of World War I and World War II, European immigrants came to the United States in vast quantities. Particularly following the end of World War I, Americans labeled European immigrants as dangerous to American culture. In 1924, the United States Congress passed the Immigration Act of 1924, which placed strict quotas on immigrants entering the United States.
From the 1960s to 1990s, the stigma labeling immigrants as "job takers" and "criminals" subsided, and instead Americans began to consider immigrants as benefactors to the American economy, culture, and political system. Although the negative labels that immigrants were given, during the first half of the twentieth century, influenced their actions in society and self-perceptions (known as labeling theory in sociology), immigrants now began to assimilate more easily into society and form strong social networks that contributed to their acquisition of social capital—the "information, knowledge of people or things, and connections that help individuals enter, gain power in, or otherwise leverage social networks".
Sociologists have studied immigration closely in the twenty-first century. Compared to the majority of European immigrants entering the United States during the early twentieth century, the twenty-first century witnessed the arrival of immigrants predominately from Asia, the Middle East, and Latin America. From 2000 to 2001, sociologists have paid particular attention to the costs and benefits of the new diversified immigration population on American institutions, culture, economic functions, and national security. After the attacks on the World Trade Center on September 11, 2001, sociologists closely analyzed the symbolism of increased anti-immigration rhetoric, directed at Middle Eastern immigrants, stemming from Americans. Structural functionalists theorists have also studied the effects of mass migration—resulting from wars, economic insecurity, and terrorism—on the social institutions of host nations, international law, and assimilation rates. Additionally, sociologists exercising social conflict theory have analyzed, in particular, labor market conflicts resulting from increased marketplace competition due to the rise in competition between immigrants and native workers for jobs and social mobility. Because rates of global immigration are continuing to increase, the field of sociology has a particular interest in monitoring twenty-first century immigration as it relates to the foundational theories of symbolic interactionism, social conflict, and structural functionalism.
In immigration studies, social scientists assign distinct definitions to various immigrant generations. In sociology, the word "generation" is used as a "measure of distance from the 'old country'". This means that people who move to, in the case of immigrants migrating to the United States, the United States from another society, as adults, are considered "first generation" immigrants, their American-born children as "second generation" immigrants, and their children in turn as "third generation" immigrants.
During the mid-twentieth century in the United States, the first, second, and third generations of immigrants displayed distinct characteristics. Second generation immigrants, due to being the parents of immigrants and witnessing the historical events unfolding in the mid-twentieth century, created a distinct social identity both in themselves and in popular American culture. In the late 1930s, American historian Marcus Lee Hansen observed "distinct differences in attitudes toward ethnic identity between the second generation and their third-generation children". Whereas the second generation was anxious to assimilate, the third generation was sentimentally invested in "ethnicity", which is defined by sociologist Dalton Conley as "one's ethnic quality or affiliation". However, twenty-first century immigrants now assimilate more than their twentieth-century predecessors, most notably in the transition to using English—among immigrants who move to the United States—as the primary language for communication.
While contemporary immigrant generations share common ethnic backgrounds and cultures, there are differences in the level of social mobility, economic achievement, educational attainment, and familial relations among the members of those generations.