Intergroup relations refers to interactions between individuals in different social groups, and to interactions taking place between the groups themselves collectively. It has long been a subject of research in social psychology, political psychology, and organizational behavior.

In 1966, Muzafer Sherif proposed a now-widely recognized definition of intergroup relations:

Whenever individuals belonging to one group interact, collectively or individually, with another group or its members in terms of their group identification, we have an instance of intergroup behavior.

Research on intergroup relations involves the study of many psychological phenomena related to intergroup processes including social identity, prejudice, group dynamics, and conformity among many others. Research in this area has been shaped by many notable figures and continues to provide empirical insights into modern social issues such as social inequality and discrimination.

While philosophers and thinkers have written about topics related to intergroup relations dating back to Aristotle's Politics, the psychological study of group attitudes and behavior began in the late 19th century. One of the earliest scientific publications on group processes is The Crowd: A Study of the Popular Mind, written in 1895 by French doctor and scientist Gustave Le Bon. Le Bon proposed that a group of individuals is different from the sum of its parts (often paraphrased as "a group is more than the sum of its parts"). This fundamental idea of crowd psychology states that when individuals form a group, this group behaves differently than each individual would normally act. Le Bon theorized that when individuals formed a group or crowd, there would emerge a new psychological construct which would be shaped by the group's "racial [collective] unconscious." Le Bon put forth three phenomena that explained crowd behavior: submergence (or anonymity), when individuals lose their sense of self and responsibility by joining a crowd, contagion, the tendency for individuals in a crowd to follow the beliefs and behaviors of the crowd, and suggestion, which refers to how the beliefs and behaviors of the crowd are shaped by a shared racial unconscious. Subsequent generations of intergroup relations and social influence researchers built from these foundational ideas and explored them through empirical studies.

The empirical study of intergroup relations, as well as the broader field of social psychology, grew tremendously in the years following World War II. The events of World War II, including the rise of Adolf Hitler and Fascism, the Holocaust, and the widespread use of propaganda, led many social scientists to study intergroup conflict, obedience, conformity, dehumanization, and other related phenomena. Social scientists were interested in understanding the behavior of the German population under Nazi rule, specifically how their attitudes were influenced by propaganda and how so many could obey orders to carry out or support the mass murder of Jews and other minority groups as part of the Holocaust. Several prominent social psychologists were directly affected by the Nazi's actions because of their Jewish faith, including Kurt Lewin, Fritz Heider, and Solomon Asch. Muzafer Sherif was briefly detained by the Turkish government in 1944 for his pro-communist and anti-fascist beliefs. These scientists would draw from these experiences and go on to make major theoretical contributions to intergroup relations research as well as the broader field of psychology.

The cognitive revolution in psychology in the 1950s and 60s led researchers to study how cognitive biases and heuristics influence beliefs and behavior. The resulting focus on cognitive processes and meaning-making represented a significant shift away from the mainstream behaviorist philosophy that shaped much of psychology research in the first half of the 20th century. During and after the cognitive revolution, intergroup relations researchers began to study cognitive biases, heuristics, and stereotypes and their influences on beliefs and behavior. Solomon Asch's studies on conformity in the 1950s were among the first experiments to explore how a cognitive process (the need to conform to the behavior of the group) could override individual preferences to directly influence behavior. Leon Festinger also focused on cognitive processes in developing cognitive dissonance theory, which Elliot Aronson and other researchers would later build upon to describe how individuals feel liking for a group they were initiated into but whose views they may not agree with.

The Civil Rights Movement of the 1950s and 60s led social scientists to study prejudice, discrimination, and collective action in the context of race in America. In 1952, the NAACP put out a call for social science research to further study these issues in light of the Brown v. Board of Education lawsuit. Gordon Allport's 1954 book The Nature of Prejudice provided the first theoretical framework for understanding and counteracting prejudice, and cemented prejudice as a central focus of social psychology. In his book, Allport proposed the contact hypothesis which states that interpersonal contact, under the correct conditions, can be an effective means of reducing prejudice, discrimination, and reliance on stereotypes. Subsequent generations of scientists built on and applied Allport's contact hypothesis to other domains of prejudice including sexism, homophobia, and ableism.