In-group favoritism, sometimes known as in-group–out-group bias, in-group bias, intergroup bias, or in-group preference, is a pattern of favoring members of one's in-group over out-group members. This can be expressed in evaluation of others, in allocation of resources, and in many other ways. This effect has been researched by many psychologists and linked to many theories related to group conflict and prejudice. The phenomenon is primarily viewed from a social psychology standpoint. Studies have shown that in-group favoritism arises as a result of the formation of cultural groups. These cultural groups can be divided based on seemingly trivial observable traits, but with time, populations grow to associate certain traits with certain behavior, increasing covariation. This then incentivizes in-group bias. Two prominent theoretical approaches to the phenomenon of in-group favoritism are realistic conflict theory and social identity theory. Realistic conflict theory proposes that intergroup competition, and sometimes intergroup conflict, arises when two groups have opposing claims to scarce resources. In contrast, social identity theory posits a psychological drive for positively distinct social identities as the general root cause of in-group favoring behavior. Origins of the research tradition In 1906, the sociologist William Sumner posited that humans are a species that join together in groups by their very nature. However, he also maintained that humans had an innate tendency to favor their own group over others, proclaiming how "each group nourishes its own pride and vanity, boasts itself superior, exists in its own divinities, and looks with contempt on outsiders". This is seen on the group level with ingroup–outgroup bias. When experienced in larger groups such as tribes, ethnic groups, or nations, it is referred to as ethnocentrism. Explanations Competition Realistic conflict theory (or realistic group conflict) posits that competition between groups for resources is the cause of in-group bias and the corresponding negative treatment of members of the out-group. Muzafer Sherif's Robbers Cave Experiment is the most widely known demonstration of realistic conflict theory. In the experiment, 22 eleven-year-old boys with similar backgrounds were studied in a mock summer camp situation, with researchers posing as camp personnel. The boys were divided into two equal groups and encouraged to bond, with the aim of fostering an in-group mentality. The researchers then introduced a series of competitive activities which pitted groups against each other for a valuable prize. Hostility and out-group negativity ensued. Lastly, researchers attempted to reverse the hostility by engaging the