When a person identifies themselves with a group, they perceive themselves and the group members as different from other groups. A Packers fan may work with a Broncos fan, where both of them hold the same position, both love pizza, and both live in the same community. But the Packers fan will view the Bronco fan as being significantly different from himself. If another coworker is also a Packers fan, but lived on the other side of town and hated pizza, he would still be viewed as more similar to the Packers fan then the Broncos fan. A partial explanation for such in-group favoritism is provided by the social identity theory. Categorization can also create a basis for ethnocentrism, or the belief that one’s own group is superior to other groups. People will put themselves into the "in-group" and all other people are in the "out-group." This leads in-group members to favor their group over the out-group and is called in-group, out-group bias (Devine, 1996). Sports fans of any kind favor their teams fans over the other team’s fans. The fans of the opposing team are not seen as friends or equals, they are the enemy. During a game, fans of each team do not socialize with the opposing fans. The seating arrangement during games is usually segregated, with, for example, the Packers fans on one side of the stadium and Broncos fans on the other. This separation contributes to favoritism of the in-group over the out-group.

    Intergroup bias is the belief that one’s own group is better than all other groups. Even when categorization is arbitrary and unrelated to psychological characteristics, intergroup bias is still established. The mere categorization of people into groups can lead to increased attraction of in-group members and devaluation of out-group members. This is called the minimal group paradigm and was pioneered by Henri Taifel. In a study done by Howard and Rothbart (1980), subjects were randomly assigned to one of two different groups in which they did not know anyone. They were then presented with a mix of positive and negative information about their group, the in-group, and the other group, the out-group. Subjects had more favorable expectations about in-group members and more negative expectations about out-group members, even though they did not know the members of either group personally. This demonstrates the minimal group paradigm, which shows why something as meaningless as being a sports fan can lead to such discrimination against fans of opposing teams. In-group members tend to behave in more prosocial ways and be more cooperative toward other in-group members than to out-group members. Sports fans obviously are more prosocial and cooperative toward the in-group members then the out-group. Rarely will fights occur between two Packers fans, but it is common to see fights between Packers and Bronco fans.

    Recent studies have shown that even the language used to refer to in-group and out-group status helps create intergroup bias. By using pronouns such as "we" or "they" and "us" and "them," in-group, out-group bias can be emphasized. Packers fans who say "We are going to win today, and they are going to loose" are viewing themselves as part of the team and demonstrating intergroup bias. The mere categorization of people into groups leads people to the processing of information about in-groups and out-groups in ways that cause intergroup differences to appear legitimate and justified (Hamilton & Trolier, 1986). A Packers fan sees all of the other members of his in-group as being similar only in positive traits. Differences between them are accentuated when they are favorable to the in-group, but are minimized if they are favorable to the out-group (Brewer & Kramer, 1985). This leads to a more positive opinion of the in-group over the out-group. Sports fans of all kinds will continue to see their in-group as being superior over the out-group, whether they know the members personally or not.