The Robbers cave study was a large-scale experiment studying intergroup relations, carried out by Turkish social psychologist Muzafer Sherif and others in 1954. The experiment took place at Robbers Cave State Park in Oklahoma (USA), where 22 white 11-year-old boys were lead to believe they were attending a normal summer camp. The boys were randomly divided into two groups, and assigned to two living areas. The two groups were isolated from each other, and none of the participants were aware of there being a second group.
Each group first spent a week doing sports and activities with members of their group, and roles were formed within the groups. The groups were then put into situations in which they were to compete against the other group for prizes and resources, the goal being to create intergroup tension. This resulted in animosity between the groups, which included raids and fist-fighting.
In a third phase of the study, the experimenters attempted to reduce the level of intergroup-conflict, by increasing the contact between the two groups. This was initially unsuccessful (as was hypothesized by the experimenters), but was more successful once super-ordinate goals were introduced to the groups. This reduced inter-group tension, and the individuals from each group became friendlier toward each-other.
Muzafer Sherif, O. J. Harvey, B. Jack White, William R. Hood, Carolyn W. Sherif (1954), “Intergroup Conflict and Cooperation: The Robbers Cave Experiment“.