Muzafer Sherif's Robbers Cave Study
Sherif assigned boys to two competitive camps in Robbers Cave, Oklahoma. By the end of the first week, both camps knew of the other and the we/they talk started to develop.
With each competitive event, the teams saw the other team as competing unfairly.
Sherif tried to set things up so that the two teams would have noncompetitive contact, but this did not work. The only thing that did work in getting the two teams to cooperate was to have them jointly solve a problem.