What is the premise of the Robbers Cave Experiment as noted in the essay collection, Unreality of Memory?

The Robbers Cave Experiment came about shortly after the publication of William Golding's Lord of the Flies. In the experiment, a group of psychologists studied the behavior of two selected groups of boys during a summer at Robbers Cave State Park. The psychologists wanted to test if hostility would arise between the groups once they introduced competition. They found that animosity arose before they even suggested competition. Gabbert uses the experiment as a throughway into her conversation about empathy and evil.