Split any group of humans into two groups, no matter how arbitrary the division, and watch them assume hostilities. Abuse, first verbal and then physical, torture, murder, extermination… Better, isolate any group of humans and watch them first split into two groups and then assume hostilities. Best, do nothing. Just watch any group of humans from a safe distance. The results are always the same, but you are mercifully left out of the picture, even as a dispassionate experimenter.