Chamove, A. S., Anderson, J. R., Nash, V. J. 1984. Social and environmental influences on self-aggression in monkeys. Primates 25, 319-325.

Simply separating the individuals off from the rest of the group ... led to a significant increase in SA [Self-Aggression]. .. It is clear that enriched environmental conditions reduced SA [self-aggression]. Allowing [group-housed] monkeys to forage through clean woodchips, even when there is no obvious incentive, substantially reduced the level of this abnormal behavior.