The proximal causes of self-injurious behavior have been widely studied in captive primates; either social or nonsocial factors can trigger this type of behavior. Social factors include changes in group composition, stress, separation from the group, approaches by or aggression from members of other groups, conspecific male individuals nearby, separation from females, and removal from the group.[3] Social isolation, particularly disruptions of early mother-rearing experiences, is an important risk factor.[2] Studies have suggested that, although mother-reared rhesus macaques still exhibit some self-injurious behaviors,[4] nursery-reared rhesus macaques are much more likely to self-abuse than mother-reared ones.[2] Nonsocial factors include the presence of a small cut, a wound or irritant, cold weather, human contact, and frequent zoo visitors.[3] For example, a study has shown that zoo visitor density positively correlates with the number of gorillas banging on the barrier, and that low zoo visitor density caused gorillas to behave in a more relaxed way. Captive animals often cannot escape the attention and disruption caused by the general public, and the stress resulting from this lack of environmental control may lead to an increased rate of self-injurious behaviors.[5]