The two heterosexual groups had their environmental complexity reduced in terms of number of compartments for a 5-week period in the middle of the study. Rates of contact aggression were low throughout the study period. In the group with housing complexity reduced to one compartment, rates of noncontact aggression and affiliative behavior rose and remained high after complexity was restored. Rates of affiliative behavior increased while noncontact aggression fell in the group whose complexity was reduced to two compartments. The all-male monkey group displayed low rates of affiliative behavior throughout the study. Despite this group's poor prognosis for socialization, it is still intact more than 3 years after formation. On the spectrum of compartment availability, which ranged from one to four, two compartments fostered affiliative behavior while four decreased the likelihood of aggression over time. Even short- term changes in housing scenarios may have had long-lasting effects on the monkeys' patterns of aggression and affiliation. 1 table, 3 figures, and 26 references