Long-Term Effects of Total Social Isolation Upon Behavior of Rhesus Monkeys

8 isolate monkeys were compared in a follow-up study to 8 sophisticated controls in brief cross-sectional pairings with 12 stimulus strangers: 4 adults, 4 age-mates, and 4 juveniles. The isolates were characterized by infantile disturbance, less environmental orality, more fear, more aggression, less sex, less play, and bizarre ritualistic movements. 12-mo. isolates were fearful and nonaggressive but threatened many attacks. 6-mo. isolates were fearful and physically aggressive. The 12-mo. isolates demonstrated practically no positive social behavior. Conclusions are: (a) 6 mo. of social isolation during the first year has negative effects on social behavior up to puberty, (b) abnormal aggression appears in 3-yr.-old 6-mo. isolates, and (c) 12 mo. of isolation suppress or delay this aggression.