Infant rhesus monkeys were reared from the first day of life under a variety of situations including total and partial isolation. During periods ranging from eighty days to as long as two years, they lived in individual wire cages, with or without access to their mothers, surrogate mothers, or other infants. 'Over all, it appears that the longer and the more complete the social deprivation, the more devastating are the behavioral effects.' Reversible, mild effects occurred after eighty days in isolation, whereas the effects of two years of total isolation were unreversed after two years of contact with other monkeys. The experimental