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Groups containing 5 or 6 infant rhesus monkeys and their mothers were formed when the infants were approximately 3 months old by random assignment from an available pool. There were 33 males and 38 females assigned to groups containing infants of both sexes (heterosexual groups); 15 males and 15 females were assigned to groups containing only infants of the same sex (isosexual groups). The social behavior of subjects in each group was observed and recorded during six 50-day periods from 3 months of age to 3 1/2 years of age. Two sexually dimorphic patterns, mounting and presenting, were recorded for each subject as indices of protosexual (i.e., patterns eventually used in adult mating behavior) development. In addition, rough play, a dimorphic response that is not protosexual, was also recorded. Both males and females in the isosexual condition were characterized by a partial inversion of the manifestation of protosexual behavior. Isosexual males showed statistically less foot-clasp mounting and more presenting than heterosexual males. Conversely, isosexually reared females showed statistically more mounting and less presenting than heterosexual females. The effect of rearing animals in same-sex groups was greater on heterotypical than on homotypical protosexual behavior. Among isosexual males, presenting responses deviated from the heterosexual male standard to a greater extent than did mounting. Among isosexual females, mounting behavior deviated from the heterosexual female standard more than presenting. Results suggested that presenting behavior by males was more easily augmented by isosexual rearing conditions than was mounting by females. At no time during the experiments did isosexual females mount at frequencies or at group proportions that were indistinguishable from those of heterosexually reared males. In contrast, males reared isosexually showed average frequencies of presenting that equaled or exceeded means for females reared heterosexually. While mounting and presenting were both modified by same-sex rearing, rough-and-tumble-play frequencies were not influenced markedly in either sex. However, isosexual males did show statistically higher frequencies of rough play than heterosexual males during the final period of observation (3 to 3 1/2 years of age), and isosexual females showed less rough play than heterosexually reared females during the first year of life. Results support the conclusion that isosexual conditions can have a selective effect on the developmental expression of protosexual responses without altering the probability of display of other sexually dimorphic social behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

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