Species-typical frequencies of copulation during the menstrual cycle differ among common chimpanzee, orang-utan, and gorilla, but all three species exhibit a midcycle enhancement associated with estrus. Thus, in the natural habitat, chimpanzees mate for 10-14 days, orang-utans for 5-6 days, and gorillas for 2-3 days. In traditional laboratory pair-tests, however, conducted in a single cage with both animals freely accessible to each other, all three species of great apes copulate more frequently than the species-typical pattern. In all three species, moreover, the increased copulation appears to result from increased male sexual initiative (aggression), male dominance over females, and the inability of the female to avoid or escape from the male within the limited spatial conditions of the free-access test. This interpretation is supported by studies using restricted-access tests in which females control sexual access. These data suggest that male sexual aggression in our closest biological affiliates commonly occurs when females are rendered vulnerable to the male by the absence of the normal social constraints and spatial prerogatives typical of the natural habitat. The possible implications of this interpretation for a biological perspective on human sexual aggression are considered.