According to primatologist SaraH Blaffer Hrdy(1981),observations of a variety of primate species have changed the view of mothers as primary caregivers and fathers as only marginally involved with offspring. Observations of a variety of species showed that male primates' involvement with infants varies from being their primary caretaker to showing benign disinterest, with researchers observing only a little dangerous aggression from adult males. Individual differences appeared in the amount of male involvement with infants within their species; some males formed numerous relationships with infants and young primates, even those they had not fathered, whereas other males of the same species were less involved with the young (Silverstein, 1993). the evidence from studies of other primate species shows the behavioral flexibility of caregiving and does not support the concept of maternal instinct (and paternal disinterest). the behavior of primates offers no evidence for invariance in biological terms of nurturing among females and lack of nurturing among males.