Intrinsic and extrinsic factors affect infant responses to maternal separation.

Responses of individuals to the loss of a primary attachment object may be quite variable. In humans, it has been suggested that only about 25% of bereavements result in substantial psychological or medical morbidity (Hamburg et al. 1975). In nonhuman primates, which are used to model responses to separation and loss, a similar estimate of about 25% has also been obtained (McKinney 1985). In addition, there are wide-ranging species differences in vulnerability with regard to the nature and severity of the response to maternal separation and/or loss. All of these findings suggest that there are important processes, intrinsic and/or extrinsic to the individual, that contribute to the probability that a loss will produce a major behavioral or physiological response. We have been systematically examining some of the factors that may account for a portion of this variability in two species of macaques (bonnet monkeys Macaca radiata; and pigtail monkeys, M. nemestrina).