A pair of small, affiliated studies that emerged from the study on the timing of maternal separation were conducted at the Oregon Health and Science University These studies investigated the role played by early experience in determining later behaviors, by examining the modulation of genetic and social influences on behavior in Japanese macaques.
One relevant early life experience in Japanese macaques is a decrease in maternal attention due to mating season or the birth of a new sibling. Factors that could influence individual differences in behavioral reaction to this experience include temperament, biological disposition (behavioral inhibition, growth hormone responsiveness) and social or environmental factors (dominance rank, social support, infant abuse). In humans, socio-economic status (SES) determines access to resources and influences vulnerability to some illnesses. One study examined whether the same may hold true for dominance rank and Japanese macaques.
The study examined whether temperament predicts response to the birth of a sibling. Only a small percentage of the monkeys studied were determined to be temperamentally inhibited. Among these monkeys, none displayed the expected responses to the birth of a sibling (decreases in social play, increases in the amount of time spent alone, agitation, and distress vocalizations, changes in levels of aggression). On the contrary, the amount of time spent receiving social contact increased compared to non-inhibited monkeys. (Inhibited monkeys received much less social contact than non-inhibited monkeys before the birth of a sibling, but received much more than non-inhibited monkeys after the birth of a sibling.) This occurs not as a result of the inhibited monkeys active seeking out of social contact, but perhaps as the result of subtle cues that lead to greater social contact from other monkeys.
The second study examined the relation between dominance rank and behavioral inhibition in predicting response to decreases in maternal attention during mating season. Behavioral inhibition was measured using four tests: a free play test, a remote-controlled car test, a human intruder test and a novel fruit test. There were no differences between high- and low-dominance individuals on the free play, car, or human intruder tests, but low-ranked monkeys did take longer to inspect the novel fruit. Other variables such as biological factors (csf monoamine metabolites, HPA activity) and early life events such as abuse may be involved.