ter mother. In contrast, dramatic differences emerged for the genetically highly reactive infants. Highly reactive infants cross-fostered to normal mothers exhibited deficits in early exploration and exaggerated behavioral and physiological responses to minor environmental perturbations. In adulthood they tended to drop and remain low in the dominance hierarchy (Suomi, 1991). Highly reactive infants cross-fostered to exceptionally nurturant females, in contrast, appeared to be behaviorally precocious. They left their mothers earlier, explored the environment more, and displayed less behavioral disturbance during weaning than both control (low-reactive) infants reared by either type of foster mother or highly reactive infants cross-fostered to normal mothers. In addition, when permanently separated from their foster mother and moved into larger groups, the highly reactive animals cross-fostered to nurturant mothers became adept at recruiting and retaining other group members as allies, and most became high dominant.
These primate studies are significant not only because they suggest an important role of parenting for modifying expression of genetically based temperamental differences but because they tie serotonergic dysfunction directly to behavioral attributes similar to those found in human offspring from abusive families. Specifically, monkeys raised without their mother (i.e., raised with peers) have difficulty moderating behavioral responses to rough-and-tumble play with peers, sometimes escalating those bouts into full-blown aggressive exchanges. Rearing with peers is also associated with a deficit of certain forms of prosocial behavior, such as less grooming among females. Peer-raised adolescent monkeys also show certain propensities for substance abuse, for example, requiring larger doses of the anesthetic ketamine to reach a state of sedation, and consistently consuming more alcohol and developing a greater tolerance for alcohol, compared to mother-raised monkeys. This pattern is predicted by their central nervous system 's serotonin turnover rates, and thus serotonin plays a key role in the normal regulation of these behaviors. In human studies, difficulty in moderating aggressive impulses, problems in developing and maintaining social relationships, and risk for substance abuse are among the outcomes most consistently seen in response to the family environment characteristics of hostility and conflict, deficient nurturing, and parental neglect (Repetti et al., in press).
The potential intergenerational transmission of these behavior patterns also warrants note. Monkeys raised by peers (rather than by their mothers) are significantly more likely to exhibit neglectful or abusive treatment of their own offspring (especially firstborns) compared with their mother-reared counterparts (Champoux et al., 1992; Suomi and Levine, 1998). Exposure to effective parenting reduces these behaviors in both animals (Suomi, 1987) and humans. For example, interventions that modify family