Caring mothers may reduce the risk behavior of monkeys genetically prone to misbehave, according to research presented by psychologist Stephen Suomi, PhD, of the National Institute of Child Health and Human Development's Laboratory of Comparative Ethology, at the "What's killing our kids?" conference (see Research-based help). The findings have implications for human youngsters.

Work by Suomi and J.D. Higley, PhD, of the National Institute on Alcohol Abuse and Alcoholism, suggests that a small subset of male rhesus monkeys--5 percent to 10 percent--act like troublemaking teens: They're impulsive, aggressive and have trouble regulating their behavior. An equal number of females show similar behaviors, though not in such obvious ways. More recently and hopefully, controlled studies show these traits can be mitigated by good maternal nurturing, said Suomi, whose findings appear in the New York Academy of Sciences December Annals (Vol. 1008) and Annals Online (

The research also indicates problem monkeys have low levels of the serotonin metabolite 5-hydroxyindoleacetic acid, or 5-HIAA, linked to impulsivity and aggressiveness. The monkeys play more aggressively than others, often starting fights, and they take more dangerous risks, like jumping long distances from treetop to treetop, he said. These monkeys often end up social isolates, rejected by potential mates and often dying before they reach adolescence, he said. In the lab, they show difficulty in delaying gratification, and they drink far more alcohol than their peers during the monkey equivalent of "happy hour," Suomi said.

Knowing that 5-HIAA tends to be highly heritable, Suomi and colleagues have recently conducted controlled studies examining whether a nurturing or non-nurturing environment influences the types of gene characteristics seen in the "trouble monkeys." They're testing two types of monkeys: one with a short version or allele of the serotonin transporter gene 5-HTT--suspected to cause problems in serotonin production--and the other with a long version of the allele--related to normal serotonin levels.

Suomi's team sorted the two types of monkeys into two groups: ones raised with peers and the others raised by mothers.

The results strongly support the importance of nurture, said Suomi. Mother-raised monkeys of both allele types showed normal 5-HIAA levels, while peer-raised monkeys with short alleles showed much lower levels of the metabolite than either group of "normal" monkeys. The same was true with aggression, with short-allele monkeys raised by mothers showing the same low level of aggression as normal monkeys in either group, and peer-raised short-allele monkeys showing high levels of aggression. Behaviorally, the peer-raised monkeys acted far more anxious, clingy and fearful, no matter what their allele type, he added.

The findings suggest a way to break the cycle of difficult behavior.

"The attachment style of monkey mothers is typically 'copied' by her daughters when they grow up and become mothers themselves," Suomi said. So if short-allele monkeys develop secure attachment--even with "foster mothers"--they're likely to become good nurturers themselves, he said.