The darker side. Although monkeys are generally amiable and cooperative, some engage in abusive parenting--a trait that is passed on to later generations through environmental influences.

Monkey See, Monkey Abuse

Children from abusive families are more likely to grow up to be abusive parents, but whether that's due to genetics or environmental influences has remained a mystery. Now, research in rhesus macaques suggests that abusive behavior is driven by early environment. If such findings apply to humans, early behavioral intervention may prove key to breaking the cycle of child abuse.

Research into how abusive behavior passes from one generation to the next has been inconclusive. One study in young boys followed through adulthood found a gene variant linked to disorder and aggression. But another in rats showed that pups frequently licked by their mothers grew up to be more nurturing to their own infants, even if the pups were not raised by their biological parents.

Dario Maestripieri, a behavioral biologist at the University of Chicago, wondered if a more straightforward study could be done on a close relative of humans. While rummaging through the records at the Yerkes National Primate Research Center in Atlanta, Georgia, Maestripieri noticed that, like humans, rhesus macaques abused as infants were likely to become abusive parents themselves-—tossing, crushing, and biting their infants. To untangle the issue of whether the pattern he observed was environmentally or genetically based, Maestripieri housed 14 female infants with adoptive abusive or non-abusive mothers.

Maestripieri followed the infants for five years and compared their behavior to that of monkeys raised by their biological mothers. He found that nine out of 16 infants raised by abusive mothers grew up to be abusive mothers themselves. However, none of the infants paired with 15 non-abusive mothers became abusive, regardless of whether they were raised by biological or adoptive mothers. "I was surprised how strong the evidence was in favor of experience [over genetics]," says Maestripieri, who publishes his findings online this week in the Proceedings of the National Academy of Sciences.

"This finding is particularly impressive because the species is close to humans," says Daphne Bugental, a psychologist at the University of California, Santa Barbara. Still, the study doesn't entirely rule out genetics, says Joseph McClay, a geneticist at the Virginia Institute for Psychiatric and Behavioral Genetics in Richmond. Some individuals may be genetically predisposed to become abusive, he says, and this may be exacerbated by early environmental influences.

Related sites
Yerkes National Primate Research Center
Dario Maestripieri Behavioral Biology Laboratory