Rhesus macaques are supplying clues about how abusive behavior passes from parents to their children, with a new study suggesting that the link is environmental, not genetic.

Like humans, rhesus macaques that are abused as infants are more likely to become abusive parents, tossing, crushing, and biting their offspring. The monkey-human parallels piqued the interest of Dario Maestripieri, a behavioral biologist at the University of Chicago.

Maestripieri housed 14 female infants with adoptive mothers, some abusive and others not. Seventeen other monkeys stayed with their biological mothers, some of whom were abusive. He followed them for 5 years. The result: Being raised by an abusive mother, adoptive or not, made a difference in their behavior as adults. Nine of 16 infants cared for by abusive mothers became abusive parents, whereas none of those paired with nonabusive mothers did. The study appeared in the 27 June online Proceedings of the National Academy of Sciences.

Still, the study doesn't 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 the trait may be exacerbated by early environmental influences.