What turns a mother into a child abuser?

The answer in humans remains unclear, but new research with monkeys suggests it may have more to do with experience than genetics.

More than half of female rhesus macaque monkeys who were abused during the first month of life became abusive mothers themselves, whether they were raised by their biological or foster mothers.

"This strongly suggests that it [child abuse] is an issue of early experience," said study author Dario Maestripieri, an associate professor of comparative human development at the University of Chicago. "We can't just blame bad genes."

The study findings appear in the July 5 online issue of the Proceedings of the National Academy of Sciences.

Just like humans, monkeys are no strangers to child abuse. Maestripieri has found that about 5 percent to 10 percent of female monkeys abuse their offspring. Among other things, the mothers drag, step on, bite and pin down their infants, he said.

"At some point, they just treat them like objects," Maestripieri said, adding that occasionally, the monkeys even kill their babies.

"The mothers who are abusive tend to be very consistent. Every year, they have a new infant and repeat the abuse," he said.

If you're wondering about male monkeys, they're not part of the picture: they're absentee fathers, uninvolved in child-rearing.

In the new study, Maestripieri designed an experiment to determine if the female monkeys learned abusive behavior from their own mothers. Researchers at the Yerkes National Primate Research Center in Georgia took monkey infants away from their biological mothers and gave them to foster mothers to see how they developed.

Foster monkey mothers tend to accept new babies under special circumstances, Maestripieri said. "An infant has to be taken away from its mother as quickly as possible after birth, within 24 to 48 hours. And the mother has to be given another infant of the same age and sex," he said.

Nine of 16 monkeys who were abused as babies went on to become abusers themselves, regardless of whether they were raised by their natural or foster mothers. However, another 16 who were raised by non-abusive mothers didn't become abusers, even if their biological mothers were abusive.

This research might offer insights into causes of child abuse in humans, Maestripieri said.

However, Joan Kaufman, a child abuse specialist and assistant professor of psychiatry at Yale University, said the picture produced by the monkey research isn't entirely clear. Since about half of the abused monkeys didn't go on to abuse their own children, factors other than childhood experience must be at play, Kaufman said. She thinks genetics play a major role.

On the bright side, the research does add to the evidence that abused children don't always become abusive parents, Kaufman said. In humans, only an estimated 20 percent to 30 percent do.

"It's not inevitable," she said, "and that's what's important."