A new study on monkeys raised by abusive mothers suggests that growing up in an abusive household can alter brain chemistry in a way that makes some youngsters prone to mistreating their own children when they grow up.
In other words, abuse is not just something that's learned from living with abusive parents, although that may have an influence, according to authors of the report, published in Thursday's issue of the journal Behavioral Neuroscience.
Suffering through abuse also appears to permanently lower the brain's production of an important regulator of emotions called serotonin, said Dario Maestripieri, the study's lead author and an associate professor at the University of Chicago in comparative human development. Low serotonin can make people more prone to acts of rejection, impulsive aggression and violence.
Of course, most children who were abused do not become abusers themselves. But some do, and the findings of the study may help explain this troubling cycle where victims of abuse later mistreat their own children.
The results emphasize the need for programs to reduce child abuse and to develop behavior-modification strategies for abusive parents. But they also open the door to the development of medications, such as antidepressants, to boost brain serotonin to normal levels in both children and mothers, Maestripieri said.
"This suggests that children who early on have differences in their brain in terms of serotonin could be treated with some of these drugs and maybe these unwanted consequences could be avoided," he said.
Other scientists said that because monkeys are not humans, such findings should be interpreted cautiously. Nevertheless, there are enough genetic and biological similarities between the two species, they said, that the results may have important implications for people.
"We know that child abuse is bad for kids and that it's associated with all sorts of mental health outcomes," said University of Wisconsin psychologist Seth Pollak, who was not involved in the research. "But we don't understand how that experience seems to get under children's skin. This emphasis on serotonin seems to really help explain a lot of the behavioral problems that these children have as they grow up."
The findings add to a growing body of scientific evidence showing that nature and nurture interact to produce behavior. Environmental experiences can significantly influence how genes act in the body, affecting behavior, while an individual's genetic makeup can help determine the impact those experiences will have.
Researchers have already documented that humans who have low serotonin levels tend to be more anxious, depressed and impulsive, and earlier studies in rodents linked infant abuse and low serotonin.
Other studies have shown that among monkeys exposed to abuse as infants, those that have a shortened version of the serotonin transporter gene appear to be more vulnerable to experiencing low serotonin levels.
And recent brain scan studies found that people with the short serotonin gene have a more active amygdala, the brain's fear center. A person who has a heightened sensitivity to fear may see threats where none exist and lash out inappropriately.
The study by Maestripieri and his colleagues involved 15 baby rhesus monkeys from a colony housed at Emory University's Yerkes National Primate Research Center. Researchers noted which adult female monkeys displayed abusive behavior to their offspring and which females were nurturing mothers.
When these monkeys became pregnant again, the babies of the abusive mothers were given to the non-abusive females to raise and vice versa. Serotonin levels were measured from the infants' cerebral spinal fluid at birth and at regular intervals into adulthood.
Researchers found that infants raised under abusive conditions tended to develop low serotonin levels and become abusive mothers themselves, even though they were born to non-abusive mothers. Infants born to abusive mothers but raised by non-abusive ones retained normal serotonin levels and were not abusive.
"What's really happening to the infants raised by non-abusive mothers is that they're getting the right input into their brain," said J. Dee Higley of Brigham Young University, who participated in the study, which was funded by the National Institute of Mental Health.
The scientists suspect that low levels of serotonin may serve as a useful survival skill in a threatening situation by making primates more vigilant. But when the level is set low right from birth and stays there, it makes them impulsively aggressive.
"The big news in the new study is that certain patterns of maternal behavior have consequences for their offspring that are not only behavioral but biological and those consequences are possibly lifelong and they appear to be passed on to the next generation," said Stephen Suomi of the National Institute of Child Health and Human Development.
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