Low serotonin levels possibly to blame for abusive parents

By Zachary Binney

Monkey see, monkey do—because of changes in brain chemistry, according to one University of Chicago researcher. The study found that monkeys abused as infants had lower levels of a neurotransmitter called serotonin, and this may have contributed to their becoming abusive mothers themselves.

“The study provides the first demonstration that naturally occurring individual differences in maternal behavior in monkeys…affect the brain development of their offspring,” said Dario Maestripieri, associate professor in Comparative Human Development at the University and the lead author of the study, which appeared in the October 2006 edition of the journal Behavioral Neuroscience.

Maestripieri noted that similar results had been shown in rats but never before in primates.

For the study, researchers used the monkey rhesus macaques (Macaca mulatta). The team observed the behavior of a group of mothers toward their infants, noting the frequency of negative behaviors like hitting and maternal rejection. Every six months the researchers tested the babies’ cerebrospinal fluid, measuring the levels of certain neurotransmitters like serotonin.

Serotonin is a chemical in the brain that helps to regulate and inhibit emotions.

“Monkeys and people with lower-than-average serotonin in their brain have difficulty controlling their impulses, are often aggressive and violent, and also anxious and depressed”—all behaviors that could foster abusive parenting, Maestripieri said.

The team continued to study the macaques as they grew up to become mothers. They found that infants who were abused had generally lower serotonin levels than those who were not. These changes were long-term: The lower serotonin levels remained unchanged as the macaques grew up.

In addition, they found that those abused macaques who grew up to become abusive adults had 10–20 percent less serotonin in their brains than those who did not grow up to become abusive or those who had not been abused as infants.

The study could have significant implications for humans as well. “I believe that this study can help us better understand the causes of abusive parenting, its consequences for child development, and the mechanisms by which abuse is transmitted across generations,” Maestripieri said.

The study suggests that childhood experiences of abuse affect the nature of the brain, and that these effects may be treatable.

“There may be opportunities for intervention, prevention, and treatment,” Maestripieri said.

The study was co-authored by J. Dee Higley, Stephen G. Lindell, and Timothy K. Newman of the National Institute on Alcohol Abuse and Alcoholism; Mar M. Sanchez of Emory University; and Kai M. McCormack of Emory University and Spelman College.