In infancy, genes are the key influence on a child's ability to deal with stress. But as early as six months of age, parenting plays an important role in changing the impact of genes that may put infants at risk for responding poorly to stress.
That's the message from a new study by researchers at the University of North Carolina-Chapel Hill, Pennsylvania State University, the University of North Carolina-Greensboro, and North Carolina State University. It appears in the September/October 2008 issue of the journal Child Development.
The researchers looked at 142 infants who had been placed in a stressful situation— being separated from their mothers — when they were three, six, and 12 months old. They measured infants' heart rates while they were exposed to the stressor, isolating a cardiac response called vagal tone. Vagal tone acts like a brake on the heart when the body is in a calm state, but during a challenging situation, this brake is withdrawn, allowing heart rate to increase so the body can actively deal with the challenge.
They also collected DNA to determine which form of a dopamine receptor gene the infants carried; specific forms of this gene are related to problems in adolescence and adulthood including aggression, substance abuse, and other risky behaviours. To assess the mothers' behaviour as high or low in sensitivity, they also videotaped the mothers and their infants playing together for 10 minutes when the babies were six months old.
What the researchers found
Both genes and parenting were found to be important to the infants' development of the way in which the brain helps regulate cardiac responses to stress. At three and six months old, those infants with the form of the dopamine gene associated with later risky behaviours did not display an effective cardiac response to the stressor (a decrease in vagal tone which takes the brake off the heart so it can respond appropriately), while those infants with the non-risk version of the gene did. At these early ages, the researchers found, it didn't appear to matter whether mothers were sensitive or not.
However, by the time the infants were 12 months old, the pattern changed. Infants with the risk form of the gene who also had mothers who were highly sensitive now showed the expected cardiac response while they were exposed to the stressful situation.
Those infants with the risk form of the gene who had insensitive mothers continued to show the ineffective cardiac response to the stressor. These findings suggest that although genes play a role in the development of physiological responses to stress, environmental experience (such as mothers' sensitive care-giving behaviour) can have a strong influence, enough to change the effect that genes have on physiology very early in life. The researchers suggest this may be because of the cumulative effect on infants of exposure to their mothers' behaviour.
"Our findings provide further support for the notion that the development of complex behavioural and physiological responses is not the result of nature or nurture, but rather a combination of the two," says Cathi Propper, research scientist at the University of North Carolina-Chapel Hill and the study's lead author. "They also illustrate the importance of parenting not just for the development of children's behaviour, but for the underlying physiological mechanisms that support this behaviour.
"Lastly, infancy is an important time for developing behavioural and biological processes. Although these processes will continue to change over time, parenting can have important positive effects even when children have inherited a genetic vulnerability to problematic behaviours." – (EurekAlert, September 2008)
Bad behaviour in the genes?
Genes blamed for child obesity