THURSDAY, Sept. 18 (HealthDay News) -- Parenting style starts to influence a child's ability to deal with stress as early as 6 months of age, a new report says.

The findings, published in the September/October issue of Child Development, show that parenting and genes influence how a child deals with stress, and that parental actions could put infants at risk of developing poor responses to such situations.

Researchers from three North Carolina universities and Pennsylvania State University measured heart rates of 142 infants during a stressful situation (separation from their mothers) to check vagal tone, a cardiac response that slows the heart when calm but allows it to pump faster in challenging situations. They also checked the babies' DNA to determine which dopamine receptor gene the infants carried; some forms of this gene are linked to later problems such as aggression, substance abuse, and other risky behaviors.

At 3 and 6 months old, those infants with the dopamine gene associated with later risky behaviors also did not have effective vagal tones to take the brake off the heart during stressful situations; infants with the non-risk version of the gene did. At these early ages, the researchers found, whether mothers were sensitive to their child's stress did not seem to affect their vagal tone.

By age 12 months, infants with the risk gene who also had mothers who were highly sensitive now showed the expected cardiac response when stressed. Those same infants with insensitive mothers continued to show an ineffective cardiac response to the stress.

These findings suggest that while genes affect development of physiological responses to stress, environmental experience (such as mothers' sensitive care-giving behavior) can have a strong enough influence to change the effect of those genes very early in life.

"Our findings provide further support for the notion that the development of complex behavioral and physiological responses is not the result of nature or nurture, but rather a combination of the two," study lead author Cathi Propper, a research scientist at the University of North Carolina at Chapel Hill, said in a news release issued by the journal's publisher. "They also illustrate the importance of parenting not just for the development of children's behavior, but for the underlying physiological mechanisms that support this behavior.

"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 behaviors."

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