For the study, the researchers from University of North Carolina-Chapel Hill, Pennsylvania State University, the University of North Carolina-Greensboro, and North Carolina State University looked at 142 infants 3, 6, and 12 months old and were placed in a stressful situation like being separated from their mother.
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tone acts like a brake on the heart when the body is in a calm state, but during a challenging situation, this brake is withdrawn, allowing heart rate to increase so the body can actively deal with the challenge.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 as 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 6 months old.
The study showed that both genes and parenting were important to the infants' development of the way in which the brain helps regulate cardiac responses to stress.
Initially, 3 and 6 months old infants with the form of the dopamine gene associated with later risky behaviours, did not display an effective cardiac response to the stressor, while those infants with the non-risk version of the gene did.
However, by the time the infants were 12 months old those 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.
The study 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.
It appears in the September/October 2008 issue of the journal Child Development.