Some babies stay calm when something changes in their life or environment, whereas others get fussy and fidget at even the slightest deviation from the norm. Researchers do not fully understand why some children are able to cope better with stress or whether kids’ response to such situations is influenced by parenting or genes. According to a new study, it is shaped by both.

Cathi Propper, a developmental psychologist at the University of North Carolina at Chapel Hill, and her colleagues studied infants at several periods over their first year of life, inducing stress by separating them from their mothers. Using an electrocardiogram, the researchers determined the babies’ vagal tone, an indicator of how strongly the vagus nerve, which runs from the brain stem to most organs in the body, is suppressing heart rate. During stress, vagal tone decreases, allowing the heart to speed up and the body to handle the stressor. But some of the babies did not show this normal decrease in vagal tone during distressing periods; the researchers found that these infants who lacked an effective response at ages three and six months shared a particular variant of the DRD2 gene, which regulates receptors for the neurotransmitter dopamine. The variant has been associated with a decreased number of dopamine receptors in the brain and linked with risk-taking behavior, such as gambling, in adults. Infants in the study who had different versions of the gene showed a more typical response to stress.

But these genes are not destiny. The researchers also evaluated the parenting styles of the infants’ mothers. “Exposure over time to sensitive parenting seems to counteract the effects” of the higher-risk version of the gene, Propper says. By 12 months of age, infants with this gene variant whose needs were consistently attended to responded to stress just as effectively as did the babies with other versions of the gene.

Note: This article was originally printed with the title, "Baby Stress".