"What was most surprising was we found the same result in three independent studies," Hankin said. "There's a lot of controversy around these kinds of genetic studies because a lot of time the results don't replicate. As scientists, when something happens three times in a row, we start to believe it."
Marta Flaum, a child psychologist in Chappaqua, N.Y., said the study highlights the importance of environment in determining whether children will become happier and more successful adults.
"As science becomes more sophisticated, we're better able to identify these genetic or biologic markers and can predict what's going to happen in kids," she said. "We know how important early intervention is, and this study points in a direction to help us intervene."
Hankin noted that most people have no idea whether their genes predispose their children toward lower brain serotonin levels, but children who seem chronically moody are likely to be affected.
"So if you're a parent, and you have a kid who has a difficult temperament, your parenting matters a lot," he said. "Being a positive parent can accomplish a lot."
But regardless of genetics, every child can benefit from warm, supportive parenting, said Rahil Briggs, a child psychologist and director of the Healthy Steps Program at Montefiore Medical Center in New York City.
"Parenting is an incredibly powerful tool for change in children, so supportive parenting is the way we want to go for any kid," Briggs said. "That holds true for all kids, even children who come into this world with little handicap in genetics and susceptibility."
The Nemours Foundation has more information about positive parenting.
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