(Medical Xpress) -- A new study from the University of Michigan and the University of Pittsburgh shows that even if a child isn't crying, frowning or displaying other negative emotions on a consistent basis, another warning sign is when a child shows fewer positive displays, like hugging a parent or smiling and laughing.
"Surprisingly, it seems that it is low levels of happiness, as opposed to high levels of sadness, what may help explain why these kids too often develop depressive disorders," said Nestor Lopez-Duran, an assistant professor of psychology at U-M and one of the study's authors.
The study involved 140 mothers and 202 children, ranging in age from late infancy to 9 years. Groups were divided into two: Children of mothers with a history of depression and healthy mothers. Children were seen annually for a laboratory assessment that was videotaped while completing tasks to elicit positive and negative emotions.
In the study, children whose mothers had a history of depression and therefore were at high risk for the disorder did not differ from their low-risk peers in the amount of negative emotions they experienced, said Nestor Lopez-Duran. However, compared to their peers, children at high risk for depression had lower frequencies of positive emotions.
A reduced capacity for positive affect may keep kids from effectively managing their negative moods, and thus may represent one source of vulnerability to depressive disorders, the researchers wrote.
Lopez-Duran said parents should not ignore the child's sadness or frustration; they just should also be attuned to their children's positive emotional displays. If parents encourage their kids to play, but their kids don't seem to be able experience happiness from any activity, it could signal a red flag for depression.
Lopez-Duran recommends that parents seek professional help if their kids show more symptoms of depression such as sleep difficulties, poor appetite, sad mood, difficulty concentrating, or irritability.
University of Pittsburgh researchers who participated in the study were lead author Thomas Olino, Maria Kovacs, Charles George, Amy Gentzler and Daniel Shaw.
The findings appear in the current issue of Journal of Child Psychology and Psychiatry.
Explore further: Kids of depressed moms more prone to behavioral problems and injury