Children perform worse in school when their parents are diagnosed with depression, suggests a study from Sweden.

The study found a significant negative link between parents' depression and kids' school performance, said senior author Brian Lee, of the Drexel University School of Public Health in Philadelphia.

"We obviously know that depression is a bad thing like any other mental health outcome," Lee said. "It's less recognized that mental health outcomes affect other people than the people themselves. So for parents or guardians, a vulnerable population would be their children."

Previous studies found children with depressed parents are more likely to have problems with brain development, behavior and emotions, along with other psychiatric problems, Lee and his colleagues write in JAMA Psychiatry. Few studies have looked at school performance, however.

For the new study, they used data from more than 1.1 million children born in Sweden between 1984 and 1994.

Three percent of the mothers and about 2 percent of fathers were diagnosed with depression before their children finished their last required year of school, which occurs around age 16 in Sweden.

Overall, when parents were diagnosed with depression during their children's lifetime, the kids' grades suffered. A mother's depression appeared to affect daughters more than sons, they note.

Lee characterized the link between parental depression and children's school performance as "moderate."

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On the range of factors that influence a child's school performance, Lee said parental depression falls between a family's economic status and parental education, which is one of the biggest factors in determining a child's success in school.

The researchers caution that depression may have been undermeasured in the population. Also, they can't say that a parent's depression actually causes children to perform worse in school.

In an editorial published with the study, Myrna Weissman points out that providing mothers with treatment for their depression - with psychotherapy or medication - has been shown to reduce problems in children.

"Therefore you should be treating the parents," said Weissman, of the New York State Psychiatric Institute and the Columbia University Department of Psychiatry in New York City. "Sometimes you have to treat the children, but you should start with the parents."

Research suggests that depression may run in families, she told Reuters Health. Additionally, childrearing is demanding and made even more difficult if a person is suffering with depression.

"Depression is a real illness," she said. "Depressed patients are awfully hard on themselves. They should be told it's not their fault."