The mental health of parents can be adversely impacted even if one of their children fared bad or are found less successful in life, says a new study.

Researchers studied telephone survey results from 633 middle-aged parents in the Philadelphia area who rated each of their grown children's achievements in relationships, family life, education and career.

The researchers asked parents whether each of their adult children had experienced a list of problems within the past two years, and to rate each child's successes compared to other adults the same age.

They also tested the parents' psychological well-being and the kind of relationship they had with their children through a series of questions.

Two kinds of problems – voluntary and involuntary – were studied. While lifestyle and behaviour problems due to the person's own actions were considered voluntary, issues such as trouble with the law, drinking or drug problems, and financial difficulties; and physical and emotional problems, such as a disability or serious health issue were considered involuntary.

Among the parents surveyed, most had two or more children. Nearly three-quarters of parents reported having a mix of children experiencing problems and children experiencing successes.

Parents who had more than one highly successful child reported better well-being. However, having even one problematic child had a negative impact on the parent's mental health, even if their other children were successful, the researchers found. The findings were the same for both voluntary and involuntary categories of problems.

"What this study really shows is that for parents, it's very hard to see your children struggle," says lead author Karen Fingerman, a professor in gerontology, developmental and family studies at Purdue University in West Lafayette, Indiana.

So far, no study has examined how parents are affected when they have a mix of problematic and successful children, says Fingerman, who co-authored the study along with researchers at the University of Michigan and Pennsylvania State University.

"We were kind of hoping for the opposite result, but it's pretty consistent with the literature on negative emotions," she said.

Even if parents have one or more successful children, it failed to enhance their well being if a problematic child was also part of the mix. "Thus, it may be true that parents are only as happy as their least happy child," the authors wrote.