Few studies, however, have looked at how a child might affect their parent’s own mental health. Kelsey Howard, a doctoral candidate at Northwestern University and a co-author of the new research, says she suspects that’s because most of the research done so far has been concerned primarily with the treatment methods themselves, not on the effects of treatment on people’s relationships. “From my own observations, and work I’ve done with families, and, you know, personal experience, it’s clear that kids affect parents,” Howard says.

Her research drew on the data of a large, foundational study from 2007 that looked at how depressed teens responded to two different types of treatments—an antidepressant drug and cognitive-behavioral therapy, a form of therapy that focuses on modifying thought processes and finding solutions—as well as a combination of the two. Sifting through the data, Howard and her colleagues at Northwestern discovered that regardless of the treatment teens received, the psychological health of their parents improved as well. And there was no difference between the type of treatment in the outcome on the parent, Howard says.

While the study wasn’t able to conclude exactly why parents got better as their children got treated, Howard has a few guesses. “It’s possible that the feedback, the control, and involvement in the treatment may have been beneficial,” she says. “It could be in how the family is interacting with each other: The kid is more pleasant to be around, the kid is making less negative statements, which can affect how other family members think.”

“Relationships are reciprocal,” says Laura Mufson, the associate director of the Division of Child & Adolescent Psychiatry at Columbia University, who was not involved in the study. “If one child isn’t doing well, if they’re having mood problems, if they’re more irritable—it’s affecting their behavior that impacts the rest of the people in the family.”

Psychological data increasingly suggests that treating an individual for a mental illness doesn’t simply help them alone. The Centers for Disease Control and Prevention, for instance, has found that depression costs employers in the United States between $17 billion and $44 billion dollars annually, with a loss of approximately 200 million work days a year. (And that’s not counting other mental illnesses such as anxiety, bipolar disorder, or schizophrenia.)

Still, a lot remains to be learned about depression’s broader effects, such as on siblings and peers. Howard—whose new research still needs to be reviewed by other researchers and published—plans to continue to gather data in the field, focusing on adolescent depression and family relationships.

Sharon Hoover, an associate professor of child psychology at the University of Maryland and a co-director of the Center for School Mental Health, studies the implementation of mental-health care in school settings. “Teachers report one of the greatest stressors is mental illness within the student population, so I can’t imagine that if these things aren’t treated, that it won’t affect the overall classroom environment and climate,” she says.