Bullying during adolescence may be responsible for almost a third of cases of depression during adulthood, new research finds.

A long-running study of British youth reveals that the people who experienced frequent bullying at age 13 had double the risk of developing clinical depression at age 18, compared with people who were never bullied.

It's impossible to say for sure whether the bullying caused the depression, said study researcher Lucy Bowes, a psychologist at the University of Oxford. But Bowes and her colleagues say they strongly suspect there is a causal relationship. They controlled for factors that might otherwise explain the depression, including baseline depression and emotional problems that might make a person more susceptible to both bullying and to later clinical depression. [10 Scientific Tips for Raising Happy Kids]

Bullying and depression

Previous studies have linked bullying with having depression symptoms over the short term, Bowes told Live Science. And a few long-term studies have shown that people who are victims of such aggression during childhood may have long-term mental health problems. For example, a study published in 2013 in the journal JAMA Psychiatry found increased risks of depression and anxiety in adulthood among bully victims, and especially among people who had both been bullied and bullied others.

But many of these previous long-term studies were limited because they couldn't control for pre-existing conditions or because their measurements of bullying lacked detail, Bowes said. In the new study, Bowes and her colleagues used data from the United Kingdom's Avon Longitudinal Study of Parents and Children, which surveyed kids at age 13 with specific questions about bullying, including whether they'd experienced physical violence, threats, lies, rumors and exclusion.

"This is an age when the influence of peers becomes paramount," Bowes said. Then, when the participants reached the other end of adolescence, they answered questions about their symptoms of clinical depression.

Long-lasting scars

About 15 percent of bully victims were depressed at 18 compared with 5 percent of those who hadn't been bullied — an almost tripling of depression risk, the researchers report today (June 2) in the British Medical Journal. When the researchers controlled for other factors that could influence depression at age 18, such as a teen's gender and pre-existing emotional problems, the link between bullying and later depression shrank, but remained notable.

Ultimately, "we found that kids who reported that they were frequently bullied at 13 were twice as likely to report being clinically depressed at 18," Bowes said.

Bowes noted that she and her colleagues also controlled for the effects of being a bully, as people who fill the roles of both victim and bully tend to have pre-existing problems that can obscure the long-term effects of bullying, she said.

The researchers also found that parents and children were often worlds apart in understanding the bullying experience. The survey of more than 3,700 families turned up 1,199 teens who reported they were frequently bullied. But among the mothers surveyed, only 229 said their children were frequent bully victims.

Meanwhile, between 41 percent and 74 percent of teens said they didn't report bullying to their teachers, and 24 percent to 51 percent said they didn't tell their parents.

"Bowes and colleagues establish a clear link between victimisation and non-reporting to teachers or family members," psychological criminologist Maria Ttofi of the University of Cambridge, who was not involved in the research, wrote in an editorial accompanying the paper in the journal. "Parents and teachers need to be aware of this and proactively ask children about school experiences beyond academic matters."

The study drives home the long-term dangers of bullying and highlights the need to stop it where it starts, Bowes said. Schools are beginning to institute anti-bullying programs, she said, and these should be studied to ensure they're helping. More programs need to involve moms and dads, too, she said.

"We know that parents' involvement is really important, and we need to design interventions that are able to bridge the gap between the home and school life," Bowes said.

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