Being bullied in adolescence may make kids more vulnerable to depression in early adulthood and explain almost a third of depression burden at that age, according to a new study in the U.K.

Among nearly 4,000 children in southwest England followed from birth, kids who were frequently bullied at age 13 were more than twice as likely to be depressed at age 18 as those who were not bullied – even after accounting for other factors that could contribute to depression risk.

“Given what we already know about bullying and other adverse health outcomes, we had anticipated that we would find a link between peer victimization in the teenage years and clinical depression,” said lead author Lucy Bowes of the experimental psychology department at the University of Oxford in the U.K.

“What was surprising was the proportion of depression that might be explained by peer victimization if this really is a causal relationship – nearly 30 percent in our sample,” Bowes told Reuters Health by email.

It was also concerning to find that up to three-quarters of children who were bullied said they never told parents or teachers about their experience, she said.

The researchers used data on some 3,700 families from a long-term study. Nearly 7,000 kids reported on their experience of bullying at age 13, when they were asked whether and how often they had experienced peer victimization, like exclusion, rumor spreading or physical violence, over the previous six months.

At age 18, some 3,900 teens returned to the clinic for a self-administered computer based depression assessment, including questions about depression symptoms over the past month.

Of the participants who had data at both time points, 683 said they had "frequently" been the victim of bullying at age 13. Almost 15 percent of this group was depressed, according to clinical criteria, at age 18, the study team reports in BMJ.

Of the 1,446 children who said they were victimized "sometimes" at age 13, 7 percent were depressed at age 18. The largest group, 1,769 kids who said they had not been bullied, had the lowest rates of depression, with less than 6 percent meeting clinical criteria at age 18.

Although this does not prove causation, it does suggest that bullying may lead to depression, Bowes said.

The researchers had complete data for 2,668 kids about family and personal factors, like past behavior problems and baseline depression levels, and adjusted their analysis for those other possible contributors to depression risk.

If bullying does cause depression, they speculate, the analysis shows that it could be responsible for 29 percent of depression at age 18.

“There is likely a vicious circle occurring – children more at risk of depression are also more likely to be bullied by their peers, which makes it even more likely that they might go on to develop depression as an adult,” Bowes said. “But even when we control for previous depressive symptoms as well as a number of other factors relating to the individual, their family circumstances and stressful life experiences, we still observed an increased risk of depression those bullied in childhood.”

Depression in adolescence is often undiagnosed and may go untreated, she said.

“Clinicians treating adolescents presenting with depression may wish to enquire about experiences of peer victimization,” especially if the bullying may be ongoing, she said.

“Given the cross national consistency in the relation between bullying behaviours and psychosocial adjustment, Bowes and colleagues’ work offers clear antibullying messages that should be endorsed by parents, school authorities, and practitioners internationally,” Maria M. Ttofi, a lecturer in psychological criminology at the University of Cambridge, wrote in an editorial alongside the new results.

Parents and teachers should note that many teens do not report their bullying experiences and should proactively ask children about school experiences beyond academic matters, Ttofi writes.

We do not know whether children habitually bullied at age 13 continue to be bullied in their late teens, though some evidence suggests that peer victimization decreases with age, Bowes said.

“Our study suggests that the majority of teenagers do not tell parents or teachers if they are being bullied, so a first step would be to raise the issue with teenagers if parents or teachers suspect that they may be experiencing bullying,” Bowes said. “There are many school-based interventions targeting bullying, but these need to be more rigorously evaluated so we can understand which are most effective at reducing bullying, and support schools in implementing these.”