Nearly a third of early adulthood depression linked to bullying in teenage years

School bullying takes its toll later in life too. Ashley Rose/Flickr, CC BY-SA

Being bullied at school is an unpleasant and relatively common experience for many young people, but does it lead to long-lasting harm? A growing number of studies have found that being bullied is associated with an increased risk of developing symptoms of anxiety and depression, self-harm and even suicidal ideation.

Many would argue that it seems obvious that bullying might cause some young people to develop depression – but how strong is the evidence that this is the case?

Does teenage bullying contribute to clinical depression?

In new research published in The British Medical Journal, we built on the work of other studies to investigate the strength of the association between being bullied as a teenager and developing depression as a young adult.

We used data from the “Children of the 90s Study” - a large birth cohort study based at the University of Bristol, which has been charting the health of families since the early ‘90s. We were particularly interested in bullying occurring during the early teenage years, given this is a time both when the influence of peers rises, and rates of depression begin to increase.

At 13 years, participants filled in detailed questionnaires about their recent experiences of different types of peer victimisation. When they were 18 years old, participants came back into the lab and took part in clinic assessments that were used to derive diagnoses of depression.

Of the 683 children who reported frequent victimisation at 13 years, 101 (14.8%) were depressed at 18 years. Of the 1446 children reporting some victimisation, 103 (7.1%) were depressed, and of the 1769 children reporting no victimisation at 13 years, 98 (5.5%) were depressed.

Children who were frequently victimised had over a two-fold increase in odds of depression compared with children who were not victimised by peers. So is this enough to show that being bullied causes depression?

Even if bullying starts earlier in life, the effects continue into later life. Ankakay/Flickr, CC BY

Sadly, some children are more at risk of being bullied by their peers than others. In particular, children who are more shy and withdrawn are more likely to be bullied than their less anxious peers.

This makes it difficult to disentangle cause and effect – does being bullied lead to an increased risk of depression, or are kids with higher levels of depressive symptoms simply more likely to be targeted by bullies?

Are there other characteristics that might explain this association? Using data on children’s mental health and their family experiences, we were able to adjust for a large number of factors that might have explained the relationship between bullying and depression.

Even when statistically controlling for these factors, we still found that teenagers who were frequently bullied were nearly twice as likely to be depressed at 18 compared to their non-bullied peers. The more frequent the bullying, the increased likelihood of developing depression as an adult.

How important is bullying as a risk factor for depression?

In our sample, up to 30% of depression at 18 years might be attributed to being bullied as a teenager if bullying really is a cause of depression. However with our type of study, we can never be certain that there isn’t some other, unmeasured variable that might explain our findings.

But given that researchers would never randomly assign children to be bullied or not in order to test whether this association is causal, we must rely on evidence from observational studies such as ours.

Also, we can’t say for sure whether our findings generalise to other populations. That’s why it is so important that research such as ours is replicated using different samples from different populations.

For now, the weight of evidence is sufficiently strong that bullying should be considered as an important risk factor for depression. There are effective school-based interventions, and schools should be supported in implementing these.