Almost 30% of early adult depression can be traced to teen bullying, according to a study published by The BMJ (British Medical Journal).
From the LA Times:
By tracking 2,668 people from early childhood through adulthood, researchers found that 13-year-olds who were frequent targets of bullies were three times more likely than their non-victimized peers to be depressed as adults.
Even when the researchers accounted for factors like a teen's record of behavioral problems, social class, child abuse and family history of depression, those who were bullied at least once a week were more than twice as likely to be depressed when they grew up.