Teens not only recover from depression when they have friends, but they may be able to prevent it in the first place if they have a strong social network, according to a new study published in the Proceedings of the Royal Society B.

The World Health Organization (WHO) reports depression is the top cause of illness and disability among young people aged 10 to 19. The good news, study co-author Frances Griffiths said, is researchers have found that a healthy mood shared among friends can significantly improve depression. Encouraging these friendships could possibly be a method of treatment.

Griffiths, who is the head of social science and systems in health at Warwick’s Medical School, used data from The National Longitudinal Study of Adolescent to Adult Health to look at more than 2,000 adolescents going to various high schools in the United States. He and a team of researchers looked to see how certain moods spread among students using a model that’s used to track infectious diseases. Interestingly, this model can also be used to track patterns of gang violence.

What the team saw was depression is not contagious, but happiness for sure is. Having enough happy friends cut the probability of developing depression in half among teens who had five “mentally healthy” friends or more. More impressive than that was the fact that a happy social network doubled the probability of recovering from depression over a six- to 12-month period among teens with 10 or more healthy friends. These probabilities were compared to having less than three healthy friends.

"In the context of depression, this is a very large effect size,” lead author Edward Hill said. “Our results suggest that promotion of any friendship between adolescents can reduce depression since having depressed friends does not put them at risk, but having healthy friends is both protective and curative.” Essentially, this study further fuels the idea that strong social support is vital to wellbeing.

In a separate study from the Massachusetts General Hospital in Boston, researchers found that the stronger patients' social relationships were — which meant frequently keeping in contact with social ties and participating in social groups — the less likely they were to commit suicide. At its worst, depression can lead to suicide.

"As a society,” researchers concluded, “if we enable friendships to develop among adolescents (for example, providing youth clubs) each adolescent is more likely to have enough friends with healthy mood to have a protective effect. This would reduce the prevalence of depression."

Source: Hill EM, Griffiths FE, House T. Spreading of healthy mood in adolescent social networks. Proceedings of the Royal Society B. 2015.