Indeed, friends can provide a protective effect against depression.
Professor Frances Griffiths, one of the study's authors, said:
"Depression is a major public health concern worldwide.The study followed over 2,000 adolescents attending high school in the US.
But the good news is we've found that a healthy mood amongst friends is linked with a significantly reduced risk of developing and increased chance of recovering from depression.
Our results offer implications for improving adolescent mood.
In particular they suggest the hypothesis that encouraging friendship networks between adolescents could reduce both the incidence and prevalence of depression among teenagers."
Researchers looked at how their moods changed in comparison to other students who were in their social network.
They found that when someone's friends were in a healthy mood, it provided a protective effect against depression.
In fact, it halved the probability of someone developing depression.
Or if they became depressed, being in a happy group doubled the chance of recovery.
Mr Edward Hill, the study's first author, said:
Dr Thomas House, another of the study's authors, said:"In the context of depression, this is a very large effect size. Changing risk by a factor of two is unusual.
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."
The research was published in the journal Proceedings of the Royal Society B (Hill et al., 2015)."It could be that having a stronger social network is an effective way to treat depression.
More work needs to be done but it may be that we could significantly reduce the burden of depression through cheap, low-risk social interventions.
As a society, 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."