Having friends who suffer from depression doesn't affect the mental health of others, according to research led by the University of Warwick. The academics found that having friends can help teenagers recover from depression or even avoid becoming depressed in the first instance.

The findings are the result of a study of the way teenagers in a group of US high schools influenced each others' mood. The academics used a mathematical model to establish if depression spreads from friend to friend.

Professor Frances Griffiths, head of social science and systems in health at Warwick Medical School, University of Warwick, said: "Depression is a major public health concern worldwide. 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."

The study has been published in the journal Proceedings of the Royal Society B entitled Spreading of healthy mood in adolescent social networks.

Using data from The National Longitudinal Study of Adolescent to Adult Health they looked at more than 2,000 adolescents in a network of US high school students. They examined how their mood influenced each other by modelling the spread of moods using similar methods to those used to track the spread of infectious diseases.

Individuals were classified as either having depressive symptoms (low mood) or not being depressed (healthy mood) according to the score cut-off associated with a clinical diagnosis of depression.

The team found that while depression does not 'spread', having enough friends with a healthy mood can halve the probability of developing, or double the probability of recovering from, depression over a six to 12 month period.