Sad friends will not bring you down but upbeat moods are contagious, according to a new study. Researchers at the universities of Manchester and Warwick studied 2000 teenagers to see if their social groups could influence how they felt about life.They found that having mentally stable, happy friends helped to improve the mood of those who were depressed. But depressed people did not seem to have an impact on the state of mind of those around them.Thomas House, senior lecturer in applied mathematics at the University of Manchester, said: "It could be that having a stronger social network is an effective way to treat depression. It might be that we could significantly reduce the burden of depression through cheap, low-risk social interventions."Edward Hill, an applied mathematician who collaborated on the study, said: "We've ensured that the method we used was not confounded by homophily - that is the tendency for people to be friends with others like themselves." ..