For teenagers, surrounding themselves with friends, particularly friends in a good mood could significantly reduce their risk of developing depression, and improve their ability to recover from it.

In a study published this week, scientists analysed the data of more than 2,000 high school students in the United States to investigate whether the moods of students influenced one another and if this could in turn impact levels of depression among teens. The teams modeled the spread of moods among the students over six to 12 months, using techniques similar to modeling the spread of an infectious disease.

"We classified people as ill (depressed) or not and looked at how that changed over time," says Thomas Moore, a lecturer in applied mathematics at the University of Manchester, who worked on the study. Moore says previous studies have found depressed people tend to group into clusters, implying that this frame of mind could be spread. But the team found the opposite.