Depression doesn't spread among friends, but happiness is contagious.

That's what researchers at the University of Warwick discovered after studying the way teenagers in a group of U.S. high schools influenced each others’ moods.

They found that having friends can help teens recover from depression or even stop them from becoming depressed in the first place.

"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," Frances Griffiths, head of social science and systems in health at Warwick Medical School, University of Warwick, said in a news release.

For the study, which was published in the journal Proceedings of the Royal Society B, researchers used data from The National Longitudinal Study of Adolescent to Adult Health, tracking how the moods of more than 2,000 teenagers spread between friends. They used methods similar to how researchers track the spread of infectious diseases.

In addition to finding that depression doesn't "spread," researchers discovered that having enough friends with a healthy mood can cut the chance of developing depression in half and double the probability of recovering from depression over a six- to 12-month period.

The research model suggests that the more healthy friends a teen has, the better his mental health will be. Teens who have five or more mentally healthy friends have half the probability of experiencing depression compared to teens with no healthy friends. And teens who have 10 healthy friends have double the odds of recovering from depression compared to teens with just three healthy friends.

So, parents, encourage those friendships.

Said University of Warwick mathematics researcher Edward Hill, lead author of the research paper: “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.”

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.