“Laugh and the world laughs with you” turns out to be more accurate than the 19th century poet Ella Wheeler Wilcox perhaps knew. A recent study shows that mood can spread through adolescent social groups by what is called social contagion. While this study involved adolescents, other studies show similar findings among grownups. If the people you’re spending time with are sad, angry or frustrated, you’ll most likely feel those emotions too. And while clinically diagnosable depression is not contagious, depressive moods — which include feelings of helplessness and worthlessness, loss of interest and concentration, sadness and tiredness — are contagious. On the flip side, your mood may lift when you’re spending time with upbeat, optimistic people — or even if you’re simply reading their posts on a social network. In fact, even if you’re not in a happy mood, if someone starts laughing, you’ll probably “catch” the giggles too.