If you’re happy and you know it, pass it on. In a study that analyzed the happiness of nearly 5,000 individuals over 20 years, researchers from Harvard Medical School and the University of California, San Diego, found that joy is a collective phenomenon that spreads through social networks. One person’s happiness sets off a chain reaction that can be measured up to three degrees of separation and can last for one year. And the closer you live to happy people, the stronger the contagious effect. The researchers found that living within a mile of a happy friend gives you a 25 percent increased chance of becoming happy, but live next door to her and your increased chance of “catching” the happiness goes up to 34 percent. So the next time you’re feeling cheerful, talk to friends, who’ll talk to friends. But don’t shun your gloomy pals—sadness doesn’t spread through social networks as quickly as happiness, according to the study.