The orginal, and still most common definition of contagion is the spread of a disease from one individual to another.  Thus, the flu is contagious, and so is a cold.   The term has also been used by social scientists to refer to the spread of a behavior, belief, or emotion from individual to individual (or group to group) as a result of social influence.  Thus, economists have detected contagion in stock markets during financial panics and among nations in the expropriation of natural resources.  Political scientists have found evidence of contagion in acts as diverse as voting and suicide bombing.  And psychologists have found contagion effects for aggression and suicide.   

A study published today (reported here) has looked at the contagion of happiness.  James Fowler, a political scientist at the  University of California, San Diego, and Nicholas Christakis, a Harvard University medical sociologist, looked at changes in happiness over time in social networks.  They used data from the Framingham Heart Study, an ongoing cardiovascular study.  Participants in that study listed contact information for their closest friends, family members and neighbors, allowing the researchers to look at the spread in happiness from one person to the next.  When a  one person became happy, that person’s friend experienced a 25 percent increased chance of becoming happy.  A friend of that friend in turn was nearly 10 percent more likely to experience increased happiness, and a friend of that second friend had a 5.6 percent increased chance of happiness.  However, the effect was only found for friends who lived within a mile of each other; presumably the closer distance made for more contact and thus more opportunity for happiness to rub off.  Also, happiness contagion wasn’t found in the workplace, so an improved mood on the job doesn’t particularly affect the mood of coworkers. 

Despite the limitations of the contagion effect, it’s nice to know that, to some extent anyway, happiness is a characteristic of social systems, not just individuals.  For those of you who are guilt-prone, the downside to this conclusion would be that the happiness of others is in your hands.   Fortunately, one other finding of the study provides some consolation; unhappiness isn’t as contagious as happiness is.