The greatest gift is the connected life

But our data also showed that dumping your friends is not the answer. In fact, when we compared people who kept their friends who became fat with those who jettisoned them, we found a startling result. People who kept their overweight buddies were less likely to gain weight than the people who stopped being friends. So while the evidence clearly showed that obesity can spread from person to person, it also showed that getting rid of obese friends did not help. The positive effect of staying connected was stronger than the negative effect of the friend’s weight gain.

Of course, a variety of negative outcomes spread in social networks. Gang vendettas generate violence that cascades as new victims are targeted by friends of the fallen. Anxiety spreads like wildfire, causing episodes of epidemic hysteria in which everyone thinks they are sick even if they are not. Bank runs topple small banks, threatening whole financial systems.

This panoply of perils that spread through our social networks could make even the most social of us consider becoming hermits. At the very least, it would tempt many to disconnect from our imperfect connections and huddle close to those we are sure won’t infect us with bad behavior. But the beneficial effects of a connected life are so strong that, on balance, the best solution is to help, not abandon, our friends and loved ones.

Take smoking, for example. Over the past 40 years, smoking among U.S. adults has decreased from 45 percent to 21 percent of the population. But people have not been quitting alone. They have been quitting together, in droves. When we analyzed the decline of smoking over that period, we found that when one person quit smoking, it had a ripple effect on the person’s friends, the friends’ friends and the friends’ friends’ friends. Whole interconnected groups of smokers quit smoking at roughly the same time, as if a wave of opposition to smoking were spreading through the social network.

And we found evidence that this was not merely the result of smoking bans and skyrocketing cigarette taxes. For example, no relationship in smoking patterns showed up between next-door neighbors. The behavior only appeared to spread between close friends and family.

But as more and more people have quit smoking, the smokers have been forced to the periphery of their networks, just as they are now forced out of doors to smoke. And it’s not just that smokers have become less popular; they also tend to be friends with people who themselves are less popular, which helps to speed up the dramatic increase in their social isolation. As a consequence, it is increasingly difficult to reach these smokers to help them kick the habit. So if you stop being friends with smokers, you are making it that much harder for the people you care about to live healthy lives.

Staying connected is also vital to your emotional well-being. When we studied the spread of happiness, we found that it spreads to three degrees of separation, just as obesity and smoking do.

But here we discovered an intriguing asymmetry. Each happy friend increases a person’s probability of being happy by about 9 percent, while each unhappy friend decreases it by only 7 percent. So the virtue of staying connected lies in playing the averages. It’s true that the best-connected individuals at the center of the social network are more likely to “catch” an unhappy wave spreading through the network, but they are even more likely to catch a happy wave.

As a result, the people who stay connected are significantly happier than people who don’t.

Another reason to keep your friends is that dropping them can change the structure of your network, which might yield unintended consequences. For example, there is evidence that, among high school students, if one person commits suicide, an epidemic can arise around him or her. But the shape of the network also matters. If your friends are friends with each other, it creates dense interconnections in your social circle, and these connections also tend to reduce the likelihood of suicide.

So, there are two reasons not to ditch your misbehaving, up-to-no-good, loser friends and family members. First, the spread of good things within networks — whether wealth, information, happiness, love or kindness — compensates for the spread of bad things such as germs, crime, slander or sadness.

Second, the network itself is its own reward. Recent research has shown that we would rather give an anonymous gift to a friend who will never repay us than give a gift to a stranger who will. The reason is that we give to sustain our network.

So as we gather for the holidays, it is important to consider the extraordinary power of our social networks. We not only help ourselves by staying connected, but we also help our whole community.

Nicholas Christakis of Harvard Medical School and James Fowler of the University of California, San Diego are the co-authors of “Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives.”