The Frontal Cortex

In recent years, it's become clear that much of our individual behavior depends on the dynamics of our social network. It doesn't matter if we're talking about obesity or happiness: they all flow through other people, like a virus or a meme. Last year, I profiled James Fowler and Nicholas Christakis in Wired, who have conducted several fascinating studies that demonstrate the power of social networks:

There's something strange about watching life unfold as a social network. It's easy to forget that every link is a human relationship and every circle a waistline. The messy melodrama of life--all the failed diets and fading friendships--becomes a sterile cartoon.

But that's exactly the point. All that drama obscures a profound truth about human society. By studying Framingham as an interconnected network rather than a mass of individuals, Christakis and Fowler made a remarkable discovery: Obesity spread like a virus. Weight gain had a stunning infection rate. If one person became obese, the likelihood that his friend would follow suit increased by 171 percent. (This means that the network is far more predictive of obesity than the presence of genes associated with the condition.) By the time the animation is finished, the screen is full of swollen yellow beads, like blobs of fat on the surface of chicken soup.

The data exposed not only the contagious nature of obesity but the power of social networks to influence individual behavior. This effect extends over great distances--a fact revealed by tracking original subjects who moved away from Framingham. "Your friends who live far away have just as big an impact on your behavior as friends who live next door," Fowler says. "Think about it this way: Even if you see a friend only once a year, that friend will still change your sense of what's appropriate. And that new norm will influence what you do." An obese sibling hundreds of miles away can cause us to eat more. The individual is a romantic myth; indeed, no man is an island.

In their latest paper, published this week in PNAS, Christakis and Fowler re-analyzed an earlier set of experiments led by Ernst Fehr and Simon Gachter, which investigated "altruistic punishment," or why we're willing to punish others even at a cost to ourselves.

Christakis and Fowler demonstrate that, when one of the students gave money to help someone else - they were cooperating - the recipients of that cash then became more likely to give their own money away in the next round. (Every unit of money shared in round 1 led to an extra 0.19 units being shared in round 2, and 0.05 units in round 3.) This leads, of course, to a cascade of generosity, in which the itch to cooperate spreads first to three people and then to the nine people that those three people interact with, and then to the remaining individuals in subsequent waves of the experiment.

The paper itself is filled with optimistic sentences, but it's worth pointing out that 1) selfishness is also contagious and 2) there's a big difference between lab experiments played with strangers and the messy social networks of real life. That said, altruistic cascades like this make me happy:

We report a chain of 10 kidney transplantations, initiated in July 2007 by a single altruistic donor (i.e., a donor without a designated recipient) and coordinated over a period of 8 months by two large paired-donation registries. These transplantations involved six transplantation centers in five states. In the case of five of the transplantations, the donors and their coregistered recipients underwent surgery simultaneously. In the other five cases, "bridge donors" continued the chain as many as 5 months after the coregistered recipients in their own pairs had received transplants. This report of a chain of paired kidney donations, in which the transplantations were not necessarily performed simultaneously, illustrates the potential of this strategy.

Update: I've gotten a few emails wondering what this means for free will. After all, if our decisions are so determined by the decisions of others, then where is there space for human autonomy? My first reaction is that the new science of social networks still leaves plenty of elbow room for individual decisions. We're talking about risk factors and tendencies and statistical correlations. Just because we're influenced by others doesn't mean we can't reject those influences. I asked James Fowler a related question last year and this was his eloquent response:

Everyone always tells me that this research is so depressing and that it means we don't have free will. But I think they're forgetting to look at the flipside. Because of social networks, your actions aren't just having an impact on what you do, or on what your friends do, but on thousands of other people too. So if I go home and I make an effort to be in a good mood, I'm not just making my wife happy, or my children happy. I'm also making the friends of my children happy. My choices have a ripple effect.