The US National Heart Institute's Framingham Study of the health conditions of 15000 odd residents and their descendents of Framingham, Massachusetts, since 1948, has yielded several interesting results (like identifying the positive role of "good" cholesterol) about the risk factors for heart disease.
Now, the NYT Magazine draws attention to the work of a pair of social scientists, Nicholas Christakis and James Fowler, who used the information collected in the study over the years, to find enough evidence to subtantiate the claim that "good behaviors — like quitting smoking or staying slender or being happy — pass from friend to friend almost as if they were contagious viruses". They argue that clusters of friends (and their behaviours) infect one another's health in both positive and negative ways. Therefore, they claim that good health is not only about about your diet and genes, but also a product of "your sheer proximity to other healthy people".
As a testament to the importance of "social contagion", they found that "groups of people would become obese together, while other groupings would remain slender or even lose weight... When a Framingham resident became obese, his or her friends were 57% more likely to become obese, too... A Framingham resident was roughly 20% more likely to become obese if the friend of a friend became obese — even if the connecting friend didn’t put on a single pound. Indeed, a person’s risk of obesity went up about 10 percent even if a friend of a friend of a friend gained weight."
They also found evidence that smoking too spreads socially — a friend taking up smoking increased your chance of lighting up by 36%, and if you had a three-degrees-removed friend who started smoking, you were 11% more likely to do the same. Drinking spread socially, as did happiness and even loneliness. They framed the "three degrees influence rule" about human behaviour that claims that an individual's influence stretched out three degrees before it faded out.
They rationalize that if a person is seated next to someone who’s eating more or has heavier friends, then he will calibrate his sense of what constitutes a normal meal or what is obese, and will therefore eat more or add pounds. They argue that happiness may result in more deeply "sub-conscious contagion" - "the spread of good or bad feelings might be driven partly by 'mirror neurons' in the brain that automatically mimic what we see in the faces of those around us — which is why looking at photographs of smiling people can itself often lift your mood".
They also find that contrary to conventional wisdom, the key to happiness and good health is not to have a few deep, long-term and heart-to-heart friendship, but lots of friendships of all kinds. The happiest people were those who had the most connections, even if the relationships weren’t necessarily deep ones. When people experience repeated emotional mirroring, their spirits are automaticaly raised by the mirroring of the spirits of their partners.
They find that happiness is more contagious than unhappiness. According to their statistical analysis, each additional happy friend boosts your good cheer by 9 percent, while each additional unhappy friend drags you down by only 7 percent.
However, there are critics who raise doubts about the contagion theory, arguing that it is is too complicated to isolate causal effects in such social issues in an inter-connected society. They point to two specific reasons - homophily and environment - to cast doubts on the work of Christakis and Fowler. The fact that people tend to gravitate towards those similar to or like themselves and shared social and psychological environments too are characterizied by similar networks tend to add to weight to the claims of these critics.
Update 1 (10/11/2010)
Scott E. Carrell, Mark Hoekstra, and James E. West examined data from the random assignment of 3,487 undergraduates to residential groups of about 30 people at the United States Air Force Academy from 2001 to 2005, and found that "poor physical fitness spreads on a person-to-person basis".