Social networks and health: Communicable but not infectious

Poet and pastor John Donne famously proclaimed "No man is an island." It was true in his day, and because society has become increasingly complex and interdependent over the ensuing 400 years, it's certainly true today. Studies in the modern era show that people can be good medicine and that individuals with strong social supports are healthier than those who are lonely and isolated. Married men, for example, are healthier than their single, divorced, or widowed peers.

It's easy to understand how face-to-face interactions can be beneficial. But research suggests that social interactions have a ripple effect that extends far beyond household and family units. Some of these effects can promote health; others are detrimental. But whether for good or ill, these communitywide effects give networking a new meaning.

Spreading problems

Obesity is an enormous health issue in the U.S. Since two of every three Americans are overweight or obese, it's no stretch to say the problem has assumed epidemic proportions. There are many explanations for our expanding waistlines, starting with insufficient amounts of exercise and excessively large portions of inexpensive, calorie-dense prepared and processed foods. But is it possible that social interactions also play a role, and that the obesity epidemic is in part a contagious disease? An important study suggests that answer is yes.

Researchers from Harvard and the University of California investigated 12,067 people who had been evaluated medically on multiple occasions from 1971 to 2003 as part of the Framingham Heart Study. They found that if one sibling became obese during the study, the chance that another sibling would become obese increased by 40%. Genetics might account for some of the parallel weight gain in siblings, but not for the fact that if a spouse became obese, the likelihood that the other spouse would follow suit jumped by 37%. Shared meals and other lifestyle habits might explain that link, but the scientists also found that if a person had a friend who became obese, his chance of growing obese rose by 57%.

The impact of networks depended more on social status than physical proximity; obesity in a neighbor had much less influence than obesity in a friend, regardless of how far away the friend lived. Friends of the same sex were particularly influential; a man who had a male friend who became obese experienced a 100% increase in his own chance of becoming obese. And when two people regarded their friendship as mutual, obesity in one member of the pair increased the other's likelihood of becoming obese by a staggering 171%.

Why does obesity spread in social networks? The effect extends far beyond the impact of genetics and shared environmental influences. The researchers did not specifically investigate diet and exercise patterns, but they did find that changes in smoking did not account for the spread of obesity in the Framingham network. Although scientists don't fully understand how obesity spreads, they suspect a major factor is that a social network influences what its members perceive as normal and acceptable. If a man sees his friends become obese over time, he may accept weight gain as natural, even inevitable. Instead of exercising more or eating less when his own weight begins to creep up, he may simply go with the flow and join the crowd.

The notion that obesity is contagious may lend new weight to folks who claim "you make me sick." But the same researchers who revealed the spread of obesity also tell us that networks can spread positive emotions as well.

Spreading happiness

Like the study of obesity, the Framingham Heart Study's database was the foundation of the happiness study. In this case, 4,739 people who were tracked between 1983 and 2003 served as the primary study population. Together, these subjects reported a total of 53,228 social ties to family, friends, neighbors, and coworkers. Because scientists have kept a close eye on the Framingham volunteers since 1971 — over three generations — detailed medical and psychological information was available for many of these people.

The researchers used the Center for Epidemiological Studies Depression Scale to evaluate happiness at the start of the study and at subsequent follow-up examinations. Just as the obesity study provided information on how weight changed over time, the happiness study focused on changes in mood over time.

The Framingham study confirmed earlier findings that the strongest determinant of whether a person will be happy in the future is how happy he has been in the past. And the study also confirmed that healthy people tend to be happier than those who are ill; men tend to be a bit happier than women; and more educated people tend to be slightly happier than those with less schooling.

Previous research had validated the everyday observation that emotions are contagious over a very brief time frame; if one person in a room starts to laugh or cry, others often follow suit, but the effect wears off in minutes. But the Framingham study added an unexpected finding: happiness can also spread more diversely and broadly across social networks.

The scientists found that if one spouse became happy, the likelihood that the other spouse would become happy increased by 8%. Siblings who became happy increased the other sibling's chance of becoming happy by 14%. But while the spread of obesity was not related to physical closeness, the spread of happiness did depend on distance. Spouses and friends only transmitted happiness to people living within a mile, and although obesity did not spread between neighbors, happiness did. But physical proximity on the job did not allow happiness to spread among coworkers.

Like obesity, happiness spread more readily between members of the same sex than between people of the opposite sex. Like obesity, the spread of happiness seemed to reach across at least three degrees of separation, spreading, for example, from a friend to the friend of a friend and then to the friend of that friend. But the impact diminishes with each degree of separation, and even within first-degree contacts, it begins to wane after six to 12 months.

Although the researchers did not discover exactly how happiness spreads across social networks, they did speculate on the positive role that spreading happiness may play. Humans are social beings, and the health and well-being of one person influences others. Since happiness and optimism are linked to better health and improved longevity, contagious happiness might have a beneficial effect on the health of an entire community. Interestingly, the Framingham study found that unhappiness does not spread across social networks. If emotions were the flu, that would mean that immunity could spread, but the virus itself could not — it's a fantasy for the flu season that's just kicking off, but it's a real possibility for the way emotions spread.

A new science

Epidemics have plagued humans throughout history. Before scientists identified the microbes responsible for an epidemic as well as the way the germs spread from person to person, people blamed things like human misbehavior, divine intervention, and supernatural forces for epidemics ranging from the Black Death of the Middle Ages to the Spanish Flu of 1918. But now that doctors understand the way infections spread through communities, they can use tools such as immunization, hygiene, and the isolation of sick individuals to control epidemics.

Social-network science is much newer than epidemiology, and its eventual impact on medicine remains uncertain. The statistical methodology used in the Framingham research on obesity and happiness has come under fire. Still, the studies raise the intriguing possibility that noninfectious phenomena can spread across communities through social networks, and researchers have added alcohol consumption and depression to the list of things that may be affected by social networks.

Natural social networks may already have a substantial impact on health, and if doctors learn to harness them to spread healthful habits, positive attitudes, and wise lifestyle choices through communities, they may be able to improve public health. It might sound farfetched, but support groups such as Alcoholics Anonymous and Weight Watchers already function as small, artificial therapeutic networks.

More research is needed. Call it a network in progress.