People are social by nature, spending about 80% of their waking hours with family, friends or co-workers. Research about the health impact of this web of personal relationships — or "social networks," as the scientists call it — suggests that friends, family members and even friends-of-friends can "spread" particular health behaviors and emotions, just as they can spread a cold or flu virus.
Two researchers, in particular, have been studying and writing about how our human networks affect everything from how much you weigh to how industrious you are, from how happy you feel to the nature of your political thinking.
Dr. Nicholas Christakis is a social scientist and physician. He became interested in social networks while helping patients and families make tough end-of-life decisions. His collaborator, Dr. James Fowler, is a political scientist at the University of California, San Diego.
Along with other researchers, they analyzed data from the original Framingham Heart Study and follow-up studies of people connected to the original participants: spouses, children, grandchildren, close friends and employers. (The Framingham Health Study began in 1948 to identify risk factors for heart disease. In 1971, researchers began the Framingham Offspring Study to study the spouses and children of the original participants. Then in 2002, a study of the third generation of participants began.)
Here's are some of their research findings about how the people around you can influence your health.
Germs spread based on physical closeness. It's about geography. Weight gain, however, seems to spread based on social contacts. For example:
Social ties influence smoking behaviors in similar ways to weight gain. If one spouse stopped smoking, the other was also more likely to quit. And whenever one individual stopped smoking, that person's friends and siblings were also more likely to kick the habit. But a next-door neighbor appeared to have little impact on a person's decision to quit smoking.
What about co-workers? It depended on the size of the company.
Both geography and social ties affect the spread of happiness. After all, face-to-face contact is likely to be important for communicating emotions.
We often think of loneliness as, well, a lonely, individual affair. But Drs. Christakis and Fowler found that this feeling, too, can spread in networks.
For example, participants were more likely to report being lonely if they were directly connected to someone else who was lonely. Here, too, the strength of the social tie and geographic closeness mattered. Friends were more likely to have an impact on feeling lonely than did acquaintances. Next-door neighbors had more impact than people living down the street.
In a surprise, however, siblings — no matter where they lived — had no impact on how lonely another sibling might feel. The researchers suggested that loneliness may reflect relationships people form on their own, rather than the ones they are born into.
Health behaviors or emotions might spread among members of a social network in three ways.
Drs. Christakis and Fowler believe that induction is the leading explanation for the health effects of social networks. They don't so much say that social networks cause obesity, smoking behaviors, or mood changes. Rather, they argue, social networks increase the effects of the environmental and other factors.
For example, genes and environment clearly influence a person's weight. Social networks create a sense of what is "normal." People who notice that their best friend has gained weight since the last college reunion, for example, might — without thinking too much about it — adjust their concept of what a normal body weight is.
Some researchers criticize this view. Some argue that these behaviors are too complicated to be meaningfully affected by such person-to-person influences. Others say that the Framingham Heart Study participants were not so random a sample. Coming from a specific region of the country, the study subjects may have been more similar than different in their habits and outlook. Scientists who stress this point suggest that homophily and environment, rather than induction, would be a more important influence.
The research confirms what many mental health clinicians know through their training and experience: Health and other behaviors have personal as well as communal effects. Social network theories help to explain why group therapies and group-based self-help interventions, such as Alcoholics Anonymous or Weight Watchers, might work.
And, looking around you, you may have your own theory about how your social network affects you. At the very least, if there is a personal habit or pattern that you're trying to understand or even change, you may want to look at the company you keep.
Michael Craig Miller, M.D. is editor-in-chief of the Harvard Mental Health Letter and an assistant professor of psychiatry at Harvard Medical School. Dr. Miller has an active clinical practice and has been on staff at Beth Israel Deaconess Medical Center for more than 25 years.