If your friends are smokers, you tend to light up too, studies show. If they're overweight, then your belt also feels a bit tight. If they're happy, chances are you're smiling too. And on and on.
Many public health leaders now believe this growing science of social networks can be used to improve health and well-being on a broad, population-sized scale. Some see the approach as a promising new front against the day's most urgent health problems, such as obesity, smoking and suicide.
"We've come to realize more and more that how people live and function in social networks is really important to health," says Deborah Olster, acting director of the Office of Behavioral and Social Sciences Research at the National Institutes of Health. What's less clear, at least so far, is the best way to nudge people toward healthy habits and away from destructive ones. Results from experiments are mixed — some efforts work, others don't. In March, the NIH issued a funding opportunity for scientists studying how to improve public health through social networks.
Public health programs could tap into social networks in two main ways, says Dr. Nicholas Christakis of Harvard Medical School, co-author with James Fowler of UC San Diego of the 2009 book "Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives."
One approach, says Christakis, is to manipulate the network of connections people have. Artificial social groups — think Weight Watchers — could be created to urge unhealthy members toward more healthful behaviors.
A second approach, which Christakis considers far more promising, is to manipulate existing networks so that positive health messages become "contagious." In this model, public health programs would target key members of a social group in an effort to influence the network at large.
When social scientists talk of "networks," they mean any social circle in which people trade ideas, share experiences and generally touch each other's lives — neighborhoods, offices, classrooms or online communities such as Facebook.
Identifying the most influential or popular people within these networks has become easy, Christakis says, due to advances in data analysis. The trick is to identify which members could give positive health efforts the best bang for the buck and to create the best design for such programs.
In a study published in 2003 in the American Journal of Public Health, led by USC professor of preventive medicine Thomas Valente, sixth-graders participated in an eight-week smoking-prevention program. The intervention was taught to groups of students (the "networks," in this case) by class-nominated peers, teacher-nominated peers or random peers. All told, class-nominated peer leaders conducted the program most effectively, with students in these networks reporting less intention to smoke and lower smoking rates one year later.
But a follow-up study, also led by Valente, had less success. This time, Southern California high schoolers took part in a 12-lesson drug-abuse intervention. The program had a greater effect on health behavior when led by an influential, student-nominated peer than when led by a teacher — but with a caveat. Drug use declined only in relatively drug-free social circles; for those whose friends included drug users, the program actually elevated drug use, the researchers reported in the journal Addiction in 2007.
The mixed findings suggest to Valente, who's done some of the leading work on network-based health interventions, that programs may need different designs based on the type of social network they're intended to help. This important point, he adds, has been entirely ignored in all work on health promotion.
"You can't divorce the content of the program from the people delivering it," he says. "The message is really the messenger."
Valente is part of a new effort, led by Peter Wyman of the University of Rochester, to study how effectively key network members can implement a suicide prevention program called Sources of Strength. The intervention familiarizes students with suicide-coping resources and encourages them to seek help for suicidal friends. Peer leaders at 36 high schools in New York and North Dakota will learn the program and then introduce it into their social circle — the "network." Over the course of the five-year study, the researchers expect that students in these networks will seek more help than students in control groups, resulting in fewer suicide attempts.
Network interventions might be effective against obesity too, says economist Scott Carrell of UC Davis. In a recent study, Carrell and collaborators examined the spread of fitness habits in students at the Air Force Academy. The researchers found that the probability of a student being classified "unfit" tripled once half the student's social network fell out of shape.
The finding underscores that network influences work in both directions — for good or for bad — but Carrell sees an opportunity in the results. "If you make that 'unfit' person more healthy," he says, "that suggests you will also increase the fitness levels of everyone else in the group."
All these efforts depend on a key scientific concern: to craft strong social-network interventions, researchers must first be certain that one person's health behavior has a direct, causal effect on another's, says Jason Fletcher of Yale University's School of Public Health. For some behaviors, such as smoking or drinking, the direct network effect is quite apparent, Fletcher says. But for others, such as obesity, the case is less clear.
In a 2007 issue of the New England Journal of Medicine, Christakis and Fowler reported that obesity spread through social ties in one's neighborhood. They analyzed a real-world network population of more than 12,000 people living in Framingham, Mass., who were followed for 32 years, and concluded that overweight people tended to create overweight friends.
That finding, though intriguing, doesn't prove cause and effect, Fletcher argues. Environmental factors could have been a big influence: Maybe two neighbors are obese not as a result of their social connection but because a fast-food chain opened at the end of their street. Friend selection matters too: Maybe overweight people seek each other and cluster together rather than directly influencing one another's behavior. (Christakis says his study did consider these factors.)
A new study, presented in June at a conference of the American Society of Health Economists, tried to tackle this cause-and-effect dilemma by studying college roommate assignments — which are random in terms of mental health. The researchers, led by Daniel Eisenberg of the University of Michigan, then tracked the health behaviors of the 775 male and 867 female participants.
They found preliminary evidence for depression contagion only in male networks and for anxiety contagion only in female groups. Additionally, Eisenberg found plenty of support for the spread of binge drinking, but none for happiness.
Such a finding could help public health leaders build depression programs specifically for college-age males, Eisenberg says, but does throw some cold water on the notion that hanging around happy people is bound to make you more happy.
Ultimately, experts agree, as understanding about the structure and flow of social networks improves, so will health interventions. Sorting out the details is a matter for science — for the average person, it's enough to know that improving the health of loved ones often means leading by example.
"An easy thing to say would be 'Choose your friends wisely,' " says Olga Yakusheva of Marquette University, who has found that weight-loss behaviors can be contagious among college women. "I'd like to say, 'Choose your behaviors wisely,' because they're going to impact people around you."