Study Contradicts Earlier Reports That Some Health Issues Are ‘Contagious’ Among Friends

Published: December 12, 2008

New Haven, Conn. — A smile may be infectious, but are obesity, smoking and even a headache?

Published research has suggested that certain health outcomes are "contagious" in close social networks — that, for instance, people who kick the smoking habit are likely to influence those in their inner circle to do the same, or that people are more likely to become obese if a friend is overweight. The same researchers most recently contended that happiness can be spread from one friend to another.

The statistical techniques used to determine these associations, however, are prone to "large biases ... that might produce effects where none exists," according to Jason M. Fletcher, assistant professor at the Yale School of Public Health.

He and fellow researcher Ethan Cohen-Cole of the Federal Reserve Bank of Boston set out to show the limitations of these methodologies by applying them to areas where "contagion" could not possibly exist among friends.

They found "surprisingly high," if implausible, social network associations in such areas as headaches, skin conditions and even height, notes Fletcher.

"Without adequately controlling for the other reasons that friends experience similar health outcomes, some of our empirical results suggest that headaches are even more contagious than obesity between friends," he says.

The statistical technique used to study these associations fails to control for two important factors, says Fletcher — namely, that people usually choose to become friends with someone who is very similar to themselves, and that close friends share an environment that can lead them both to make similar health decisions.

When Fletcher and Cohen-Cole statistically controlled for the shared environment of friends, the social network associations they had found earlier — headaches, acne and height — all disappeared.

"Our findings do not prove that social connections are unimportant. To the contrary, we believe that friends do in fact influence individuals' decisions and health to a considerable degree," says Fletcher. "Our results are meant to show the difficulties of producing convincing evidence that friends can make us fat, cause us to quit smoking, or pursue other healthy and unhealthy behaviors. Our results suggest caution in attributing similarities of health outcomes between friends to a ‘contagious' process."

Details of the research will be published in the British Medical Journal.

— By Michael A. Greenwood, Yale School of Public Health

PRESS CONTACT: Michael Greenwood 203-737-5151