Body Blog: Are Bad Habits Contagious?


Is obesity contagious? What about smoking and drug use? Is it possible to unconsciously pick up the bad habits and health problems of your friends? First introduced in 2007, this idea has sparked a debate among researchers and doctors that is still heating up.

A controversial study led by researchers from Harvard and the University of California, San Diego, concluded that behaviors such as overeating and cigarette smoking could be contagious within close social networks. The researchers came to this conclusion by analyzing data from the Framingham Heart Study, a long-running, detailed survey that examined over 12,000 participants. The Framingham Study included information about body weight and drug use habits, and also indicated which subjects were friends or family members. Thanks to this unique feature, researchers were able to take a look at rates of obesity and other health risks among friend groups.

Dr. Nicholas Christakis and Dr. James Fowler, the leaders of the study, found that if one spouse became obese, the other was 37 percent more likely to become obese as well. If a man became obese, his brother’s risk rose by 40 percent. And the risk was even higher among friends  – the doctors found that someone with an obese friend was between 51 and 171 percent more likely to become obese.

Dr. Christakis and Dr. Fowler provided three possible explanations for their findings. The first explanation was a natural human tendency to choose friends who are similar to ourselves. The second was the fact that friends may be similarly affected by the environments they share. The third explanation, which attracted the most attention, was called contagion – the possibility that a person’s ideas of acceptable weight and portion size might be influenced by how much their friends weigh or eat. This was the explanation that doctors Christakis and Fowler focused on as the cause of obesity.

But not all scientists agree with this theory. A recent paper attacked the findings of doctors Christakis and Fowler, saying that their study contained serious statistical errors. Other doctors don’t doubt the integrity of the study, but say that it is impossible to prove that contagion was the cause of high rates of obesity among friends and families. A more recent study that focused specifically on women did find that social relationships had an influence on bodyweight, but the influence was very small. The researchers were also unable to pinpoint one specific explanation for their findings.

So, the verdict is mixed: problems like obesity may or may not spread among friends. But even if behaviors like overeating and smoking aren’t contagious, the question of friends and behavior is an interesting one for college students. When we go away to college and have to make new friends, do they influence our behavior? Do we change and become more like our new friends? Or do we choose friends who are more like us to begin with? What do you think?

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