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Are Healthy (And Unhealthy) Habits Contagious?

What is quite startling, however, from the social network research is that obesity spreads almost like an infectious disease, not just to friends but to friends of friends, and further.
05/29/2011 10:16am ET | Updated July 29, 2011
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Have you ever caught a healthy habit from a friend? Ever been inspired to go to the gym or to eat healthy because one of your friends is doing it? Friends can really make a difference in our lives.

It turns out that we can catch unhealthy habits from our friends too.

We can see this by looking at a Harvard-led study that examined a social network of more than 12,000 people. They found that if one of your friends gained enough weight so that their body mass index (BMI) placed them in the obese category, it increased the chances of you also becoming obese by 57 percent.

Eating regularly with a friend who starts to eat junk food might cause you to indulge more often yourself, or you might start skipping gym sessions because a friend doesn't feel in the mood any more.

The researchers suggested that seeing a friend become overweight can also alter our social perceptions.

A person might previously have a negative opinion about obesity, but once a friend becomes obese they realize that it's still the same person, still their friend, just a little heavier. It can alter how they think about weight. They might now be less strict with their own diet and lifestyle because in their mind, gaining weight isn't such a bad thing, increasing the likelihood that they might gain weight.

What is quite startling, however, from the social network research is that obesity spreads almost like an infectious disease, not just to friends but to friends of friends, and further.

If Rachael gained weight, for instance, it would increase the chances of some of her friends gaining weight. But say her friend, Claire, has other friends that are not friends with Rachael. Could Rachael's weight gain have an impact on their weight?

It certainly can. If Claire had a friend, Samantha, then according to the research Rachael's weight gain increased the risk of Samantha becoming obese by 20 percent.

But it doesn't stop there. Obesity was found to spread outwards to three degrees of separation. Many are familiar with the six-degree rule, which says that you know someone (1) who knows someone (2), who knows someone (3), etc, etc, who knows President Obama (6) or even HH the Dalai Lama (6). You might even have played the popular game, 'Six Degrees of Kevin Bacon' and figured out how many social steps you are away from him.

The study showed that one person becoming obese increased the risk of their friends' friends' friends also becoming obese, and by 10 percent. So even if Samantha had a friend, Julie, who had never met Rachael, nor would ever be likely to meet her, Rachel could well impact Julie's health.

Could healthy habits also be contagious? Almost certainly so! You only need to think of a time when one of your friends chose a healthy meal because you did, or vice versa.

In the network study, the researchers found that non-obese people clustered together, suggesting that their behaviors were affecting each other.

Another study examined the effect of a weight loss program on 357 overweight patients with Type 2 diabetes. Each patient and their spouse were weighed at the start of the study and again after one year. The diabetes patients lost weight but so did their spouses, shedding an average of five pounds each.

So just as we can catch a cold from a friend, it seems that we can also catch their habits. And, of course, we also spread our own. If we develop healthy habits, it might just be a blessing in disguise for our friends.

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You can read more on contagious obesity, contagious emotions, and even contagious kindness in my latest book, "The Contagious Power of Thinking."