July 25, 2007
Obesity Socially Contagious?

Using over 12,000 people studied over 32 years as part of the Framingham Heart Study Harvard and UCSD researchers find that people are more likely to become obese if people they are close to become obese.

Are your friends making you fat? Or keeping you slender? According to new research from Harvard and the University of California, San Diego, the short answer on both counts is “yes.”

Appearing in the July 26 issue of the New England Journal of Medicine, a study coauthored by Nicholas Christakis of Harvard Medical School and James Fowler of UC San Diego suggests that obesity is “socially contagious,” spreading from person to person in a social network.

The study – the first to examine this phenomenon – finds that if one person becomes obese, those closely connected to them have a greater chance of becoming obese themselves. Surprisingly, the greatest effect is seen not among people sharing the same genes or the same household but among friends.

If a person you consider a friend becomes obese, the researchers found, your own chances of becoming obese go up 57 percent. Among mutual friends, the effect is even stronger, with chances increasing 171 percent.

Christakis and Fowler also looked at the influence of siblings, spouses and neighbors. Among siblings, if one becomes obese, the likelihood for the other to become obese increases 40 percent; among spouses, 37 percent. There was no effect among neighbors, unless they were also friends.

Nicholas Christakis at Harvard thinks the evidence is strong enough to demonstrate a causal relationship.

“What we see here is that one person’s obesity can influence numerous others to whom he or she is connected both directly and indirectly,” says Nicholas Christakis, MD, PhD, a professor in Harvard Medical School’s Department of Health Care Policy. “In other words, it’s not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship.”

Over the last 25 years, the incidence of obesity among U.S. adults has more than doubled, shooting from 15 to 32 percent. In addition, roughly 66 percent of U.S. adults are considered overweight.

In a way this makes sense. You do not feel as strong a need to maintain some form of appearance if the people around you let go.

The guys are less likely to follow their fat friends into fatness than the gals are.

Gender played an important role in how these statistics broke down. In same-sex friendships, individuals experienced a 71 percent increased risk if a friend of theirs became obese. This pattern was also observed in siblings. Here, if a man’s brother became obese, his chances of becoming obese increased by 44 percent. Among sisters, the risk was 67 percent. Friends and siblings of opposite genders showed no increased risk. While the researchers note that correlations among siblings provide evidence for a biological, and possibly even a genetic, component to obesity, patterns seen among friends indicate that there’s more than biology at work.

So if you are going to have fat friends make sure they are of opposite sex.

What to do with this information if it is true? Maybe virtual reality will help. See all your friends as skinny in virtual reality and you'll become more likely to keep off the pounds. Any other ideas?

Do people who are obsessed with skinny celebrities stay skinnier than those who do not suffer such obessions?

Share |      Randall Parker, 2007 July 25 07:59 PM  Brain Appetite

sean said at July 25, 2007 8:06 PM:

Claiming causation is quite bold.

Could there be a lurking variable hiding somewhere?

RueHaxo said at July 25, 2007 8:24 PM:

Eating is often a social event. If the obese friend in the posse is eating alot, the others will snack and eat more too.

bee said at July 26, 2007 6:28 PM:

Using the word causation in this manner is not scientific. Yes there is a correlation. The mechanism as defined here is social relationship. This is pure trash.

Mirco said at July 27, 2007 10:11 AM:

The relation could be not behavioural but microbical.
There are theories that link obese people and different microbical populations in their guts.
Friends eat together and usually the same foods. The types of food to be eated select for different microbial populations.

Craig said at July 28, 2007 8:52 PM:

Being around fat people can make you fat? They needed a study for this? It is, after all, just a symptom of peer pressure. Which leads me to connect connect some dots...

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