Like a Virus

by Russ Roberts on July 27, 2007

in Health

Gina Kolata writes on the front page of the New York Times:

can spread from person to person, much like a virus, researchers are
reporting today. When one person gains weight, close friends tend to
gain weight, too.

It’s much like a virus, you see. It’s not a virus. We know what a virus is. Obesity is not a virus. But it’s like a virus. It’s much like a virus. You see, the more it’s LIKE a virus, the more increasing obesity is like an epidemic rather than a failure of personal responsibility or merely a pleasant experience, say, of eating more ice cream and being a little less trim. The more it is like a virus, the less it is a personal choice, the more justified is government involvement on "public health" grounds. And in case you didn’t get the drift, check out the headline of the article:

Find Yourself Packing It On? Blame Friends

So blame your friends. Don’t blame yourself. Never blame yourself. After all, you’re standing in the path of a tsunami (scroll to the bottom.) There’s nothing you can do. But if we can just get more regulation to protect you from your friends, we can save you. We’ll start by saving your friends, first, of course. That will save you, eventually.

Here’s the summary of the study:

Their study, published in The New England Journal of Medicine,
involved a detailed analysis of a large social network of 12,067 people
who had been closely followed for 32 years, from 1971 to 2003.

investigators knew who was friends with whom as well as who was a
spouse or sibling or neighbor, and they knew how much each person
weighed at various times over three decades. That let them reconstruct
what happened over the years as individuals became obese. Did their
friends also become obese? Did family members? Or neighbors?

answer, the researchers report, was that people were most likely to
become obese when a friend became obese. That increased a person’s
chances of becoming obese by 57 percent. There was no effect when a
neighbor gained or lost weight, however, and family members had less
influence than friends.

It did not even matter if the friend was hundreds of miles away, the
influence remained. And the greatest influence of all was between close
mutual friends. There, if one became obese, the other had a 171 percent
increased chance of becoming obese, too.

You see it’s even worse than a real virus. It can spread over the phone or across the country covering hundreds of miles!

The real lesson here is that if you see your best friend gaining weight, stop being friends with your best friend. Dump your fat friends. You don’t want to catch the obesity "virus." In fact, make friends with people who are thinner than you. What a great study. All those people who judge people on their looks were right after all! It turns out that looking for thin, fashionable friends is actually good for you.

And it turns out the researchers actually have thought of this, though as you might expect, it isn’t a pleasant thought to have. The article in the Times continues:

If the new research is correct, it may say that something in the
environment seeded what some call an obesity epidemic, making a few
people gain weight. Then social networks let the obesity spread rapidly.

It may also mean that the way to avoid becoming fat is to avoid having fat friends.

is not the message they mean to convey, say the study investigators,
Dr. Christakis and his colleague, James H. Fowler, an associate
professor of political science at the University of California, San Diego.

do not want to lose a friend who becomes obese, Dr. Christakis said.
Friends are good for your overall health, he explained. So why not make
friends with a thin person, he suggested, and let the thin person’s
behavior influence you and your obese friend?

Beautiful isn’t it? If you have a fat friend, the two of you need to befriend a thin one. Or maybe two thin ones. After all, you risk exposing the new thin friend to the "virus." Obviously this is too risky. We need to quarantine fat people to protect the rest of us from the "epidemic."

At the bottom of the first page on the web version of the story, the author gives us a little more info about the magnitude of the changes we can expect from "exposure." Turns out it’s not quite as dramatic as it sounds:

On average, the investigators said, their rough calculations show that
a person who became obese gained 17 pounds and the newly obese person’s
friend gained five. But some gained less or did not gain weight at all,
while others gained much more. Those extra pounds were added onto the
natural increases in weight that occur when people get older.

What usually happened was that peoples’ weights got high enough to push
them over the boundary, a body mass index of 30, that divides
overweight and obese. (For example, a 6-foot-tall man who went from 220
pounds to 225 would go from being overweight to obese.)

And the last part of the article talks about how the study can never be replicated because it’s based on a one-in-a-lifetime data set, the Framingham Study. It turns out that the conclusions are based on the residents of a single town, Framingham, Massachusetts. I wonder if they controlled for time trends and economic factors correctly.


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