Remember back in junior high school when you were warned about peer pressure? You were warned that your “friends” would pressure you to stay out late, drink alcohol, take drugs, and even eat Twinkies!
Okay, chances are you weren’t warned about the Twinkies, but are your friends responsible if you’re obese? (And if everyone else jumped off a bridge, would you?)
In the last few decades there have been more obese and overweight people than ever in human history. Not coincidentally, there are now more extraordinary ways of trying to make fat people skinny than ever before: extreme diets, drugs, radical exercise regimes, gastric bypass…
But what about the big question? Why? Why are there more obese and overweight people?
A few main reasons have been suggested, such as:
- cheap food
- lack of exercise
- your friends
It’s the farmers’ fault
How cheap food could lead to weight gain is pretty straightforward. You eat more because you can afford more food, more food causes you to gain weight and eventually you get fat –- makes sense. But I think there’s more to the cheap food argument than just eating more food. I think obese people not only eat more food, but food that is calorie-dense and nutrient poor. Extra lean ground beef, spinach, and brown rice are all pretty cheap; you could eat as much of them as you’d want and you still probably wouldn’t end up obese. On the other hand, a regular diet of doughnuts, chips and cola will fatten you up fairly quickly.
It’s your car’s fault
The next possibility, lack of exercise, is also pretty self explanatory. People are overweight because they don’t exercise enough. The argument is that in an average day, the average person has fewer physical demands than their ancestors. Compared to a generation or two ago, people walk less, drive more, and spend more time online and watching TV than riding a bike or taking a walk. Again, this argument makes sense. Most folks in the PN community would probably agree that both poor nutrition and inactivity cause obesity.
It’s Darwin’s fault
Another argument for the modern cause of obesity is genetics. The idea is that in recent years there has been an increase in the genes that cause obesity. That’s right: People have gotten obese because suddenly there has been an increase in “fat genes”. How exactly this has happened is unclear. There are two big problems with the genetics argument:
- why and how would there be natural selection for people to be overweight or obese (especially considering how much it probably limits their survival options)?
- how could it happen in little over a generation?
It’s my expert opinion that this is hooey! I would say that the genetics argument goes against the basic principles of natural selection:
- that there is an advantage to the change
- that it occurs over long periods of time – centuries and millennia, not in one generation.
But how about your friends? That’s the subject of this week’s review: does your social network, the people you regularly spend time and interact with, affect your weight?
Cohen-Cole E and Fletcher JM. Is obesity contagious? Social networks vs. environmental factors in the obesity epidemic. Journal of Health Economics 2008 Vol 27(5):1382-1387 (If you want the full text, you can download an early working paper draft in PDF here)
The spread of obesity in society – Christakis and Fowler
This study is a bit different than the others I’ve reviewed. This study is in direct response to an earlier study down by Christakis and Fowler in 2007.1 You could look at Cohen-Cole and Fletcher’s study as a rebuttal to the first study. While both studies found that obese people were more likely to have obese friends, the first study came up with three possible reasons for the results: 1) causal, 2) contextual and 3) correlative. Cohen-Cole and Fletcher responded by doing their own analysis and proved that one of the three reasons doesn’t stand up to further inspection -– can you guess which one? I’ll tell you at the end.
Causal – You made me do it.
Before we go into the details it’s important to clarify the terminology. What do we mean by causal, contextual and correlative? First, I’ll explain causal and correlative, since they come up more often and they are often inappropriately applied when interpreting data.
Causal relationships are relationships when one thing (A) causes another (B).
For example, if I hit you in the head with a baseball bat, I caused your headache. In the case of obesity, a causal explanation would say that if you have obese friends they cause you to change your eating and exercise habits. For instance, perhaps your friends only socialize in fast food restaurants or in a pub and by spending time with them you’re more likely to overeat and not exercise.
Correlative – We just happened to do it at the same time.
Meanwhile, correlative relationship is when one thing (A) happens at the same time as another (B), but A is not responsible for B. For example, let’s say I meet you while holding my baseball bat, but sit quietly without whacking you in the noggin, and you just happen to get a headache at the same time. In this case, although me and my blunt instrument may look suspicious, we didn’t cause your headache.
Many times there is evidence for correlation but no evidence for causation. It’s much more difficult to prove that one thing is responsible for or causes another than just proving they occur at the same time. For years the tobacco industry argued that the relationship between smoking and lung cancer was correlative — they proposed that it just so happened that people who were predisposed to smoking were also predisposed to lung cancer. It wasn’t until researchers carried out carefully designed studies that it was proved that yes, indeed smoking causes lung cancer.
In the case of obesity, the idea is you select friends with similar likes and dislikes to yourself. You’re already on the path to obesity and you just happened to find friends on the same path. Perhaps you are frequenting your local ice cream shop for the fourth time that week and you notice a familiar face -– they come to the ice cream shop at least as often as you. You eventually strike up a conversation, become friends and become obese together.
Contextual – My environment made me do it.
The last relationship between friends and obesity is contextual or environmental. Contextual influences are things like a fast food place opening up near your neighbourhood, where your friends just happen to live as well. You and your friends then start frequenting the fast food place and gain weight. So you all gain weight, but not because of each other. Or you all live in the suburbs and spend hours commuting each day so you don’t have time to exercise –- again it’s the context or circumstance you all are in, not the individuals.
A second look
Cohen-Cole and Fletcher wanted to look more closely at these three explanations to see if they could figure out which was the most likely. They were most interested in looking at the causal relationships and contextual influences on obesity. By using more extensive controls they found that nearly all of the causal effects were actually contextual (environmental).
It’s clear that you are a product of your environment and your relationships, but in the case of obesity it seems that your environment (contextual influences) is the bigger influence. The authors suggest that “induction”, or becoming obese, is due to environment, but they also point out that weight loss programs are more successful when your friends and family members help support your weight loss. Funny thing is that the authors of the last study promote the idea of dealing with obesity from a public-health perspective instead of a clinical one.
While I believe their data I can’t say I support the conclusion. Why? If peer pressure can help you lose weight then how can it not help you gain weight? I guess the biggest problem with teasing apart environment and friends is that your friends are a part of your environment. So, I’ll wait for more studies to look at this question. After enough come out supporting one side I’ll believe, but until then I’ll believe my junior high school teacher and avoid peer pressure.