03/22/2013 02:07 pm ET | Updated May 22, 2013

Can Our Beliefs About Exercise Make Us Fat?

Everyone is an expert when it comes to weight and weight control, and I'm no exception. I am what's known as an "exercise theorist." That is, I ascribe to the lay theory that sedentary lifestyle is a major cause of obesity, and that regular exercise is the cure. That's one of the reasons I show up at the gym most days -- and nag others to come with me.

Not everyone agrees with this. In fact, so-called "diet theorists" believe that food is much more important than exercise. These everyday theorists believe that the obesity epidemic sweeping the U.S. and other developed countries is a consequence of portion size and fattening food choices.

The problem with lay theories is they may or may not dovetail with scientific evidence. And the problem with my (passionately held) theory is that, well, it's wrong. The best medical science just doesn't support my belief in exercise as the key to healthy weight. It's not that exercise is unimportant to weight control -- and it clearly has other benefits -- but it is much less important than what we eat and how much.

So what are the consequences of such lay theories? And more important, what are the consequences of embracing the wrong theory? University of Michigan psychologist Brent McFerran has come to believe that our naïve theories of weight control are not entirely harmless, and indeed that they could undermine our own efforts to achieve a healthy weight. Working with Anirban Mukhopadhyay, a colleague at the Hong Kong University of Science and Technology, he has run a series of experiments around the world to explore this possibility.

McFerran suspected that, regardless of which theory we ascribe to, we use that theory to guide our actual eating. And because overeating is the main culprit in weight gain, then our lay theories should predict how overweight we are in fact. That is, exercise theorists don't think diet is important so they don't heed what they are eating; they eat more as a result, and over the long haul put on more fat. Here's how he tested this idea.

He started by asking a group of Korean volunteers to identify the primary cause of obesity -- eating too much, not exercising enough, or genetics. They included genetics even though a vast majority of people believe that personal choices are more important than heredity in weight and obesity, and the science concurs. And in fact more than 90 percent of this sample chose either diet or exercise. They also measured the volunteers' height and weight, which they converted in body mass index, or BMI, a widely used metric for healthy and unhealthy weight. A simple correlation revealed that diet theorists had much lower BMIs than did exercise theorists. Or to slice it a different way, nearly twice as many overweight volunteers were exercise theorists -- even though there were more diet theorists overall.

So these results supported McFerran's basic hypothesis -- that beliefs do matter. But McFerran was aware that most lay theorists are not purists. I may call myself an exercise theorist, for instance, but I still believe diet is important -- and pay attention to what I eat. So in real life someone might think a sedentary lifestyle is a major cause of obesity, but that diet and other things are important as well. To test for this nuance, McFerran ran a different kind of experiment. He recruited a group of middle-age Americans and asked them to indicate the relative importance of various factors, including stress and education and medical conditions, in addition to exercise and food.

The results were clear-cut. Those who believed more strongly in the diet theory had lower BMIs, while the more passionate exercise theorists were more likely to be overweight. Again, those who were actually overweight or obese were much less likely to indict diet as the culprit in weight gain. They ran a similar study in France, and got basically the same results.

So what's going on here? What's the psychological mechanism connecting belief about weight gain with actual weight gain? To investigate this, McFerran recruited a group of Canadian volunteers, and asked them about their theories regarding weight. While they were completing some surveys (unrelated to the study), McFerran offered them chocolates and measured their consumption. He predicted that exercise theorists, because they discounted the importance of food, would eat more chocolate.

And that's what he found. As reported in an article to appear in the journal Psychological Science, exercise theorists ate many more chocolates. In a similar and final experiment, this one in Hong Kong, McFerran primed volunteers' thinking by having them read scientific articles touting one theory or the other. Those primed to think about the importance of exercise ate more chocolate than did those primed to contemplate diet. This is the first evidence that everyday beliefs about obesity actually influence food consumption.

So what we believe about obesity -- for whatever reason -- appears to be an important psychological antecedent of obesity itself. Obesity is a huge and growing public health concern, and the evidence here suggests that an effective public health campaign might well start by targeting unhealthy everyday theorizing.

Wray Herbert's blogs -- "Full Frontal Psychology" and "We're Only Human" -- appear regularly in The Huffington Post.

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