Brent McFerran

Brent McFerran Ph.D.

I'll Have What She's Having

Beliefs About the Causes of Obesity

And how they predict actual body mass.

Posted Jan 02, 2013

Weight loss or maintenance is one of the most common goals people have, and the aim of many in their New Year’s resolutions. However, if you ask people how a person loses weight (i.e., what specific steps they should take), you get little consensus. This likely stems from the fact that people encounter advice that is often outright contradictory – some “experts” encourage greater exercise, some advocate reduced calorie intake, and others lay the blame on genetics. A new paper shows there are consequences to these mere beliefs – they predict an individual’s actual body mass.

In a series of studies forthcoming in Psychological Science, Anirban Mukhopadhyay (Hong Kong University of Science and Technology) and I examine what regular laypeople (i.e., not medical professionals) believe causes obesity. We term these beliefs “lay theories of obesity”. We test samples from five countries on three continents, and find a roughly equal split between people primarily blaming overconsumption and those blaming a lack of exercise. More interestingly, however, those who indicted a lack of exercise were actually heavier, and were more likely to be overweight or obese.

This effect is consistent with work on lay theories more generally. Research consistently shows that lay theories influence goal-directed behaviors, meaning our beliefs guide our actions. If we believe overeating causes obesity, and we want to lose weight, we should cut back our intake. If we believe weight gain stems from a lack of exercise, we try to increase our activity. However, relative to the other, one of these paths is much more likely to be effective – reducing caloric intake. While we show a test of this concept, it is not a new finding to medicine. If we survey physicians, for instance, they are much more likely to blame overconsumption than are laypeople.

What’s the logic behind the “magic” of one’s diet? The reasoning is as follows. First, people often reward themselves for their exercise with an indulgent treat. Sometimes we end up consuming more as a reward than the exercise burned in the first place. As a result, calorie intake and exercise often increase together, even with the best intentions. Second, people persistently overestimate the amount of calories burned during exercise and underestimate calories in the food they eat, and this exacerbates the problem above. So you believe you burned 500 calories in that run? You are probably guessing too high. And that muffin has probably 200 calories, right? Here you are probably giving too low. Third, most people do not have the time in the day needed to truly “exercise off” the extra calories they eat. Think you can “work it off later”? Perhaps not. If you eat a 3000 calorie lunch, it could take most of your waking hours to truly burn off the excess calories. But few people have the time to do this, and when they try, they are subject to the same estimation biases noted previously.

These findings from other studies help explain our key conclusion – that one’s beliefs about the cause of obesity predict his or her actual body mass index (BMI). While this relationship is a correlation, we also show that it holds even after controlling for numerous other correlates of obesity, and that reverse causality is unlikely. Consistent with our theorizing, we also show that people who believe obesity is caused by diet tend to consume less food.

What’s the prescription here? The health of the body often depends on the health of the mind, and this research is one of the first to identify a true psychological antecedent of obesity. The fact that those who believe diet causes obesity are less likely to be overweight supports the medical conclusion that the most effective path to reducing obesity is indeed dietary change. We should note however, that we are not advocating ceasing your exercise regiment, or that exercise does not help lose weight. It does – but only if calorie intake doesn’t also increase alongside.

Would simply informing people that eating too much is the main cause of weight gain actually impede the obesity epidemic? Our work suggests it could. We demonstrate that people do have different beliefs about the causes of obesity, and these views differ from medical professionals. People’s beliefs can change in response to public health campaigns from trusted sources, and hence our research supports initiatives like the Small Plate Movement, which focus attention on consumption quantities as the best path to healthier public.

McFerran, Brent and Anirban Mukhopadhyay (in press), “Lay Theories of Obesity Predict Actual Body Mass”, Psychological Science.  Paper available on my website:

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