An interesting paper in the August, 2013 issue of Psychological Science by Brent McFerran and Anirban Mukhopadhyay demonstrates that people’s beliefs about the causes of affect their own weight.

Based on their pretests, McFerran and Mukhopadhyay find that most people believe that obesity either reflects poor or low levels of exercise with a minority of people believing that obesity is largely genetic.

An initial set of studies done in South Korea, France, and the United States examined the relationship between these beliefs and people’s body mass index (BMI), which is a measure of the amount of weight people are carrying relative to their height. High BMI values indicate that someone is overweight. Across these initial studies, a greater belief that weight gain is caused by diet was associated with lower BMI, and a greater belief that weight gain is caused by a lack of exercise was associated with higher BMI. That is, people who believed that obesity is caused by a lack of exercise were more overweight on average than those who believed that obesity is caused by poor diet. These studies controlled for many other variables that also influence weight gain like , use of medications, socioeconomic status, and amount of sleep that people get each night.

Why are these beliefs related to people’s weight? Two additional studies suggest that people who believe that obesity is caused by a lack of exercise eat more than those who believe that obesity is caused by poor diet.

In one study, participants performed a series of unrelated experiments. At the start of the study, they were given a cup full of candies and were told that they could snack on them if they wanted. At the end of the study, participants were asked about their beliefs about the causes of obesity. The more strongly people believed that obesity is caused by poor diet, the fewer candies they ate. The more strongly they believed that obesity is caused by a lack of exercise, the more candies they ate.

Finally, one study manipulated people’s beliefs. Participants read a passage that summarized scientific research. One group read a passage that suggested obesity is caused by poor diet. A second group read a passage suggesting obesity is caused by a lack of exercise. A third group read a passage unrelated to obesity. Throughout the rest of the study, participants were able to eat candies. Participants who read that obesity is caused by a lack of exercise ate more candies than those who read that it is caused by poor diet, with the control group landing in between.

The finding that a stronger belief that obesity is caused by poor diet leads to lower BMI is not that surprising, though it is comforting. Presumably, this belief makes people more aware of the relationship between what they are eating and their weight.

The finding that a stronger belief that obesity is caused by a lack of exercise is related to higher BMI is more surprising. While exercise does matter in controlling weight, it is much harder to burn off calories that you have consumed than it is to overeat. People often overestimate how many calories they burn through exercise. Thus, they do not internalize the value of a moderate diet in maintaining a healthy weight.

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