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What Makes Us Fat?

Researchers from the University of Michigan wanted to see whether people’s personal beliefs about the causes of obesity—which the researchers termed lay theories of obesity—were affecting their weight. They designed a series of studies that were conducted in the US and four other countries.

In one study, adults were asked what they believed was the primary cause of obesity. About 50% of participants said poor diet was to blame (the researchers called these people the “diet theorists”)…41% of participants blamed lack of exercise (the “exercise theorists”)…the rest laid the blame on genetics (the “genetics theorists”).

Then researchers analyzed participants’ beliefs in relation to their body mass index (BMI)—and discovered that in each country studied, diet theorists were the thinnest and exercise theorists were the fattest, while genetics theorists fell in the midrange between those two. For example, in the US, average BMI was 25.5 for diet theorists and 27.7 for exercise theorists. Keep in mind: According to many health professionals, a BMI of 25 is where “overweight” begins, and a BMI of 30 is where “obese” begins—so the differences seen in this study are significant!

In another experiment, US participants used a 100-point scale to rate the relative importance of all three factors (diet, exercise, genes) in contributing to obesity, rather than having to choose just one. Again, people’s beliefs were reflected in the their weight—with diet theorists significantly less likely than exercise theorists to be overweight.


Next, researchers looked at how the lay theories affected how much people ate. Participants in Canada were invited into a lab and were asked to complete some questionnaires—meanwhile, there were seven individually wrapped candies available on the table in front of them. The final question (and the only one that the researchers really cared about) involved rating what causes obesity on a sliding scale ranging from one (eating too much) to seven (exercising too little), with the number chosen indicating the relative importance of diet versus exercise.

Results: Participants who thought that obesity was primarily due to lack of exercise ate an average of 3.4 candies…while those who blamed overeating consumed just 2.7 candies, on average.

For the final experiment, done in Hong Kong, the participants’ beliefs were “primed.” One group read about a fictitious study stating that obesity is caused by overeating…a second group read another fake study blaming insufficient exercise for obesity…a third group read about an unrelated topic (fingerprinting). While they were reading the studies, the participants were offered chocolates to snack on. Afterward, they answered the question, “Obesity is caused more by…” using the same seven-point sliding scale as in the Canadian study.

Findings: The fake studies did prime the participants’ beliefs, as indicated by their responses to the obesity question—and those beliefs affected how much they ate. The average number of chocolates eaten was 3.7 among participants primed to believe the exercise theory, but just 2.5 among those primed to believe the diet theory. (The group that read about fingerprinting ate an average of 3.3 chocolates.)


Most scientific evidence indicates that diet is indeed the most important factor affecting weight. In the US, we are consuming on average 200 more calories per day than we did in 1980—and despite steady or increasing exercise levels, more Americans are overweight or obese than ever before.

Of course, no one is saying that exercise doesn’t help weight loss at all or that genes are irrelevant. But when it comes to weight control, the biggest determinant by far is what you put in your mouth. Believe that truth…and you may find it easier to keep your weight where you want it.

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