Lay theories of obesity predict actual body mass.

Obesity is a major public health problem, but despite much research into its causes, scientists have largely neglected to examine laypeople's personal beliefs about it. Such naive beliefs are important because they guide actual goal-directed behaviors. In a series of studies across five countries on three continents, we found that people mainly believed either that obesity is caused by a lack of exercise or that it is caused by a poor diet. Moreover, laypeople who indicted a lack of exercise were more likely to actually be overweight than were those who implicated a poor diet. This effect held even after controlling for several known correlates of body mass index (BMI), thereby explaining previously unexplained variance. We also experimentally demonstrated the mechanism underlying this effect: People who implicated insufficient exercise tended to consume more food than did those who indicted a poor diet. These results suggest that obesity has an important, pervasive, and hitherto overlooked psychological antecedent.