Weight loss is one of the most common goals people have. However, when looking for ways to lose weight, 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.

As a result, there are differing beliefs about the best way to lose weight (ie. what specific steps one should take).

New research shows that these beliefs have real consequences - they predict an individual's actual body mass. In a series of studies forthcoming in the journal , Brent McFerran of the University of Michigan and I examined what regular people believe causes obesity. We asked over 1,200 people from five countries on three continents about their beliefs regarding the primary cause of obesity, and found a roughly equal split between people primarily blaming overconsumption of calories and those blaming a lack of exercise. More interestingly, those who blamed a lack of exercise were more likely to be overweight, and indeed ate more food.

Why is this? People's beliefs guide their actions. If we believe overeating causes obesity, and we want to lose weight, we cut our intake. If, on the other hand, we believe weight gain stems from insufficient exercise, we try to increase our activity.

However, relative to the increased exercise, reducing caloric intake is actually much more likely to be effective.

This is not news for the medical profession. A recent editorial in the stated it quite simply: "Clearly, environmental causes for obesity are far more influential than genes. Obesity results from overnutrition and the primary therapeutic target is preventing or reversing overeating. Exercise is associated with weight loss but its duration or intensity has minor effects on weight loss relative to diet." Consistent with this, we surveyed physicians at the University of Michigan, and found that they too were more likely to blame poor diet.

Diet control is more effective for many reasons. First, people often reward themselves for their exercise with an indulgent treat, and end up consuming more calories 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 generally overestimate the amount of calories burned during exercise and underestimate calories they eat.

So you believe you burned 500 calories in that run? You are probably guessing too high. And that bubble tea has 200 calories, right? Here your estimation is probably too low. Third, most people do not have the time in the day needed to truly "exercise off" the extra calories they eat. If you eat a 2,000 calorie lunch, it could take most of your waking hours to burn off the excess calories.

So what's the prescription here? The health of the body often depends on the health of the mind, and our finding that those who believe poor diet causes obesity are less likely to be overweight supports the medical conclusion that the most effective path to reducing obesity is indeed diet control. We are not advocating ceasing your exercise regimen, or that exercise does not help lose weight. It does - but only if calorie intake doesn't also increase alongside.