The studies asked subjects to eat at certain establishments and then estimate the calorie content of the foods they had consumed. On average, in one study patrons of Subway underestimated their meals by 151 calories as compared to those eating at McDonald's. This difference could lead to a possible weight gain of nearly five pounds a year if such meals were eaten on a regular basis.

Dr. Pierre Chandon of the research institution INSEAD in Fontainebleau, France, and Dr. Brian Wansink of Cornell University in Ithaca, New York, suggest that thinking objectively about calorie content will help diners avoid getting swayed by the perception of one food being healthier than another ("People Eat More in ‘Healthy' Restaurants," Journal of Consumer Research, October 2007).

Of course, if we are aware of this tendency, we can be realistic about the calorie content. It is much better for our health to get calories filled with vitamins and minerals than empty calories from fat and sugar, for example.