Don't blame me!
But simple correlations between restaurant visits and overeating may conflate the impact of changes in supply and demand. People choose where and how much to eat, leaving restaurant consumption correlated with other dietary practices associated with weight gain.
A key question say Michael L. Anderson and David A. Matsa in Are Restaurants Really Supersizing America is whether the growth in eating out is contributing to the obesity epidemic, or whether these changes merely reflect consumer preferences.
The interesting causal parameter is how much more an obese person consumes in total because he or she ate at a restaurant. To the extent that changes in preferences are leading consumers to eat out more, regulating restaurants may only lead consumers to shift consumption to other sources rather than to reduce total caloric intake.
In part using data food intake data collected by the U.S. Department of Agriculture (USDA) and US Census data, the Berkeley pair found there is selection bias in who eats at restaurants; people who eat at restaurants also consume more calories than other consumers when they eat at home. "Second, when including individual fixed effects," they write, "we find that people who eat large portions in restaurants tend to reduce their calorie consumption at other times during the day. After accounting for these factors, we find that although the average restaurant meal contains approximately 250 calories more than the average meal eaten at home, the existence of restaurants increases BMI by only 0.2 BMI points for the typical obese consumer"
The abstract for the journal article concludes:
While many researchers and policymakers infer from correlations between eating out and body weight that restaurants are a leading cause of obesity, a basic identification problem challenges these conclusions. We exploit the placement of Interstate highways in rural areas to obtain exogenous variation in the effective price of restaurants and examine the impact on body mass. We find no causal link between restaurant consumption and obesity. Analysis of food-intake micro-data suggests that consumers offset calories from restaurant meals by eating less at other times. We conclude that regulation targeting restaurants is unlikely to reduce obesity but could decrease consumer welfare.