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That's the provocative finding of a new working paper from Michael Anderson and David Matsa, economists at UC Berkeley and Northwestern, respectively.

It has become a truism that more people are eating out, and that the higher caloric content of food consumed at hamburger and pizza joints is making people a lot chunkier than they used to be. In fact, a number of studies have found a strong correlation between body weight and eating out.

But as any statistician will tell you, correlation doesn't prove causation.

To investigate the link, Matsa and Anderson turned to a natural experiment unknowingly created by the government decades ago. The Federal Highway Act of 1956 paved the way for over 42,000 miles of turnpike to be constructed in the United States. One of the unintended consequences of the new Interstate system was that, in rural areas, shopping centers that wanted to attract traveling drivers began to cluster around highways. For the residents of those areas, this development meant easier access to restaurants.

To see if this new supply changed obesity rates, Matsa and Anderson compared the Body Mass Index levels of those living close to highways (0 to 5 miles) with those living farther away (5 to 10 miles) and less accessible to restaurants. Using survey data from 11 states, Matsa and Anderson found no difference in the number of overweight, normal, and underweight people in the two areas, suggesting that access to restaurants was not making people fatter.

How can this be? If people are eating out more, and eating more high-calorie meals, why aren't they getting chubbier than those without easy access?

Matsa and Anderson next looked at data on individual eating habits from a survey conducted between 1994 and 1996. When eating out, people reported consuming about 35 percent more calories than when they ate in on average. But importantly, people reduced their caloric intake at home on days they ate. Overall, eating out increased daily caloric intake by only 24 calories.

Other research into the causes of rising obesity has centered on the role of technological change in modern economies. This paper from Darius Lakdawalla of RAND and Tomas Philipson of the University of Chicago estimates that 40 percent of the rise in obesity from the 1970's to the 1990's can be attributed cheaper food thanks to a more efficient agricultural industry. The other 60 percent was chalked up to more sedentary lifestyles.

Matsa and Anderson's findings suggest that New York City's move to force many restaurants to list the caloric content of menu items will likely have little to no effect on obesity levels.
 
The findings also suggest that I shouldn't feel so bad about having Taco Bell (NYSE:YUM) for dinner three times a week.

Source: Provocative Finding: Fast Food Doesn't Make You Fat