It's no secret that Americans have been getting fatter over the last several decades. But in fact, weight has been rising for more than 150 years, as shown by the economic historians Dora Costa and Richard Steckel. From the Civil War to the 1990s, the weight of a 6-foot-tall American male increased by about 30 pounds on average.
These historical trends are not hard to understand. As we have gotten wealthier and more technologically advanced, food has gotten cheaper and work more sedentary. Both these factors have contributed to rising weight over the time-frame of centuries, and the recent rise in obesity has likewise been fueled by reductions in the price of food.
Since 1976, food has fallen in price by more than 12% compared to other goods. My colleague Tomas Philipson and I have shown that this reduction in price can explain at least half the recent growth in obesity. Shin-Yi Chou, Michael Grossman and Henry Saffer reached similar conclusions about the importance of price. In addition to its overall price, they stressed the increasing availability of food service establishments.
While it is not entirely clear whether restaurants make people heavier, or heavier people attract more restaurants, there is no question that eating is cheaper and easier than it used to be. As if that were not enough, the most calorie-dense foods have seen the biggest price reductions. David Cutler, Edward Glaeser, and Jesse Shapiro have shown that technological advances have especially lowered the price of processed and snack foods -- like french fries and vending machine treats -- which are particularly high in calories.
The evidence above suggests that obesity is a by-product of prosperity and technological advance.
(Source: Greg Mankiw's Blog)