Why have Americans become more obese?

Americans have become considerably more obese over the past 25 years. This increase is primarily the result of consuming more calories. The increase in food consumption is itself the result of technological innovations which made it possible for food to be mass prepared far from the point of consumption, and consumed with lower time costs of preparation and cleaning. Price changes are normally beneficial, but may not be if people have self-control problems. This applies to some, but not most, of the population.