The number of Americans who weigh more than 20 percent of their recommended weight has grown from 20 percent of the population to 33 percent. Physicians are concerned because of the increasing incidence of weight-related diseases, such as diabetes and high blood pressure.

On the brighter side, economists say increasing average weight is a sign of the increasing wealth of society -- and the increasing cost of exercise. Consider the impact on caloric consumption of these changes in economic life:

• The price of food has declined tremendously in the past few decades, making it cheaper to consume calories.

• Physical activity has decreased as work has become more productive and sedentary.

• Most people can afford to drive rather than walk.

• The value of time has increased, increasing the price people must pay to exercise.

Economics also explains why Americans weigh comparatively more than Europeans. Average U.S. income is higher than in any European country, based on purchasing power parity. Food and gas are cheaper in America than in Europe. And the quality of television is higher in the U.S. Thus, Americans eat more, drive more places and watch more TV. By contrast, Europeans are poorer, eat less, walk to work and shop, and have fewer couch potatoes.

Source: "The Economics of Obesity," Economic Intuition, Winter 2000. Based on: "Tomas J. Philpson and Richard A. Posner, "The Long-Run Growth in Obesity as a Function of Technological Change," Working Paper No 7423, November 1999, National Bureau of Economic Research.