Carson Chow, an investigator...

...at the National Institute of Diabetes and Digestive and Kidney Diseases, says in an article in the Times that one in three Americans are overweight. Also (my emphasis): 

Between 1975 and 2005, the average weight of Americans had increased by about 20 pounds. 

Why? 

The epidemic was caused by the overproduction of food in the United States. 

Beginning in the 1970s, there was a change in national agricultural policy. Instead of the government paying farmers not to engage in full production, as was the practice, they were encouraged to grow as much food as they could. At the same time, technological changes and the “green revolution” made our farms much more productive. The price of food plummeted, while the number of calories available to the average American grew by about 1,000 a day. 

Well, what do people do when there is extra food around? They eat it! This, of course, is a tremendously controversial idea. However, the model shows that increase in food more than explains the increase in weight. 
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Society has changed a lot. With such a huge food supply, food marketing got better and restaurants got cheaper. The low cost of food fueled the growth of the fast-food industry. If food were expensive, you couldn’t have fast food. 

People think that the epidemic has to be caused by genetics or that physical activity has gone down. Yet levels of physical activity have not really changed in the past 30 years. As for the genetic argument, yes, there are people who are genetically disposed to obesity, but if they live in societies where there isn’t a lot of food, they don’t get obese. For them, and for us, it’s supply that’s the issue. 

P. S. Carson Chow -- what a great name for someone interested in food!