Beginning in the 1980s, the average weight of Americans began to soar, and by 2002, 31 percent were classified as obese. Although the reasons for this epidemic remain controversial, researchers have implicated at least four developments: The first is the decrease in energy expenditure throughout the 20th century, following the introduction of automobiles and the replacement of high-energy blue-collar work by low-energy office occupations. The second is the growing affluence of Americans, who could now afford more and better food. The third is the technological transformation of the food industry in the past 30 years that has made available cheap, convenient, high-calorie and tasty foods. The last is the decline through much of the 20th century in extended breast-feeding, which tends to reduce an infant's chances of obesity in later life.

As a consequence of these shifts, the average weight of Americans increased throughout the century. But because many were underweight in the earlier years, the epidemic of obesity did not become apparent until the 1980s, when these historical developments apparently reached critical mass.Dietary fat has not played a major role in the epidemic. Although clinical trials have shown that reducing dietary fat leads to weight reduction, the average amount consumed has declined in recent decades. Heavy television viewing by children is linked to obesity, yet there is no definitive proof of a causal relation. The decline in smoking and the popularity of eating out--restaurants typically serve extra-large portions--have also been blamed, but the evidence remains contradictory.