Height, while determined largely by genetics, also is influenced by childhood nutrition. Adults in the early 1960s grew up during tougher times when they may not have had enough to eat, Klein said.

"Things were not so plentiful here," he said. In recent years, there have been "very few starving kids." On the contrary, many are being overfed.

The weight gain trend is typically reported as what portion of all children or all adults are overweight. Those numbers are also alarming. In 1999-2002, 31 percent of adults had a BMI of 30 or more, double the rate from the early 1960s.

About two in three adults in 1999-2002 were considered overweight.

The explanations center on too much food or not enough exercise.

More television channels and remotes to surf them. Computers and video games. E-mail. Technological advances often mean people move around less.

"Everyone has a leaf blower. Ten years ago, people had rakes," David Katz, director of Yale University's Prevention Research Center.

Changes in neighborhoods can contribute to the trend. People stay inside if it is not safe on the streets and may not go to the store if sidewalks are lacking.

Food is to blame, too. Portions have gotten bigger. People go out to eat more. Junk food that stays fresh for a long time is more readily available, meaning it is easier to find a bag of cookies or potato chips in the cupboard than an orange, which may go bad in a few days.

Among the report's other findings:

—For men, the increase in weight was most dramatic among older men. Those 60 to 74 were nearly 33 pounds heavier in 1999-2002 than men that age in the early 1960s.

—For women, the difference was starkest among the young. Women 20 to 29 were nearly 29 pounds heavier.

—The average weight for a 10-year-old boy went from 74.2 pounds in 1963 to nearly 85 pounds by 2002. The average girl's weight rose from 77.4 pounds to nearly 88 pounds.

—An average 15-year-old boy weighed 135.5 pounds in 1966 and 150.3 pounds by 2002. The average weight for a teenage girl went from 124.2 pounds to 134.4 pounds.

The report, "Mean Body Weight, Height and Body Mass Index, United States, 1960-2002," was based on data from the National Health and Nutrition Examination Survey, which uses actual body measurements.