Dr. Friedman points to careful statistical analyses of the changes in Americans' body weights from 1991 to today by Dr. Katherine Flegal of the National Center for Health Statistics. At the lower end of the weight distribution, nothing has changed, not even by a few pounds. As you move up the scale, a few additional pounds start to show up, but even at midrange, people today are just 6 or 7 pounds heavier than they were in 1991. Only with the massively obese, the very top of the distribution, is there a substantial increase in weight, about 25 to 30 pounds, Dr. Flegal reported.

As a result, the curve of body weight has been pulled slightly to the right, with more people shifting up a few pounds to cross the line that experts use to divide normal from obese. In 1991, 23 percent of Americans fell into the obese category; now 31 percent do, a more than 30 percent increase. But the average weight of the population has increased by just 7 to 10 pounds since 1991.

Dr. Friedman gave an analogy: "Imagine the average I.Q. was 100 and that 5 percent of the population had an I.Q. of 140 or greater and were considered to be geniuses. Now let's say that education improves and the average I.Q. increases to 107 and 10 percent of the population has an I.Q. of above 140.

"You could present the data in two ways," he said. "You could say that the average I.Q. is up seven points or you could say that because of improved education the number of geniuses has doubled."