Ask anyone: Americans are getting fatter and fatter. Advertising campaigns say they are. So do federal officials and the scientists they rely on.

But Dr. Jeffrey Friedman, an obesity researcher at Rockefeller University, argues that contrary to popular opinion, national data do not show Americans growing uniformly fatter.

Instead, he says, the statistics demonstrate clearly that while the very fat are getting fatter, thinner people have remained pretty much the same.

Let it be said that Dr. Friedman, a Howard Hughes Medical Institute investigator and the discoverer of the gene for leptin, a hormone released by fat cells, is not fat. He is tall and gangly, with the rumpled look of an academic scientist.

As an obesity researcher, he might be expected to endorse the prevailing view that obesity in this country is out of control. But Dr. Friedman said he was outraged by the acceptance of what he sees as a hurtful myth, one that encourages people to believe that if you are fat, it is your fault.

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.''

He added, ''The whole obesity debate is equivalent to drawing conclusions about national education programs by saying that the number of geniuses has doubled.''

Not everyone agrees.

''It' s one thing to talk about statistics and another to talk about what's happening to individuals,'' said Dr. Marion Nestle, a professor of nutrition, food studies and public health at New York University. ''Everyone notices that there are more overweight people now.''

Dr. Friedman, however, begs to differ. The statistics let scientists get beyond impressions and focus on the evidence.

He is, in a way, an unexpected figure to insert himself into the highly charged politics of obesity. He left clinical medicine in 1980 after discovering that his true passion was the laboratory. By 1981, he had begun his scientific career, and within a few years he was taking on what seemed like an impossibly onerous task, finding a gene whose absence made mice grow massively obese.

He keeps mementos from those days. He still has the purchase order, from December 1986, for the first batch of mice he used for the experiment. Hanging on his office wall is a framed strip of white paper with black blotches, the data that on Sunday morning, May 8, 1994, revealed he had found the gene that he named leptin.

''To me, those data are as beautiful as the Mona Lisa,'' he said.

Over the years, Dr. Friedman says, he has watched the scientific data accumulate to show that body weight, in animals and humans, is not under conscious control. Body weight, he says, is genetically determined, as tightly regulated as height. Genes control not only how much you eat but also the metabolic rate at which you burn food. When it comes to eating, free will is an illusion.

''People can exert a level of control over their weight within a 10-, perhaps a 15-pound range,'' Dr. Friedman said. But expecting an obese person to decide to simply eat less and exercise more to get below the obesity range, below the overweight range? It virtually never happens, he said. Any weight that is lost almost invariably comes right back.

The same goes for gaining weight in general, Dr. Friedman argued. A person who has the genes to be thin is not going to get fat because portion sizes increase. It makes no scientific sense, he said.

But isn't it true that we can decide to eat or not, choosing to skip dinner, say, or pass up dessert? Isn't that free will? Not really, Dr. Friedman said. The control mechanisms for body weight operate over months, even years, not day to day or meal to meal.

''People live in the moment,'' he said. ''They lose weight over the short term and say that they have exercised willpower,'' but over the long term, the body's intrinsic controls win out. And just as willpower cannot make fat people thin, a lack of it does not make thin people fat.

No one, he says, can consciously calibrate their food intake as precisely as the body does naturally. Most people's weights remain steady, within about 10 pounds, year in and year out. But when people count calories, they typically err by about 10 percent. For someone who eats 750,000 calories in a year, that 10 percent error would add up to 75,000 calories, or about 25 pounds.

Obesity, Dr. Friedman says, is a problem; fat people are derided and they have health risks like diabetes and heart disease. But it does no one any good to exaggerate the extent of obesity or to blame the obese for being fat.

''Before calling it an epidemic, people really need to understand what the numbers do and don't say,'' he said.