A. J. LIEBLING, THE CELEBRATED AUTHOR of The New Yorker’s “On Press” column, once wrote that he had enunciated a journalistic truth with such clarity that it was suitable for framing. “There are three kinds of writers of news in our generation,” Liebling wrote. “In inverse order of worldly consideration, they are:

1. The reporter, who writes what he sees.

2. The interpretive reporter, who writes what he sees and what he construes to be its meaning.

3. The expert, who writes what he construes to be the meaning of what he hasn’t seen.

“To combat an old human prejudice in favor of eyewitness testimony,” Liebling wrote, “the expert must intimate that he has access to some occult source or science not available to either reporter or reader. He is the Priest of Eleusis, the man with the big picture…. All is manifest to him, sincehis conclusions are not limited by his powers of observation.”

Leibling was talking about journalism, but a similar ranking holds true in medicine. In fact, the medical experts have the further advantage that they can disseminate their opinions with considerably greater influence. They can make their case with the imprimatur of the institutions that employ them—the American Medical Association, for instance, or Harvard University. They can easily attract the media’s attention. Physicians’ case reports and the patients’ anecdotal experience have a fundamental role in medicine, but if these conflict with what the experts believe to be true, the experts’ opinions win out.

This conflict between expertise and observational evidence has had a significant influence in the science of obesity. Reliable eyewitness testimony has come only from those who have weight problems themselves, or the clinicians who regularly treat obese patients, and neither group has ever garnered much credibility in the field. (The very assumption that obesity is a psychological disorder implies that the obese cannot be trusted as reliable witnesses to their own condition.) But it is these individuals who have the firsthand experience. When Hilde Bruch reported in 1957 that a fine-boned girl in her teens, “literally disappearing in mountains of fat,” lost nearly fifty pounds over a single summer eating “three large portions of meat” a day, it was easier for the experts to ignore the testimony as a freakish phenomenon than to contemplate how such a thing was possible. But the process of discovery in science, as the philosopher of science Thomas Kuhn has put it, only begins with the awareness that nature has violated our expectations. Often it is the unconventional events—the anomalous data, as these are called in science—that reveal the true nature of the universe.