In 1983, Jules Hirsch of Rockefeller University framed this enigma in the form of two alternative hypotheses. One was the common belief “that obesity is the result of a willful descent into self-gratification.” The other was the “alternative hypothesis that there is something ‘biologic’ about obesity, some alteration of hormones, enzymes or other biochemical control systems which leads to obesity.” Because no such biologic abnormality had been unambiguously identified, Hirsch believed, “it is perhaps better to maintain the illusion that obesity is not an illness. It is more pleasant to believe that it is no more than an error of good judgment and that better judgments and choices will eventually lead” to a better outcome.

Here is another apparent contradiction: it may be true that, “for the vast majority of individuals, overweight and obesity result from excess calorie consumption and/or inadequate physical activity,” as the Surgeon General’s Office says, but it also seems that the accumulation of fat on humans and animals is determined to a large extent by factors that have little to do with how much we eat or exercise, that it has a biologic component.

The deposition of fat in men and women is distinctly different. Men tend to store fat above the waist—hence the beer belly—and women below it. Women put on fat in puberty, at least in the breasts and hips, and men lose it. Women gain weight (particularly fat) in pregnancy and after menopause. This suggests that sex hormones are involved, as much as or more than eating behavior and physical activity. “The energy conception can certainly not be applied to this realm,” as the German clinician Erich Grafe observed in 1933 about this anatomical distribution of fat deposits and how it differs by sex.

Fat, or lack of it, runs in families and even does so, noted the pediatrician-turned-psychiatrist Hilde Bruch in 1957, with such characteristic shapes or body types that “this similarity may be as striking as facial resemblance.” And if girth has a genetic component, then that means it is regulated by biological factors—perhaps tilted in one direction for those who gain weight easily, and tilted in another for those who don’t. “It is genetics, and not the environment, that accounts for a large proportion of the marked differences in individual body weight in our population today,” wrote the Rockefeller University molecular biologist Jeffrey Friedman in 2004. If obesity does have such a significant genetic factor—“equivalent to that of height, and greater than that of almost every other condition that has been studied,” according to Friedman—then how does this figure into the equation of overeating and sedentary behavior?