The same conclusion will be reached if our obese subject undergoes bariatric surgery. “This procedure alters gastrointestinal anatomy to reduce caloric intake beyond what could be achieved volitionally,” explains Jeff Friedman of Rockefeller University in a recent issue of Nature Medicine. “Although people who undergo bariatric surgery lose a significant amount of weight, nearly all remain clinically obese.” We will now have two individuals of more similar size and weight, one of whom needs a surgically altered gastrointestinal tract to reduce calorie intake so much that he can stay at that weight, and the other who doesn’t and can eat to his heart’s content. Our surgical patient is perceived as defective in character, having had to rely on surgery to curb his appetite. Our naturally lean subject is not, despite the possession of an identical appetite. “The implication,” as Friedman noted, “is that something metabolically different about morbidly obese individuals results in obesity independently of their caloric intake.”

Whatever the accepted wisdom, making obesity a behavioral issue is endlessly problematic. “Theories that diseases are caused by mental states and can be cured by will power,” as Susan Sontag observed in her 1978 essay Illness as Metaphor, “are always an index of how much is not understood about the physical terrain of a disease.” This is certainly the case with obesity. One goal of any discussion of the cause of obesity must be a way to think about it that escapes the facile and circular reasoning of the overeating/sedentary-behavior hypothesis and permits us to proceed in a direction that leads to real progress, to find a way of discussing the condition, as the philosopher of science Thomas Kuhn might have put it, that allows for a “playable game.”