The principle hypothesis, generally accepted, is that obesity is caused by eating more calories than are expended, the excess being stored as fat. Reduce intake, increase output, or both, and fat comes off. It's a tidy equation. Overeating causes obesity. Suppose you come up with a competing hypothesis that says that over or under eating, and/or low or high energy output are caused by the accumulation of fat, i.e., a hypothesis that at first glance seems more complicated, but is actually -- Occam's Razor style -- simpler. What if, for whatever reason, a body simply accumulates fat, and overeating and sedentary behavior are in response to it? Can you see how that's simpler? So then the question becomes: what causes fat accumulation, which then sets off what in some ways is a positive feedback mechanism, including behaviors that are widely seen as causal rather than effects? Well I'm no expert at this, but Gary Taubes has spent the last several years pouring over studies going back as far as the 1800s. Rather than rehash it, I've got links for you accumulated from Chris at Conditioning Research. Big Fat Lie; Telegraph The Scientist and the Stairmaster; New York Magazine Interview with...