It may be easier to believe that we remain lean because we're virtuous and we get fat because we're not, but the evidence simply says otherwise. Virtue has little more to with our weight than our height. When we grow taller, it's hormones and enzymes that are promoting growth, and we consume more calories than we expend as a result. Growth is the cause - increased appetite and decreased energy expenditure (gluttony and sloth) are the effects. When we grow fatter, the same is true as well. We don't get fat because we overeat; we overeat because were fat.