Being fat isnt genetic, but instincts and tendencies are hereditary. It may be a gene that the rest of us may carry that make us more able to not be gluttonous and lazy. If this is completely false then it has been proven that stupidity causes obesity and intelligence is very much hereditary. But while not all stupid people are fat, it will still weed out a large portion of stupid people who can't control themselves.  You are right that being fat isn't genetic obviously, but with a change in environment it is the people who are able to adapt to it that survive. We are the ones that have been able to stay fit and healthy even with a surplus of cheap, processed food. Self control and discipline are taught but we may just be better than fats by default. If that is not the case, we are at least more intelligent.