Citation needed.
The ability of human beings to control their biology through willpower is limited (if you don't believe me, try holding your breath for 30 minutes, or not drinking anything for a few days). Willpower itself is limited, and motivation is itself influenced by mental health. And diseases like Prader-Willi and Emery-Dreifuss demonstrate situations in which motivation and willpower are irrelevant.
And that's only what we know.
Sometimes when I have dinner with friends, I'll observe a situation where two of them will order the same thing, the bigger one will wolf down the dinner and the thinner one will pick at it and eat maybe half. I don't see two people equally tempted, where one gives it to his cravings and other is morally strong. I see two people with two different drives.
Why do different people respond so differently the same meal? What is it our society that leads so many people to consume a calorie surplus? It's disappointing the number of people that don't even want to investigate the biological and psychological origins of such phenomenon, instead claiming (without evidence) that it's all about moral failure.
In the 19th century, when painkillers first started becoming available, physicians would refuse to give them to woman in childbirth, because the Bible says childbirth is supposed to be painful. Women had sinned, therefore they must suffer. That's nonsense, of course, but people really wanted to hold onto their moral judgements, even at the expense of biology, good medicine, and relief of suffering. I think we should be suspicious of any impulse to use moral judgement to avoid investigation into the causes of medical conditions. Like obesity.