There were some good comments to my earlier posts that I wanted to address. First, I am not talking about food reward. I am talking about eating when not hungry, whether that food is high reward or low reward.
Here's a new study to illustrate what I'm talking about. The study was designed to look at dietary compensation in solid foods versus beverages. But here I'm focused only on the solid food part. Adults were fed additional fruits and vegetables (equal to 20% of daily caloric needs) to their diet. In the lean adults, there was full dietary compensation and no weight gain. In the overweight group, there was incomplete compensation and weight gain. So as weird as it sounds, additional fruits and vegetables led to weight gain, even though they are very low reward compared to junk food.
I'm focused on just the general hunger mechanism - if someone eats when they are not hungry, it seems likely they will gain weight. As shown in that study, even overconsuming healthy (Paleo) food in the absence of physiological need can cause weight gain.