Addiction: Rich Foods

One study found that when rats were given unrestricted access to tasty, calorie-rich food they ate twice as much and gained substantial weight.

Measurements in their brains indicated that the threshold for experiencing reward was elevated (in other words, they worked harder for the brain stimulus) and their overeating was not diminished by electrical shock to their feet. Eating too much rich food seems to trigger addiction-like responses in the brain circuits and most likely causes the development of compulsive eating habits. 

PositiveTip: In humans, restricted access to fattening foods for prolonged periods can help the brain reactivate its reward centers to respond to healthier foods.