​Evaluate the adequacy of the set-point model and the positive incentive model to explain obesity.

​A. The set-point model hypothesizes that weight is homeostatically controlled.
1. Deviations in either direction activate feelings of hunger that prompt eating or lack of hunger that motivate weight loss.
2. The model explains why people can maintain a constant weight, why people who lose weight often gain it back, and why some people find it difficult to gain weight.
3. The model fails to explain individual variations in set-point or why some people's set-point should be at the obese level, but it is the most comprehensive of the models.
B. Genetic explanations of obesity come in two varieties.
1. One version explains the tendency to gain weight because of a "thrifty" metabolism that protected against starvation in human prehistory.
2. One version examines twins and adopted children and has determined that there is a genetic component for weight and fat distribution.
C. The positive incentive model holds that a variety of positive reinforcers can accompany eating, and these reinforcers may lead to overeating and consequently to obesity.
1. People have several types of motivation to eat, including personal pleasure, pleasant social surroundings, and biological factors.
2. Personal pleasure can come from the act of eating, including the taste of food and how pleasurable eating is at any particular time.
3. The social context of eating includes the cultural background as well as the other people present and whether they are eating or not.
4. The biological factors include the length of time since the person's last ingestion of food as well as blood glucose levels.
5. The positive incentive model predicts a variety of body weights, depending on food availability, personal experiences with food, cultural encouragement to eat various kinds of food, and the cultural view of ideal weight.
6. The positive incentive model does a better job than the set-point model in explaining individual variations in body weight.