“In the Redish model, the excess dopamine provides additional value, no matter what. Marks et al. (2010) directly tested this hypothesis in an elegant experiment, where rats were trained to press two levers for a certain dose of cocaine (both levers being equal). One lever was then removed and the other provided smaller doses of cocaine. The Redish theory predicts that the second lever should gain value, while expectation of homeostatis theories would predict that the second lever should lose value (because animals would learn the second lever was providing smaller doses). The Marks et al. data was not consistent with the Redish excess-delta model. However, a key factor in drug addiction is that not everyone who takes drugs loses control over their drug use and becomes an addict. Studies of drug use in both human and nonhuman animals suggest that most animals in self-administration experiments continue to show elasticity in drug-taking, stopping in response to high cost, but that a small proportion (interestingly similar to the proportion of humans who become addicted to drugs) become inelastic to drug-taking, being willing to pay excessive costs for their drugs. One possibility is that the homeostatic models are a good description of nonaddicted animals, which have a goal of maintaining a satiety level, but that addiction is different.”