Comparative studies of the preference for free vs response-produced reinforcers

Neither domesticated hooded rats nor wild black rats responded appreciably for water on a CRF schedule when free water was available. However, four out of five crows showed substantial responding for food in the presence of identical free food. Previous reports also indicate that type of reinforcer and species are variables which significantly influence this phenomenon. Nevertheless, the overall findings seem to be amenable to an explanation based upon incentive-motivation theory.