The preference of rats for free or response-produced food

Three experiments were performed to investigate the preference of rats for either free or response-produced food. Rats were trained to free feed in an operant chamber and then to leverpress for food. Subsequently, they were given a choice between continuing to leverpress for food on CRF, FR 2, and FR 10 schedules or free feeding. In three experiments the independent variable was the method of free-food presentation: A fixed free-food dish was used in the first experiment, a movable dish in the second, and a large flat dish in the third experiment. The results of the three experiments were very similar, with most of the rats showing a preference for the free food. This preference increased further when more than one response was required to produce a food pellet. These results contradict any conclusion that rats have a generalized tendency to prefer response-produced to free food but provide no answer to the question of why rats do on occasion respond for food in the presence of free food.