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Since Jensen (1963), the phenomenon of "contrafreeloading" corresponds to the fact that animals which have been conditioned to perform a task (by skinnerian conditioning for instance) continue to behave as they have been taught to even when a more simple and effortless solution is made available to them (for instance free access to abundant food in skinnerian situation). This behaviour has been considered as an exception to the "law of the least effort" (Tolman, 1955). To explain it, two types of theories have been put forward. The first one assumes the existence of positive reinforcers connected with the acquired activity, the other one of aversive stimulus connected with the most effortless solution (such as neophobia in the case of cup of abundant food). A reconsideration of the analysis of operant behaviour has led us to take into account the localisation of reinforcing agents and to integrate it in our experiments. The results that have been obtained cannot be accounted for by the theories mentioned above.