To some it may seem far fetched that the that the alternative explanations to secondary reinforcement described above can be given serious consideration. Yet we only need remind ourselves of the proven potency of “novel” stimuli as reinforcers to realize the need for incorporating appropriate controls in our experiments. Monkeys will respond with great persistence when the only reinforce is an occasional chance to peek at the experimenter, an opportunity to manipulate gadgets, or other such “investigative behavior” (see review by Butler 1965).