The theory that learning is dependent on the alleviation or satisfaction of a drive; more specifically, that an organism will acquire new responses only when it is motivated by a need and receives a reward that meets that need.This principle was first proposed by the psychologist Clark Hull in 1943 to explain the conditions under which learning takes place. If we want to teach a dog to stand up when we hold up our hand, we first make sure he is hungry by not feeding him for some time, then we hold a piece of food high enough so that he has to stand up to look at it, and finally we drop it into his mouth while he is standing up. By repeating this process a number of times he will learn to associate the hand signal with standing and will eventually stand when we hold up our hand even though he is no longer rewarded with food. The theory holds that this association is firmly established and reinforced by repeatedly giving him a reward that reduces (i.e. satisfies) his hunger drive.The central idea of the theory is that no learning can take place unless the organism (animal or human being) is goaded by a drive that makes it tense and uncomfortable, and receives a reward of some kind that relieves this discomfort. The drive reduction hypothesis has inspired a great deal of experimentation since it permits the conditions for learning to be carefully regulated. Hunger, thirst, sex, and other drives can be increased or reduced to see how these changes affect the speed of learning—for example, when rats were kept thirsty for six hours they were found to learn more rapidly than when they were deprived of water for two hours. This technique is also used in attempting to discover the effects of rewards on physiological processes, and the changes that occur in an organism when learning takes place. Studies of hunger and thirst have contributed a limited amount of information on this aspect of the problem.Recent research indicates that the Hullian theory does not fully explain the learning process. Both animals and human beings have been shown to acquire new responses when there appears to be little possibility of drive reduction. Rats, for instance, will learn when the only “reward” is an increase in illumination (Roberts, Marx, Collier, 1958), and monkeys learn when the only reward is the opportunity to look at another monkey or to hear a colony of monkeys (Butler, 1953, 1957). Other animals have acquired new responses when they were merely given an opportunity to explore a maze (Montgomery, 1954). Electrode implantation experiments have shown that animals will learn to press levers and perform other operations when correct responses are followed only by electrical stimulation of certain areas of their brain.All these experiments show that learning can apparently occur in animals without involving the satisfaction of physiological needs. This is even more true of human learning. Many people engage in study and research because of the intrinsic satisfaction that learning itself brings, and not because of drives that make them tense or uncomfortable. We even acquire information without realizing we are doing so and without having a problem to solve, a phenomenon called incidental or latent learning.