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In operant conditioning, reinforcement is the presentation of a stimulus contingent on a response which results in an increase in response strength (as evidenced by an increase in the frequency of response). This concept has been criticized as circular, since it appears to argue that response strength is increased by reinforcement while defining reinforcement as something which increases response strength. Non-circular definitions have been proposed; for example, reinforcement can be defined as consummatory behaviour contingent on a response. Reinforcement is the key concept and procedure in the experimental analysis of behavior.

Whether the definition is circular or not, the study of reinforcement has produced strong, reproducible results. The effects of different schedules of reinforcement have been extensively studied. These schedules are:

Ratio schedules produce higher rates of responding than interval schedules, with variable ratio scales producing the highest rates of response. Variable ratio schedules produce the greatest resistance to extinction, which is the decline in response strength following the cessation of reinforcement.

Positive reinforcement is the contingent presentation of a stimulus following a response, resulting in an increased likelihood of the response occurring in the future. Negative reinforcement is the contingent withdrawal of a stimulus following a response, resulting in an increased likelihood of the response occurring in the future. Conditioned reinforcement, also called secondary reinforcement, is the presentation of a stimulus which has acquired reinforcing power through association with primary reinforcers (stimuli which are inherently reinforcing). Social reinforcement is a form of conditioned reinforcement in which the reinforcer involves some sort of interaction with others.

Successive approximation is the presentation of reinforcers after increasingly accurate productions of the desired response. In training rats or pigeons to depress a lever or peck a key; for example, reinforcement will initially be contingent on simply turning toward the lever or key. As training progresses, the response reinforced becomes progressively more like the response desired by the trainer. Behaviours developed through the reinforcement of successive approximations to the eventual desired behaviour are called shaped behaviours and the process is called shaping.