A reinforcer is a stimulus that follows some behavior and increases the probability that the behavior will occur. For example, when a dog's owner is trying to teach the dog to sit on command, the owner may give the dog a treat every time the dog sits when commanded to do so. The treat reinforces the desired behavior.
In operant conditioning (as developed by B. F. Skinner), positive reinforcers are rewards that strengthen a conditioned response after it has occurred, such as feeding a hungry pigeon after it has pecked a key. Negative reinforcers are stimuli that are removed when the desired response has been obtained. For example, when a rat is receiving an electric shock and presses a bar that stops the shock, the shock is a negative reinforcer— it is an aversive stimulus that reinforces the bar-pressing behavior. The application of negative reinforcement may be divided into two types: escape and avoidance conditioning. In escape conditioning, the subject learns to escape an unpleasant or aversive stimulus (a dog jumps over a barrier to escape electric shock). In avoidance conditioning, the subject is presented with a warning stimulus, such as a buzzer, just before the aversive stimulus occurs and learns to act on it in order to avoid the stimulus altogether.
Punishment can be used to decrease unwanted behaviors. Punishment is the application of an aversive stimulus in reaction to a particular behavior. For children, a punishment could be the removal of television privileges when they disobey their parents or teacher. The removal of the privileges follows the undesired behavior and decreases its likelihood of occurring again.
Reinforcement may be administered according to various schedules. A particular behavior may be reinforced every time it occurs, which is referred to as continuous reinforcement. In many cases, however, behaviors are reinforced only some of the time, which is termed partial or intermittent reinforcement. Reinforcement may also be based on the number of responses or scheduled at particular time intervals. In addition, it may be delivered in regularly or irregularly. These variables combine to produce four basic types of partial reinforcement. In fixed-ratio (FR) schedules, reinforcement is provided following a set number of responses (a factory worker is paid for every garment he assembles). With variable-ratio (VR) schedules, reinforcement is provided after a variable number of responses (a slot machine pays off after varying numbers of attempts). Fixed-interval (FI) schedules provide for reinforcement of the first response made within a given interval since the previous one (contest entrants are not eligible for a prize if they have won one within the past 30 days). Finally, with variable-interval (VI) schedules, first responses are rewarded at varying intervals from the previous one.
See also Behavior modification