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Although the term reinforcement has many common uses and associated meanings, its meaning is precise when used by behavior analysts and behavior therapists. The act or process of making a reinforcer contingent on behavior is termed positive reinforcement, and a reinforcer is any object or event that, when delivered following some behavior, increases the probability that the behavior will occur again. A typical example might evolve from a laboratory experiment with rats. A rat is placed in a small plastic chamber. The rat can press a lever located on one wall of the chamber. When the rat presses the lever, a small food pellet drops into a dish. If the rat returns to the lever and continues to press it would be said that the food pellet functions as a reinforcer that the behavior is maintained by positive reinforcement.

There is often confusion between positive reinforcement and negative reinforcement. Negative reinforcement occurs when a behavior results in terminating an aversive stimulus. In the case of the rat, the negative stimulus might be a loud noise. A lever press turns off the stimulus. If the rat continues to press the lever, it would be said that loud noise functions as a negative reinforcer and the behavior is maintained by negative reinforcement. Thus, both positive and negative reinforcement refer to increases in behavior, but differ in whether a pleasant stimulus is presented as the result of some behavior (positive reinforcement). Negative reinforcement is also referred to as escape (if the response turns off the stimulus each time it appears) or avoidance (if the response can postpone presentation of the stimulus).

It is important to note that reinforcement is a concept that refers to the relationship between behavior and its consequences. Stimuli or events are not assumed to have inherent reinforcing effects. For example, although most people like money and will continue to exhibit behavior that results in obtaining money, it cannot be assumed that money functions as a reinforcer for everyone. For example, money might not serve as a reinforcer for a monk devoted to an ascetic lifestyle. The defining characteristic of reinforcement depends on how a behavior is changed and not on the types of things that serve as reinforcing events (Morse & Kelleher, 1977). Factors that help determine whether a given object or event is reinforcing or punishing for a given individual include that individual's previous experiences and other features of the environment that coexist and are associated with the object or event. The upshot is that different things may function as reinforcers for different people.

DRUGS can serve as reinforcers that maintain drug-seeking and drug-taking behaviors. This can be observed in the prevalence of drug use among humans and has also been shown in laboratory research with animals. In a typical laboratory experiment, the animal such as a rat or monkey has a catheter placed in a vein and connected to a pump-driven syringe. The animal can press a lever to activate the pump, and this results in a dose of a drug such as COCAINE, HEROIN, NICOTINE, or ALCOHOL being infused into the vein. If the animal continues to press the lever to obtain the drug, then the drug is said to serve as a reinforcer. Interestingly, those drugs which lead to ADDICTION in humans also serve as reinforcers in animals. The only exception is MARIJUANA (THC), which is used fairly extensively by humans but does not function as a reinforcer in animals. It should be noted that drugs that serve as reinforcers under one condition may not serve as reinforcers under other conditions. For example, nicotine serves as a reinforcer only at low doses and when doses are properly spaced. Nevertheless, the observation that drugs of abuse generally function as reinforcers in experimental animals has brought the study of drug-seeking behavior and drug abuse into a framework that allows carefully controlled behavioral analyses and the application of well-established and objective behavioral principles (Schuster & Johanson, 1981).

The acquisition of drug use in humans predominantly involves positive reinforcement, whereas the maintenance of drug use can involve both positive and negative reinforcement. The ability of a drug to serve as a positive reinforcer is usually associated with its pleasurable subjective effects (e.g. a "rush", a "high", or other feelings of intoxication). But again, given the definition of reinforcement, it is not necessary for a drug to be subjectively reinforcing or pleasurable in order for it to maintain behavior. Many drugs are also associated with symptoms of WITHDRAWAL when abstinence is initiated following a period of regular use. In this case, taking the drug again may terminate the aversive state of withdrawal; in this way, drug use is maintained by negative reinforcement. Drug use can also be influenced by sources of reinforcement other than the direct effects of the drug. For example, social encouragement and praise from a peer group can play an important role in the development of drug use by teenagers. Biological factors may also come into play. For example, some individuals may be more or less susceptible than others to feeling and recognizing the pleasurable effects of drugs. When drug use is viewed as a behavior maintained by the reinforcing effects of drugs, it suggests that this behavior is not amoral or uncontrolled but rather that it is the result of normal behavioral processes.