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When using reinforcement in animal training, we often think of using food, like meat‐flavored treats. Food is a biologically based reinforcer, along with others such as water, shelter, and mating, and these are all called primary or unconditioned reinforcers. The same goes for punishers. Some stimuli are unconditioned and function as punishers because of their inherent aversiveness, such as a painful electric shock.

It is obvious, especially when analyzing human behavior, that most of what influences behavior is not a piece of food or access to a mate. Instead, human behavior is often influenced by stimuli that are more complex. For example, students study to get good grades, employees work for money, and children draw silly cartoons for their parent’s approval. These stimuli (i.e., grades, money, and approval) get their reinforcing efficacy through the individual’s prior learning experience. Without an associative learning history, a good grade or a dollar bill are unlikely to produce any behavior changes. In this respect, they begin as neutral stimuli. Neutral stimuli acquire reinforcing function by being paired with an already established reinforcer. After repeated pairings of the neutral stimulus and a reinforcer, the neutral stimulus becomes a conditioned reinforcer. This should sound familiar! The classical conditioning process of stimulus‐stimulus pairings results in the capacity for neutral stimuli to become conditioned reinforcers or conditioned punishers (Williams 1994).

Conditioned reinforcement has been thoroughly investigated in the behavioral laboratory. In the laboratory, when pigeons earn food reinforcers by pecking a key, a grain dispenser, also called a food hopper, is made accessible for a certain period of time so that the pigeon can consume the primary reinforcer (grain). When the food hopper activates, it produces a distinct sound. After repeated pairings of the sound and food, the sound itself becomes a conditioned reinforcer and thus can strengthen behavior (Kelleher 1961). This means that the pigeon will peck at the key just to produce the hopper sound!

Conditioned reinforcers have been shown not only to be effective in strengthening or maintaining behavior, but they can also establish new behavior (Alfernik et al. 1973). A dog is not born wanting to play with toys, but when that toy is paired with primary reinforcers such as social interaction, the toy itself can reinforce a response. The toy can be used to reinforce behaviors the dog already knows as well as behaviors that the dog is learning.

Although conditioned reinforcers can maintain learned responses and establish new ones, they are at risk of losing their reinforcing value if they aren’t periodically paired with the unconditioned reinforcer. If the pigeon’s key pecks produced only the sound of the hopper but no food, after a while the pigeon would stop pecking. The sound will only function as a reinforcer if it is occasionally paired with food. Similarly, money maintains its reinforcing value because it can be exchanged for goods and services. If someone tried to use Canadian dollars in the United States, the Canadian dollars will lose their reinforcing value quickly because they are no longer paired with other reinforcers.

The same concepts that apply to conditioned reinforcers also apply to conditioned punishers. For example, some dog owners use invisible fencing systems to keep their dogs within the boundaries of their yard. When the dog approaches a boundary, the dog’s collar emits a tone and then shortly thereafter a shock. After some experience hearing the tone and then experiencing the shock, the tone alone is aversive to the dog, and the dog refrains from approaching the boundaries. Once a stimulus becomes a conditioned punisher, it can successfully diminish behaviors beyond the context in which they were first paired. That same collar tone could be used to suppress barking, jumping, or potentially any other operant behavior. However, the punishing effects of the tone will eventually wear off if the shock no longer accompanies it. For a conditioned punisher to maintain its suppressive effects, it too must occasionally precede the unconditioned punisher.

Readers experienced in animal training may wonder why we don’t discuss clickers in this section. For those non‐animal trainers, clickers are hand‐held devices that, when pressed, make a clicking sound. Clickers and similar devices (such as whistles) are discussed by animal trainers as conditioned reinforcers because they are paired with food. However, this function has been questioned (Dorey and Cox 2018), and more research needs to be conducted to make this claim.