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An important aspect of learning is emitting certain behaviors at certain times or in certain contexts. Otherwise, energy and time are wasted emitting behaviors when the desired consequence is unlikely to happen. We answer the phone when it rings, a cat runs to the sound of the can opener, a trained scent detection dog sits when they smell the target scent, and drivers stop at intersections when the light is red. A stimulus that precedes operant behavior, called an antecedent, can become correlated with the consequences that follow behavior. When an antecedent stimulus exerts control over whether or not a behavior occurs, it is said that behavior is under stimulus control. Stimulus control explains why animals don’t engage in random behavior all the time—an antecedent stimulus that an animal experiences at any given moment signals the animal to behave in ways that are likely to produce reinforcement and avoid behaving in ways that are likely to result in punishment or extinction. In other words, the antecedent stimulus “controls” the occurrence of behavior because it signals that the behavior will be reinforced, punished, or extinguished. A thorough analysis of behavior in terms of operant conditioning usually entails looking at the ABCs: antecedents, behaviors, and consequences (see Table 3.3).

Table 3.3 The ABCs of behavior analysis.

	Description	Example
Antecedent	A stimulus that precedes a response	Mailperson walks down the street
Behavior	The organism’s response to the antecedent	The dog barks
Consequence	The stimulus change that follows the behavior (addition or removal of a stimulus)	The mailperson crosses the street and thereby reinforces the dog’s barking behavior

In technical terms, if the presentation of a stimulus reliably evokes an operant response, the stimulus is called a “discriminative stimulus.” In application, a discriminative stimulus is often called a “cue.” For a stimulus to reliably function as a discriminative stimulus, the same rules for creating strong associations apply. The cue needs to reliably and consistently signal a certain consequence if a behavior occurs. Naive trainers sometimes attempt to train their pet to sit by repeatedly saying “sit.” After saying “sit” a dozen times, the pet sits and gets a treat. Unfortunately, “sit” never becomes a reliable cue because the pet did not sit most of the time the cue was presented. However, after a few pairings of the trainer saying “sit” once and the dog sits, and the dog is unlikely to sit when the trainer refrains from saying “sit,” then the behavior can be said to be under stimulus control.

Antecedent stimuli can reliably evoke behavior after being correlated with reinforcement. Antecedent stimuli can also reliably inhibit behavior by signaling that there is no chance that reinforcement will occur following a behavior (i.e., extinction). A stimulus that signals extinction is termed s‐delta. Stimuli that signal whether a behavior will be reinforced or not are ubiquitous and very effective in guiding behavior. An “Out of Order” sign on a vending machine tells us that using the machine will not yield us any goodies. In animal behavior, research has found that dogs don’t beg from people who aren’t looking at them (Udell et al. 2011). A person not looking at the dog is effectively an “Out of Order” sign to the dog that means if the dog begs, it is highly unlikely that she will get food. If the person is making eye contact with the dog, it is more likely that food will be given.

One area of training in which “tight” stimulus control is desired is landmine detection. A non‐profit organization called Anti‐Persoonsmijnen Ontmijnende Product Ontwikkeling (APOPO) employed the use of giant pouched rats to help with de‐mining areas of Africa (Poling et al. 2010). The rats are given extensive training on detecting the odor of a landmine and emitting a behavior to indicate that they found an explosive. So that each landmine is identified and removed safely, it is imperative that the explosives have perfect stimulus control over the indicator behavior. That is, the behavior must occur each time a landmine is found, it must never occur when there is no landmine present, it must never occur in response to a different stimulus, and no other behavior should occur in the presence of the landmine (Pryor 1999). Otherwise, it would be a waste of resources to dig for a landmine that isn’t there and would put lives at risk if a landmine is missed.