Translated from the Portuguese original. Read the original (Português)
A behavior is under stimulus control when it becomes more (or less) likely in the presence of a given signal in the environment. This happens because, in the past, that behavior was reinforced in the presence of that signal and was not reinforced in its absence. The signal that "announces" that acting is worthwhile is the discriminative stimulus (SD); the one that announces it is not worthwhile is the S-delta (SΔ). Important: the signal does not force or trigger the behavior — it only makes the occasion more favorable.
Stimulus control occurs when the rate, latency, duration, or intensity of a behavior is altered by the presence of an antecedent stimulus. This control is the product of a learning history: the behavior was reinforced in the presence of a certain stimulus (the discriminative stimulus, SD) and was not reinforced in its absence (S-delta, SΔ). The response is then said to have come under the control of that antecedent.
The two signals: SD and SΔ
Imagine a traffic light. The green light doesn't push you into the street — but, in its presence, crossing "works out" (you reach the other side safely). The red light doesn't hold you in place either — but, in its presence, crossing "doesn't work out." Over time, your behavior comes to respond differently to each color. That is exactly what stimulus control is.
- Discriminative stimulus (SD): the signal in the presence of which the behavior has already been reinforced. It signals that reinforcement is available — like the green light.
- S-delta (SΔ): the signal in the presence of which the behavior has not been reinforced. It signals that reinforcement is not available — like the red light.
How stimulus control is taught
No one is born knowing how to obey a traffic light — that is learned. The procedure that produces stimulus control is called discrimination training: the behavior is reinforced in the presence of the SD and stops being reinforced (extinction) in the presence of the SΔ. Because the consequence now depends on the signal, the person begins to respond differently to each situation — which we call discrimination.
Tap the cards to see the three concepts behind this idea:
The signal doesn't "trigger" the behaviorA common mistake is thinking that the SD elicits (sets off) the response, the way a scare triggers a jump. That is not the case: that would be respondent behavior. The SD only makes the occasion more likely for an operant behavior — the person remains free to act or not. Signaling is not forcing.
Discrimination and generalization: two sides of the same coin
Stimulus control lives in a useful tension. On one hand, we want the person to tell apart situations (discrimination). On the other, we want them to transfer what they learned to similar situations (generalization). Explore both:
Responding differently to different stimuli
A child learns to say "dog" only in the presence of dogs — not cats. The behavior becomes narrowly controlled by certain features of the stimulus. When control is too narrow in the wrong way (e.g., the child only responds to the therapist, and to no one else), we call it restricted stimulus control, something to be avoided in teaching.
Responding the same way to similar stimuli
After learning to cross at one traffic light, a person crosses at any green traffic light — even ones they've never seen before. The response "spreads" to stimuli that share properties with the original SD. In teaching, generalization is a desired goal: learning is of little use if it only works in the therapy room.
Where this shows up in practiceTransferring control from a prompt to the natural stimulus is the essence of much of structured teaching: you start by pointing out the correct response and gradually withdraw the help (fading), until the everyday stimulus alone is enough. This is also how colors, letters, matching tasks, and independence routines are taught.
Test your understanding
Quick quiz
Three questions to reinforce the concept. You'll see the explanation right away.
Further reading
- Cooper, J. O.; Heron, T. E.; Heward, W. L. Applied Behavior Analysis. 3rd ed. Pearson, 2020 (chapter on stimulus control).
- Skinner, B. F. Science and Human Behavior. New York: Macmillan, 1953.
- Catania, A. C. Aprendizagem: Comportamento, Linguagem e Cognição. 4th ed. Porto Alegre: Artmed, 1999.
- Moreira, M. B.; Medeiros, C. A. Princípios Básicos de Análise do Comportamento. 2nd ed. Porto Alegre: Artmed, 2019.
This entry is educational and does not replace assessment, diagnosis or treatment by a qualified health professional. Every person is unique — for a specific case, seek individual guidance.