Translated from the Portuguese original. Read the original (Português)
The three-term contingency — also called the ABC model — is the simplest way to describe a behavior without taking it out of context. There are three links: the Antecedent (what happens before and sets the stage), the Behavior (what the person does), and the Consequence (what happens after and changes the chance that it will happen again). It is the “basic unit” of Behavior Analysis: to understand why someone does something, we look at all three together — never at the behavior in isolation.
In Behavior Analysis (Skinner; Cooper, Heron & Heward), the three-term contingency is the fundamental unit of analysis of operant behavior. It describes the relation among three terms: an antecedent (a stimulus or event that precedes and signals the occasion), a response (the behavior), and a consequence (the stimulus that follows and alters the future probability of the response). The word contingency names exactly this “if… then…” dependency relation between behavior and consequence.
The three links: A, B, C
The acronym comes from English: Antecedent, Behavior, Consequence. The big idea is simple: no behavior happens in a vacuum. There is always a context that comes before and an effect that comes after — and it is that effect that decides whether the behavior tends to happen again.
What each term means
Each link plays a different role. Tap to explore:
Antecedent — what sets the stage
This is the stimulus or event that happens immediately before the behavior and signals that the behavior might “pay off” at that moment. The phone that rings, the light that turns green, someone's question, a colleague's presence. The antecedent does not force anything: it simply makes the behavior more likely right then. When an antecedent comes to indicate that reinforcement is available, it is called a discriminative stimulus.
Behavior — what the person does
This is the response itself: answering the phone, crossing the street, answering the question, asking for help. To be analyzed, it needs to be described in an observable way — something you could see or hear, not a vague label (“she was agitated”). The clearer the description, the easier it is to see its relation to what comes before and after.
Consequence — what comes after
This is what follows the behavior and changes the chance that it will happen again. If the consequence strengthens the behavior, that was reinforcement; if it weakens it, that was punishment. The key point: the consequence acts on the future — it teaches the organism what to do (or stop doing) next time.
“Contingency” means dependencySaying that the consequence is contingent on the behavior means it depends on it: if the behavior occurs, then that consequence follows. It is this “if… then…” relation — not mere succession in time — that gives the ABC model its power to explain and predict.
Three everyday examples
The same A → B → C skeleton shows up in very different situations. Flip the cards:
Why this matters in practice
The ABC model is the lens that turns “behavior that is hard to understand” into something analyzable. Instead of asking “why is she like that?”, we ask:
- What happened right before? (antecedent)
- What exactly did the person do? (behavior, described in an observable way)
- What came right after? (consequence)
Observing many episodes this way reveals patterns — this is the heart of applied behavior analysis and of assessing challenging behaviors. Discovering the function of a behavior (what it “achieves” as a consequence) makes it possible to teach a better path to the same goal, instead of just trying to suppress it.
The basic unit of operant behaviorIt was B. F. Skinner who formalized the three-term contingency as the unit of analysis of operant behavior. This A-B-C arrangement is the basis of the ABC recording used in functional assessment and one of the most well-established concepts in the field.
Sometimes we talk about four termsBefore the antecedent, there can be a motivating operation — something that, at that moment, changes the value of the consequence (being thirsty makes water worth more). When we include this link, we speak of a four-term contingency. The ABC remains the skeleton; the motivating operation explains why the consequence matters more or less at each moment.
Test your understanding
Quick quiz
Three questions to help the concept stick. You'll see the explanation right away.
Further reading
- Skinner, B. F. Ciência e Comportamento Humano (Science and Human Behavior). 1953.
- Cooper, J. O.; Heron, T. E.; Heward, W. L. Applied Behavior Analysis. 3rd ed., 2020 — chapters on the three- and four-term contingency.
- Catania, A. C. Aprendizagem: Comportamento, Linguagem e Cognição. 4th ed.
- Michael, J. “Motivating operations”. In: Concepts and Principles of Behavior Analysis, 2004.
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.