Motivating operations (MOs) and discriminative stimuli (SDs) are both antecedents, and both can make a behavior more likely right now. That shared effect is exactly why they get confused. The difference lies in why each one evokes behavior.
Why it matters
When you misread an antecedent, you pick the wrong intervention. If a child's requests depend on deprivation, adding more visual cues won't help much; you need to arrange motivation. If requests happen only with one therapist, adding snacks won't fix it; you need to bring requesting under the control of more people and places.
The distinction also shapes verbal behavior programming. Mands are controlled mainly by MOs, while tacts are controlled by nonverbal SDs. Practice vignettes often ask you to name what an antecedent is doing, with a tempting "SD" option for something that is really an MO.
Key ideas
- An SD is about availability. A stimulus becomes an SD when a response has been reinforced more often in its presence than in its absence. It evokes the response because it signals that the reinforcer can be obtained now.
- An MO is about effectiveness. An MO changes how valuable a consequence is at the moment (the value-altering effect) and changes how often behavior that has produced that consequence occurs (the behavior-altering effect).
- Both evoke, so "it made the behavior more likely" doesn't settle it. Ask what changed: the chance of getting the reinforcer, or how much the reinforcer is worth right now.
- The availability test. Imagine the antecedent is absent. If the response would still be reinforced when it occurs, the antecedent did not signal availability, so it is not an SD. If it changed how much the consequence mattered, it is an MO.
- They work together. An SD evokes behavior strongly only when a relevant MO is in effect. A "Water" sign at a park pulls you toward the fountain mainly when you are thirsty.
- Learning history differs. SD control always comes from a history of differential reinforcement. Unconditioned MOs (such as food deprivation, pain, or temperature changes) alter value without any learning; conditioned MOs depend on a learning history.
Definitions
- Discriminative stimulus (SD): a stimulus in whose presence a response has been reinforced and in whose absence it has gone unreinforced or reinforced less often, so it now evokes that response.
- Motivating operation (MO): an environmental variable that alters the reinforcing (or punishing) effectiveness of a stimulus and alters the current frequency of behavior that has been reinforced (or punished) by that stimulus.
- Value-altering effect: the increase (establishing operation) or decrease (abolishing operation) in a consequence's effectiveness.
- Behavior-altering effect: the immediate increase (evocative) or decrease (abative) in behavior that has produced that consequence.
- Transitive CMO (CMO-T): a stimulus that makes another stimulus valuable as a reinforcer, such as a locked cabinet making its key valuable.
- Reflexive CMO (CMO-R): a stimulus that has preceded some worsening (or improvement) and so makes its own removal (or continuation) a reinforcer.
Examples from a session
Same behavior, different antecedent
- Non-example of an SD (an MO at home): Aiyana, 7, skipped her afternoon snack. Before dinner she asks "Cracker?" again and again. Her parents give crackers on request every day, so crackers were available all along; deprivation made them more valuable.
- SD at a clinic: Ravi, 4, asks for the music toy only when Tomás, his therapist, is in the room. Tomás is the only adult who has handed the toy over for requests. Tomás's presence signals availability, so it is an SD.
- CMO-T in a community cooking class: Grace, 28, is making soup with an instructor who hands over any utensil she asks for. When she picks up a sealed can, she asks for the can opener. The instructor would have given it at any time, so the can didn't signal availability. It made the can opener valuable. The instructor's presence is the SD for asking.
Tip
A quick script for vignettes: "Without this antecedent, would the reinforcer have been delivered anyway?" If yes, look for an MO. If no, because responding is reinforced when the antecedent is present and not (or less often) when it is absent, you have an SD.
Common mistakes / exam traps
- Calling deprivation an SD. Going without food doesn't signal that food is available. A satiated person who asked for food would still get it; it just wouldn't function as a reinforcer.
- Assuming MOs only increase behavior. Satiation is an abolishing operation with an abative effect. MOs can lower value and suppress behavior too.
- Calling an S-delta an abolishing operation. Both lower responding. An S-delta signals that responding goes unreinforced; if the person still takes the reinforcer eagerly when it is offered, its value has not dropped.
- Calling every warning stimulus an SD. In avoidance, a signal that precedes something aversive is often labeled an SD, but Michael's analysis treats it as a reflexive CMO: it makes its own termination reinforcing.
- Thinking an SD works regardless of motivation. A strong SD can fail to evoke behavior when the relevant MO is absent, which can look like a skill deficit when it is really a motivation issue.
Exam trap
Both MOs and SDs have evocative effects. Options that distinguish them by "which one increases behavior" or "which one comes first" are wrong. The real difference is value (MO) versus availability (SD).
Exam trap
Transitive CMO vs. SD: the sealed can or locked door is not an SD for asking, because the item it calls for would have been handed over at any time. It makes that item valuable (CMO-T). The person who has handed the item over when asked is usually the SD.
Quick check
At Lincoln Middle School, the student store hangs an 'Open' sign only during hours when it sells snacks. Jaylen, 12, walks to the store when the sign is up and rarely when it is down. What function does the sign serve?
Related topics
- Motivating operations in action (B.16)
- Recognizing stimulus control (B.12)
- Recognizing stimulus discrimination (B.13)
- Kinds of reinforcers: unconditioned, conditioned, and generalized (B.7)
- The verbal operants: mand, tact, echoic, intraverbal, and more (B.19)
- Building motivating operations and discriminative stimuli into interventions (G.5)