ABA glossary
Discriminative stimulus
Also called SD
Definition
An antecedent stimulus in whose presence a response has been reinforced and in whose absence it has not, so the response becomes more likely when the stimulus is present. Abbreviated SD. It signals that reinforcement is available; it does not change how much the reinforcer is wanted.
In plain words
An SD is a signal learned through experience. Because a behavior has paid off when this stimulus was around (and not when it was absent), the behavior now occurs more often in its presence. In teaching, the instruction and materials, such as "Touch the cup" with three cards on the table, are the planned SD. In daily life, an "Open" sign is an SD for walking into a shop and a ringing phone is an SD for answering. The SD makes the response more likely because of that history, not because it forces the response.
Example from a session
In discrete-trial teaching, the RBT puts three pictures on the table and says, "Touch dog." The instruction plus the dog picture is the SD for touching the dog. When Kai touches the dog he earns a token; touching the other cards does not. After many trials Kai reliably touches the dog when asked and not when the RBT says "Touch cat." The RBT presents the SD the same way each time, as written in the program.
For BCBA candidates and supervisors
The SD is the first term of the three-term contingency (SD, response, consequence); the four-term contingency adds the motivating operation in front of it. Analysts build stimulus control on purpose: they choose which stimulus should control the response, reinforce the response in its presence and not in S-delta conditions, and then transfer control from prompts to the natural SD by fading prompts. They also guard against faulty stimulus control, such as a learner responding to card position or the teacher's glance instead of the instruction. Telling an SD from an MO (task B.17) is a frequent exam discrimination.
Often confused with motivating operation, S-delta
An SD makes a response more likely because it signals that reinforcement is available; an MO does so because it changes how valuable the reinforcer is. A child who asks for juice only when Dad is in the kitchen is responding to Dad as an SD. A child who asks for juice more after a salty snack is under the control of an MO. An S-delta is the opposite of an SD: it signals that the response will not be reinforced.
Exam tip
Ask whether the stimulus tells the learner the reinforcer is available, or makes the reinforcer more wanted. Available means SD; wanted means MO. A stem that describes a history of reinforcement only with a particular person, place, or instruction is pointing to an SD.
Can you spot discriminative stimulus in a scenario?
Practice BCBA B.12 with exam-style questions and instant feedback.