ABA Concepts Explained: What Is a Discriminative Stimulus (SD) in ABA?

Written By: Tara Karen, M.S. Ed, BCBA, LBA

Discriminative Stimuli are central to the concept of operant conditioning and Applied Behavior Analysis. In technical terms, an SD is an environmental cue or event that signals that reinforcement is available for a particular behavior. It can be a direction, the sight of something enticing, or the behavior of another person.

Understanding Discriminative Stimuli: The Basics

To fully grasp this concept, it is necessary to explain the basics of operant conditioning.

The Role of Discriminative Stimuli in Operant Conditioning

Operant conditioning is a type of learning where behavior is increased or decreased based on the consequences that follow a behavior that was performed. If a behavior is followed by a desirable outcome, we are more likely to repeat that behavior (this is reinforcement). If a behavior is followed by an undesirable outcome, we are less likely to repeat the behavior (called punishment).

The “3-term contingency” is a tool used in ABA to analyze behavior and focuses on the relationship between the behavior (action) and what happens before and after it occurs. It can be broken down into the ABC model:

• A – Antecedent: What happens immediately before the behavior.
• B – Behavior: The observable and measurable action an individual performs.
• C – Consequence: What happens immediately after the behavior.
  
  

How Discriminative Stimuli Function as Environmental Cues

So where does the discriminative stimulus fit in? The SD is a type of antecedent, meaning it occurs before the action/response/behavior. It is a signal that reinforcement is likely to follow a particular behavior performed, based on the learner’s past experiences. In simpler terms, it tells the individual, “If you do this behavior right now, there is a good chance something desirable will happen.”

It is important to emphasize that the SD does not cause a behavior—it simply increases the probability that the behavior will occur because, in the past, that behavior has been reinforced under similar conditions. A discriminative stimulus functions as a cue in the environment, informing the learner that a specific response is likely to be successful. Think of a neon light flashing “reinforcement is available (should you choose to act)!” This analogy captures a key idea: the SD presents an opportunity, not a requirement. The individual retains the choice to act or not.

The presence of a discriminative stimulus does not compel a response—it signals that a particular behavior is likely to be effective if chosen. For example, I am driving home from work and I see those Golden Arches on the side of the road. The McDonald’s sign is the SD for me to go in and purchase delicious French fries. The decision is mine on whether I drive through to get the fries or drive straight home to eat leftovers.

Another example: A young child sees their parent enter the room. Based on their history of interaction, the child has learned that reaching toward their parent or vocalizing often results in comfort, such as a hug or soothing words. The parent’s presence becomes an SD: it signals that certain behaviors (e.g., reaching or babbling) are likely to result in reinforcement (the hug or soothing words). The child is not required to act at that moment, but the opportunity is there.

Discriminative Stimuli vs. Prompts: Key Differences

Many times, a discriminative stimulus and a prompt are confused. While they are both antecedents that increase the possibility of a behavior occurring, they have fundamental differences in purpose and function. A Discriminative Stimulus signals that reinforcement is available for a specific behavior, whereas a prompt offers support to increase the likelihood of a correct response. An SD does not tell the learner specifically how to respond, and should be present in the final, independent behavior. A prompt specifically will help the learner respond correctly and should be faded over time so the learner is independent.

Example in Therapy: A therapist holds up an orange and says, “What is this?” This is the SD —it indicates that answering “orange” may lead to praise, a toy, or a high five. If the child does not respond, the therapist may then give a vocal model for the word “orange.” This added support is a prompt temporary strategy to assist the learner in producing the expected behavior. As the child becomes more independent, prompts are gradually faded so that the child learns to respond directly to the SD alone.

Examples of Discriminative Stimuli in Everyday Life

Discriminative stimuli are not exclusive to ABA therapy—they appear throughout daily life. Recognizing them can help providers and families support learning in natural settings. Microwave Beeping: The beep serves as an SD for opening the microwave and retrieving food. This action has likely been reinforced many times with a warm meal. Presence of a Teacher: A student may raise their hand more often when a teacher is nearby, having learned that attention or help is more likely in that context.

Best Practices for Using Discriminative Stimuli in Therapy

In ABA therapy, SDs should be carefully selected to help children learn when and how to use new skills. A discriminative stimulus should be clear, consistent, use a neutral and natural tone, given only once (not repeated), and tell the person what to do (instead of “Don’t hit, we should say “hands down.” Over time, the therapist may vary the SDs to promote generalization such as saying “Find the, show me the, touch the, give me the item.”

For example, in this receptive identification program the therapist places a ball, book, and puzzle in front of a child and says, “Touch the book.” This instruction is the SD that signals reinforcement will occur for selecting the correct item.

Promoting Independence with Discriminative Stimuli

The therapist may initially provide prompts to support success but will fade them over time as the learner responds more independently. The reinforcement that follows the correct response should be individualized for the child to keep them motivated.

Conclusion: The Power of Discriminative Stimuli in ABA

The discriminative stimulus (SD) is a foundational tool in ABA that helps signal when a behavior is likely to be reinforced. It plays a significant role in teaching, learning, and navigating everyday environments. Understanding what an SD is—and what it is not—enables providers and caregivers to support learners in a way that is consistent, respectful, and effective. When used thoughtfully, SDs allow individuals to build skills, increase independence, and engage with their environment on their own terms. Whether in a therapy session or daily routine, the presence of an SD is not about control, it is about opportunity.