SD vs S-Delta: The One Mix-Up Even BCBAs Make on Camera

A discriminative stimulus (SD) is the green light. An S-Delta is the red light. Brian Middleton learned this the funny way when he hit record on his phone, lifted his baby out of her car seat with the family dogs circling for crumbs, and said "S Delta" out loud while pointing at the very thing that was actually an SD. He caught it on the playback, never published the clip, and brought it to his talk on purpose. The mix-up is that easy to make, even for a BCBA who teaches this stuff for a living.

This page is for the times you have to explain SD vs S-Delta to a new RBT, a parent, or a school team in plain words. We will keep it short and concrete, and we will use Middleton's own examples to make it stick.

What an SD does in a session#

An SD, short for discriminative stimulus, is a cue that has a history. When that cue shows up, a specific behavior has paid off before. So the behavior is much more likely to happen now.

Picture an RBT who always pulls out the iPad right after she sits down at the table. The kid learns fast. The RBT sitting down becomes the SD for "ask for the iPad." It is not magic. It is a track record. The cue and the payoff have walked together enough times that the cue alone gets the behavior going.

A few quick clinic-floor SDs:

• The crinkle of a fruit snack wrapper is an SD for "hand out, please."
• A timer ding is an SD for "stand up and switch tables."
• Mom picking up her car keys is an SD for "shoes on, head to the door."

In every case the cue is not the reinforcer. The cue is the heads-up that the reinforcer is on the table if the behavior happens now.

What an S-Delta does in a session#

An S-Delta is the opposite signal. It is a cue that says the same behavior is not going to pay off right now. The behavior is still in the kid's repertoire. It just goes quiet because past experience says it is not worth the effort here.

The grocery store self-checkout with the "closed" light on is an S-Delta for scanning your items. You still know how to scan. You walk past anyway.

In session, the office door being closed can become an S-Delta for "ask the supervisor a question." The kid handing you a card with no picture on it is an S-Delta for "honor that mand." A communication device that is powered off is an S-Delta for tapping the screen. The behavior does not get punished. It just does not get reinforced, so over time it does not happen in that condition.

Here is the part people miss. S-Delta is not the same as "do not do that." It is more like "save your energy, not here, not now."

The dogs-circling-the-baby example, and why the dogs are the SD#

Middleton's home video is the cleanest version of this lesson you will find.

Here's a good example of my goof and you'll catch it pretty quickly because I say S Delta instead of SD. This right here is a perfect example of an SD. I'm taking my baby out of her baby seat and the dogs start circling. Because as you can see, there is a possibility of cleanup on aisle four. From the talk — Brian Middleton

Let us slow down what is happening behaviorally.

The dogs have a history. Babies drop food. Babies drop more food during transitions. When dad lifts the baby out of the seat, crumbs are about to hit the floor. The "dad lifts baby" cue has paid off before. So the cue makes circling and sniffing much more likely. That is a textbook SD for the dogs.

If the dogs were sitting in the other room with the door shut, that closed door would be the S-Delta. The behavior of circling is still in their playbook. It just does not get reinforced through a closed door, so they do not bother.

On the recording, Middleton calls it an S-Delta in the moment, then corrects himself in the talk. Same person, same example, two names, one right answer. The dogs circling are responding to an SD.

Why even practicing BCBAs swap these terms on camera#

Three reasons this slip is so common.

First, the names are almost identical. SD and S-Delta share three letters and a hand-wavy bit of behaviorese. When you are recording yourself with one hand and lifting a baby with the other, the wrong one slides out of your mouth.

Second, most of us learned SD with the green-light frame and S-Delta as a footnote. The SD got drilled in DTT examples. The S-Delta got one slide. So under pressure, the brain reaches for "SD" or fumbles the opposite.

Third, the situations that contain both happen in the same five seconds. The kid sees the iPad on the table (SD for "ask"). The iPad screen is dark (S-Delta for "tap to play"). You are watching the same scene, naming two cues at once, and your tongue picks the wrong label.

It's okay to mess up. I'm going to actually give an example of me messing up that I never distributed. It's still on my phone because I recorded it and I'm like, oh crap, I need to do this. I need to fix this. From the talk — Brian Middleton

That is the bar. If a BCBA who teaches dissemination for a living wants to re-record before sending it to families, you can do the same with a script before parent training.

A 30-second test to know which one you are looking at#

Ask yourself one question about the cue in front of you.

When this cue is here, does the behavior you care about usually pay off?

• Yes, it pays off when this cue is here. That cue is an SD.
• No, it does not pay off when this cue is here. That cue is an S-Delta.

That is the whole test. Two answers, two labels.

A second question if you are still stuck. Does the behavior happen more often when this cue is around? If yes, SD. If the behavior fades when this cue is around, S-Delta.

You do not need to know the function in 30 seconds. You just need to know which direction the cue is pulling the behavior. Up means SD. Down means S-Delta.

How to explain the difference to an RBT in one sentence#

Here is the one-liner Middleton-style.

"An SD is the cue that says go for it because it usually works here. An S-Delta is the cue that says save your energy because it does not work here."

If the RBT wants a picture, tell them green light versus red light. Then walk to a table with toys and pick out two real cues in the room before lunch. You teach the distinction faster on the rug with real cues than you do with a slide deck.

Frequently asked questions#

Is an S-Delta the same thing as extinction?

No, and this trips a lot of people up. Extinction is a procedure where a behavior that used to be reinforced stops getting reinforced, and the behavior fades out over time. S-Delta is a cue that signals a behavior will not be reinforced right now, in this specific condition. The behavior is still healthy in other conditions. Think of S-Delta as "not here, not now." Extinction is "not anymore, period."

Can the same stimulus be an SD for one behavior and an S-Delta for another?

Yes, and it happens constantly. A closed office door is an SD for knocking and an S-Delta for walking in. Mom holding the iPad is an SD for "ask for it" and an S-Delta for "grab it." The stimulus does not have a built-in label. The label depends on which behavior you are tracking and what has paid off before.

Do you need to teach the S-Delta directly or does it form by default?

Both happen, and which one shows up depends on the learner and the contrast. If the SD gets reinforced and the S-Delta does not, the S-Delta usually forms on its own through differential reinforcement. For some learners and some skills, you have to plan and run discrimination trials so the contrast is sharp. New RBTs, complex multi-step skills, and learners with a thin reinforcement history almost always need explicit S-Delta trials.

Pull this off the page and into a real session#

The fastest way to lock SD vs S-Delta in is to say it out loud to another person while pointing at the cue. Middleton makes the point at the end of his talk.

It's okay to get stuck. Connect ideas, language and behavior analytic principles. So we're looking for those connections. We're looking for the relational networks and frames. And then of course we testing it out. And again, it's okay. If you fall flat, practice, practice, practice. This is where our verbal community comes in. From the talk — Brian Middleton

So pick one session this week. Before you start, name two SDs and two S-Deltas in the room out loud to your RBT. Get one wrong. Catch it. Fix it. That is the whole loop. It is the same loop a BCBA on camera with a baby and three dogs ran, and it is the loop that makes the difference stop slipping the next time you have to script it for a parent.

If you want to hear Middleton walk through the goof himself and the rest of the translation-fluency talk, the full session is one hour and earns one BACB CEU.