An unconditioned stimulus (US) is a stimulus that automatically triggers a reflexive response, with no prior learning needed.
No experience is required. Food placed in an animal’s mouth causing salivation, or a sudden loud noise causing a startle, are both examples: the reaction is innate, present from birth.
The unconditioned stimulus is one of four elements in classical conditioning, alongside the unconditioned response, the conditioned stimulus and the conditioned response.
Of these, the US is the engine. It is the only one whose power to produce a response comes from biology rather than experience.
Unlike a conditioned stimulus (CS), you do not have to learn to respond to the unconditioned stimulus; rather, the response occurs automatically.
Examples of unconditioned stimulus
Pavlov’s Dogs Experiment
Ivan Pavlov discovered classical conditioning in the early 1900s. He was researching the digestive reflexes of dogs at the time.
Ivan Pavlov noticed something odd. His dogs’ salivation shifted earlier and earlier, appearing before the food itself ever arrived.
The dogs were not responding to the smell or sight of food. They were responding to the sounds that reliably came just before it.
- Aim: To study the salivary reflex objectively, and to test whether an automatic food reaction could transfer to an arbitrary stimulus.
- Method: Pavlov presented food (the unconditioned stimulus) and measured salivation (the unconditioned response), sounding a bell just before the food on each trial.
- Results: Food reliably produced salivation with no training at all. After repeated pairings, the bell alone came to produce salivation, even with no food present.
- Conclusion: A reflex can be transferred from its natural, unconditioned stimulus to an arbitrary learned signal purely through pairing.
The food is the unconditioned stimulus and salivating for food is the unconditioned response: both occur automatically, without any learning required.
Little Albert (Watson & Rayner) Experiment
Behaviorist John B. Watson and graduate student Rosalie Rayner were the first psychologists to apply classical conditioning to human emotion. Their study, known as Little Albert, conditioned a fear response in a healthy infant.
Albert initially showed no fear of a white rat or the other neutral objects Watson and Rayner presented to him.
- Aim: To test whether fear could be classically conditioned in a human infant by pairing a neutral stimulus with a naturally frightening one.
- Method: Conditioning trials began when Albert was about 11 months old. Watson and Rayner paired a white rat with a loud clanging noise, the unconditioned stimulus that made Albert startle and cry.
- Results: After several pairings, the rat alone made Albert cry, and his fear generalised to other white, furry things, including a rabbit and a fur coat.
- Conclusion: A phobia-like fear can be manufactured by association, supporting a learned-acquisition account of emotional disorders.
The study’s limits are well known. It is a single-case demonstration with no control group, and it would be ethically impermissible today.
Advertising
Classical conditioning is an effective tool in marketing and advertising. Advertisers pair a product with positive features. Catchy music, bright colours and attractive spokespeople all help the ad create an enjoyable response.
The consumer later sees the product. They should feel that same positive feeling and be more likely to buy it.
In this example, the advertisement serves as the unconditioned stimulus (US), and the enjoyment from watching the advertisement serves as the unconditioned response (UR).
Because the product being advertised is mentioned in the advertisement, it becomes associated with the US, and then becomes the conditioned stimulus (CS).
This is a useful teaching simplification. The genuinely unconditioned power usually sits in the ad’s embedded elements, such as its music or faces, rather than in the advertisement as a whole.
Many of these elements are themselves already-learned, positively-charged stimuli. This is why the case is better described as evaluative conditioning: the transfer of liking from one stimulus to another.
The same logic applies below. One example of this kind of conditioning is seen in the “Drake Sprite: The Spark Commercial,” a Sprite commercial starring rapper Drake.
In this one minute advertisement, Drake is not able to rap very well and is feeling uninspired. A sip of Sprite changes that.
He is instantly reinvigorated. Here, the unconditioned stimulus is Drake, and the conditioned stimulus is drinking Sprite.
The conditioned response is thinking that a Sprite will leave you feeling refreshed and inspired, just like Drake.
Smoking
According to the theories of classical conditioning, drug-related stimuli can become associated with the rewarding aspects of using. This applies directly to smoking.
For example, drug paraphernalia, such as cigarette packets, bongs, and pipes, and environments where drug use occurs can invoke craving and drug-seeking responses in users. For smokers, just the sight of a cigarette packet can invoke the feeling of wanting to smoke.
This paraphernalia becomes a conditioned stimulus. Nicotine itself is the unconditioned stimulus.
Cue reactivity is the theory that people associate situations, such as meeting friends, or places, such as a pub, with the rewarding effects of nicotine. These cues can then trigger a feeling of craving on their own.
These factors become smoking-related cues. Prolonged use creates an association between them and smoking.
Nicotine is the unconditioned stimulus (UCS). The pleasure from the resulting rise in dopamine is the unconditioned response (UCR).
Littel and Franken (2012) studied this directly. In a higher-order conditioning experiment, smokers showed pronounced associative learning to neutral cues that had simply been paired with smoking-related stimuli. In other words, the neutral object alone came to trigger smoking-related associations.
Properties of an Effective Unconditioned Stimulus
Not every unconditioned stimulus conditions equally well. Three properties of the US itself govern how readily a neutral stimulus paired with it becomes a conditioned stimulus: its intensity, its biological significance, and its novelty.
US Intensity
A stronger unconditioned stimulus supports faster, stronger conditioning and a higher ceiling of responding. Annau and Kamin (1961) demonstrated this directly.
- Aim: Annau and Kamin (1961) tested how the strength of conditioned fear depends on the intensity of the unconditioned stimulus.
- Method: Rats received a tone paired with an electric shock. Different groups received shocks of different intensities, and fear was measured by how much the tone suppressed ongoing bar-pressing for food.
- Results: Stronger shocks produced stronger conditioned fear. Weak shocks produced little suppression, while intense shocks produced near-complete suppression.
- Conclusion: A US is not an all-or-none trigger; its magnitude sets a ceiling on how much fear a cue can come to command.
This magnitude property was later formalised as the parameter λ (lambda) in the Rescorla-Wagner model of conditioning, the maximum associative strength a given US can support.
Biological Significance and Preparedness
A second property is the US’s biological fit with particular cues.
Classical conditioning was traditionally assumed to obey equipotentiality, the idea that any neutral stimulus could become a conditioned stimulus for any US with equal ease. Garcia and Koelling (1966) broke this assumption.
Rats drank flavoured water, then became ill. They later avoided the taste, not the lights and sounds that had accompanied it.
Rats that were instead shocked showed the opposite pattern. They avoided the lights and sounds, not the taste.
Equipotentiality is false. An illness US belongs naturally with a taste; a pain US belongs with external sights and sounds.
Seligman (1971) called this preparedness. Organisms are evolutionarily predisposed to form some US-based associations far more readily than others. This is one reason human phobias cluster around ancestral dangers, such as snakes and heights, rather than modern hazards like cars.
Novelty and the US Pre-Exposure Effect
Conditioning proceeds best when the unconditioned stimulus is still new to the organism. Novelty matters.
If a US is presented alone, repeatedly, before conditioning begins, its later power to support conditioning is weakened. This is called the US pre-exposure effect.
The animal has already learned that the US is inconsequential. It is then slower to treat the US as significant, in a sense partly learned to ignore it.
This is the US-side counterpart of latent inhibition, the analogous slowing produced by pre-exposing the conditioned stimulus instead. The theme is a wider one. What an organism has already learned about a US changes how that US goes on to teach it.
A novel US is a more effective teacher than a familiar one, just like a novel taste in the taste-aversion studies above.
Learning Check
- Which of the following is an example of a conditioned response?
- Feeling hungry when smelling food.
- Jumping at the sound of a loud noise.
- Sweating when the temperature is high.
- Feeling anxious when hearing a dentist’s drill, after several dental visits.
2. Which of the following can be an unconditioned stimulus in classical conditioning?
- A bell that rings every time a dog gets food.
- A light that turns on every time a rat receives a shock.
- A piece of food that causes a dog to salivate.
- A tone that sounds every time a pigeon receives food.
3. What does an unconditioned response trigger automatically?
- A neutral response.
- A learned response.
- A reflexive or biological response.
- An unlearned response.
4. Which of the following is a conditioned stimulus?
- A sudden loud noise startles a person.
- The smell of food makes someone feel hungry.
- A bell sound that, after being associated with food, makes a dog salivate.
- The sensation of pain when touching something hot.
5. What is the difference between an unconditioned stimulus and a conditioned stimulus?
- An unconditioned stimulus requires learning, while a conditioned stimulus does not.
- An unconditioned stimulus is always food, while a conditioned stimulus is always a bell.
- An unconditioned stimulus naturally triggers a response, while a conditioned stimulus triggers a response after being associated with the unconditioned stimulus.
- There is no difference between an unconditioned stimulus and a conditioned stimulus.
Answers:
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The correct answer is D) Feeling anxious when hearing a dentist’s drill, after several dental visits. This anxiety is a conditioned response, as it is learned through repeated association of the drill sound with the uncomfortable experience of dental work.
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The correct answer is C) A piece of food that causes a dog to salivate. The piece of food is an unconditioned stimulus because it naturally, without prior learning, causes the dog to salivate.
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The correct answer is C) A reflexive or biological response. An unconditioned response is automatic and does not require learning or conditioning.
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The correct answer is C) A bell sound that, after being associated with food, makes a dog salivate. The bell sound has become a conditioned stimulus because it causes salivation after repeated association with the unconditioned stimulus (food).
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The correct answer is C) An unconditioned stimulus naturally triggers a response, while a conditioned stimulus triggers a response after being associated with the unconditioned stimulus. The unconditioned stimulus is inherent and naturally induces a response, while the conditioned stimulus only causes a response after learning or conditioning.
References
Annau, Z., & Kamin, L. J. (1961). The conditioned emotional response as a function of intensity of the US. Journal of Comparative and Physiological Psychology, 54(4), 428–432. https://doi.org/10.1037/h0042199
Chang, L. M., Wieser, M. J., & Wong, A. H. K. (2025). Increase in avoidance of learnt fear via unconditioned stimulus revaluation. Behaviour Research and Therapy, 193, 104813. https://doi.org/10.1016/j.brat.2025.104813
Clark, R. E. (2004). The classical origins of Pavlov’s conditioning. Integrative Physiological & Behavioral Science, 39 (4), 279-294.
Garcia, J., & Koelling, R. A. (1966). Relation of cue to consequence in avoidance learning. Psychonomic Science, 4(1), 123–124. https://doi.org/10.3758/BF03342209
Gorn, G. J. (1982). The effects of music in advertising on choice behavior: A classical conditioning approach. Journal of marketing, 46 (1), 94-101.
Harris, B. (1979). Whatever happened to little Albert?. American psychologist, 34 (2), 151.
Hawkins, D., Best, R., & Coney, K. (1998.) Consumer Behavior: Building Marketing Strategy (7th ed.). Boston, MA: McGraw-Hill.
Littel, M., Franken, I.H. Electrophysiological correlates of associative learning in smokers: a higher-order conditioning experiment. BMC Neurosci 13, 8 (2012).
Pavlov, I. P. (1897/1902). The work of the digestive glands. London: Griffin.
Pavlov, I. P. (1928). Lectures on conditioned reflexes. (Translated by W.H. Gantt) London: Allen and Unwin.
Pavlov, I. P. (1927). Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex . Translated and edited by Anrep, GV (Oxford University Press, London, 1927).
Pavlov, I. P. (1955). Selected works. Moscow: Foreign Languages Publishing House.
Rehman I, Mahabadi N, Rehman CI. Classical Conditioning . In: StatPearls[Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470326/
Rescorla, R. A. (1974). Effect of inflation of the unconditioned stimulus value following conditioning. Journal of Comparative and Physiological Psychology, 86(1), 101–106.
Reynolds, G., Field, A. P., & Askew, C. (2015). Preventing the development of observationally learnt fears in children by devaluing the model’s negative response. Journal of Abnormal Child Psychology, 43(7), 1355–1367. https://doi.org/10.1007/s10802-015-0004-0
Seligman, M. E. P. (1971). Phobias and preparedness. Behavior Therapy, 2(3), 307–320. https://doi.org/10.1016/S0005-7894(71)80064-3
Vladtv, YouTube(2010, November). Drake Sprite: The Spark Commercial. Retrieved December 15, 2019, from https://www.youtube.com/watch?v=mh6vHoyBs58 .
Watson, J.B. (1913). Psychology as the behaviorist Views It. Psychological Review, 20, 158-177.
Watson, J. B., & Rayner, R. (1920). Conditioned emotional reactions. Journal of experimental psychology, 3 (1), 1.
Critical Evaluation
The unconditioned stimulus is a well-supported construct, but it is not the simple, fixed trigger it can appear to be in textbook examples.
- A precise, measurable foundation: The US is a stimulus whose innate response, such as salivation or a startle, can be measured objectively, which let reflex learning be studied experimentally.
- Explains where learned triggers get their power: The US supplies the response that a conditioned stimulus later borrows, which is why removing the US causes extinction.
- Generative for theory: Analysing the US’s intensity, surprise value and revaluability led directly to the Rescorla-Wagner model and to modern reward-prediction research.
- Not an inert switch: What an organism learns depends on how surprising the US is, and the response it later produces depends on the US’s current, revaluable value (Rescorla, 1974).
- Equipotentiality is false: A US does not condition every cue equally; biological preparedness means some US-cue pairings form far more readily than others.
- Restricted to reflexive responding: The US-UR relationship covers involuntary reactions and does not by itself explain voluntary, consequence-driven behaviour.
Contemporary Research
The most active recent work on the US treats it as a malleable, updatable representation rather than a fixed input, with direct implications for anxiety and its treatment.
- Aim: Chang, Wieser and Wong (2025) tested whether avoidance of learned fear could develop through revaluing the unconditioned stimulus, without any new frightening experience with the learned cue itself.
- Method: In a sample of 66 participants, two neutral cues were each linked to a low-intensity US. One US was then increased in intensity for a Revaluation group, while a Control group’s US stayed unchanged.
- Results: The Revaluation group showed stronger avoidance of the cue linked to the revalued US than the Control group did. Conditioned fear to the cue itself did not reliably increase.
- Conclusion: Avoidance can grow purely by inflating the value of a US after the fact, a lab model of how anxiety may arise from reappraising a past event as worse than it was.
The reverse also holds. Reynolds, Field and Askew (2015) found that deflating a frightening US, by showing children a calm rather than fearful reaction, significantly reduced the fear and avoidance the children had learned.
Together, these findings point to devaluing a US as a possible route to treating fear, such as through exposure therapy or systematic desensitisation. The evidence remains at the experimental stage rather than a proven therapy.
Key Takeaways
- Definition: A US triggers an automatic response with no learning needed, such as food causing salivation or a loud noise causing a startle.
- US-UR Pair: The unconditioned response is the automatic reaction the US produces; a conditioned stimulus later borrows this power through pairing.
- Key Properties: A stronger, more biologically significant, or more novel US produces faster and stronger conditioning.
- Preparedness: Some US-cue pairings, such as illness with taste, form far more readily than others.
- Revaluation: Changing a US’s value after conditioning changes the response it produces, even with no further pairing.
- Modern Evidence: Recent research treats the US as an updatable representation, with implications for how anxiety develops and might be treated.
Unconditioned stimulus vs. conditioned stimulus?
An unconditioned stimulus naturally and automatically triggers a response without any learning. For example, food causes salivation in dogs.
On the other hand, a conditioned stimulus is a previously neutral stimulus that, after being repeatedly associated with the unconditioned stimulus, eventually triggers a similar response.
For example, if a bell is rung every time food is presented, the bell becomes a conditioned stimulus as it can cause salivation even without the food.
If you pair a neutral stimulus (NS) with an unconditioned stimulus (US) that already triggers an unconditioned response (UR), that neutral stimulus will become a conditioned stimulus (CS), triggering a conditioned response (CR) similar to the original unconditioned response.
What is the difference between the unconditioned stimulus and the discriminative stimulus?
A discriminative stimulus refers to something, like a person or an event, that precedes a behavioral response. So, the discriminative stimulus comes first and then, the behavior follows as a direct result of this stimulus.
A discriminative stimulus is created when the response is reinforced, whereas an unconditioned stimulus is a stimulus that leads to an automatic response without any reinforcement or prior learning.
What is the difference between the unconditioned stimulus and the neutral stimulus?
A neutral stimulus is a stimulus that does not produce an automatic response at first whereas an unconditioned stimulus is quite the opposite – an unconditioned stimulus is a stimulus that does produce an automatic response.
In classical conditioning, the neutral stimulus will become conditioned when consistently paired with an unconditioned stimulus.
When it is initially presented, the neutral stimulus does not affect behavior. However, as it is repeatedly paired with an unconditioned stimulus, it will begin to cause the same response as the unconditioned stimulus.
What is the difference between the unconditioned stimulus and the unconditioned response?
An unconditioned stimulus is a stimulus that leads to an automatic response. An unconditioned response is that automatic response to the stimulus. The dogs salivating for food is the unconditioned response in Pavlov’s experiment.
How does an unconditioned stimulus become a conditioned stimulus?
An unconditioned stimulus becomes a conditioned stimulus after the neutral stimulus becomes associated with the unconditioned stimulus and the subject has learned to associate the stimulus with a given outcome.
Is alcohol an unconditioned stimulus?
Yes, alcohol is an unconditioned stimulus in classical conditioning. The effects of alcohol (intoxication and inhibition) do not have to be learned. They are universal, consistent, and automatic.
