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Unconditioned Stimulus in Classical Conditioning – Psychology

| Unconditioned Stimulus In Classical Conditioning | Examples | Unconditioned Stimulus vs Neutral Stimulus | Unconditioned Stimulus vs Conditioned Stimulus | Little Albert Experiment |

What is an unconditioned stimulus?

In classical conditioning, an unconditioned stimulus (US or UCS) is defined as any stimulus that can naturally and automatically trigger a response without prior learning or practice. It is also called the primary reinforcer. The involuntary response is a reflex triggered whenever the UCS is present.

Unconditioned stimulus in classical conditioning

Russian physiologist, Ivan Pavlov, first discovered classical conditioning when he was feeding his dogs. The dogs would smell the food and automatically salivate. This natural response did not require any prior learning. So the food’s smell was the UCS​1​. This phenomenon is also called the Pavlovian conditioning.

So, how to find unconditioned stimuli?

A UCS can trigger a response naturally. This response is a biological reaction. A person or animal usually does not have control over this behavior​2​.

A dog begs for food with a bowl of dog food in front of it, is an example of unconditioned stimulus definition

Unconditioned stimulus examples

Here are some examples of the unconditioned stimulus.

  1. Touching a hot iron
    Touching a hot iron makes you withdraw your hand right away.
    The hot iron is the UCS.
  2. Eating
    Putting food into your mouth causes your mouth to water.
    The food is the UCS.
  3. Hurting your foot
    Dropping a rock on your foot makes you scream in pain.
    The rock is the UCS.
  4. Inhaling dust
    Dust entering your nose causes you to sneeze.
    Dust is the UCS.

The Difference Between Unconditioned Stimulus and Neutral Stimulus

An unconditional stimulus elicits a natural, reflexive response, called the unconditioned response (UCR).

A stimulus that doesn’t naturally elicit a response is a neutral response. For example, food is a UCS for dogs and can cause salivation. But ringing a bell by itself doesn’t trigger the same response. The bell’s sound is hence a neutral stimulus.

dust enters nose causing woman to sneeze example of unconditioned stimulus

The difference between Unconditioned Stimulus and Conditioned Stimulus

A neutral stimulus initially doesn’t trigger any particular response. However, when a neutral stimulus is presented together with a UCS, an association can form.

Classical conditioning happens when a neutral stimulus is paired with an UCS repeatedly to create associative learning. The previously neutral stimulus becomes a conditioned stimulus (CS) and can trigger the same response as the UCS.

So initially, the neutral stimulus does not affect a specific behavior. But after repeatedly presented together with the UCS, it becomes a CS, and the person or animal subconsciously learns to react with the same response when it’s present. This response is then called a conditioned response (CR) or learned response.

For example, in Pavlov’s experiments, he sounded a bell whenever he brought food to his dogs. After multiple repetitions, the dogs learned to expect food and naturally salivated when they heard the bell’s sound even when they did not see the food. When UCS (food) was paired with a previously neutral stimulus (the sound of a bell), the neutral stimulus became a CS.

child tries to touch white rat unconditioned stimulus psychology definition

Little Albert Experiment

Another example of using an unconditioned stimulus to condition a subject is the infamous “Little Albert Experiment.”​3​ 

In this experiment, psychologist John B. Watson exposed the infant to a white rat. When the infant attempted to touch the rat, he would make a loud noise to scare him. Several repetitions later, the child cried upon seeing the rat alone.

In this case, the loud noise was an unconditioned stimulus. The rat served as a neutral stimulus, and the child’s cries were unconditioned reflexes.

Psychologists suggest that this type of fear conditioning by an unconditioned aversive stimulus is responsible for psychological disorders such as anxiety disorder and PTSD (Post-Traumatic Stress Disorder)​4​.


  1. 1.
    Kimmel HD. Inhibition of the unconditioned response in classical conditioning. Psychological Review. Published online 1966:232-240. doi:10.1037/h0023270
  2. 2.
    Clark RE. Classical Conditioning and Brain Systems: The Role of Awareness. Science. Published online April 3, 1998:77-81. doi:10.1126/science.280.5360.77
  3. 3.
    Harris B. Whatever happened to little Albert? American Psychologist. Published online 1979:151-160. doi:10.1037/0003-066x.34.2.151
  4. 4.
    Maren S. Neurobiology of Pavlovian Fear Conditioning. Annu Rev Neurosci. Published online March 2001:897-931. doi:10.1146/annurev.neuro.24.1.897

About Pamela Li

Pamela Li is a bestselling author. She is the Founder and Editor-in-Chief of Parenting For Brain. Her educational background is in Electrical Engineering (MS, Stanford University) and Business Management (MBA, Harvard University).


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