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20 Unconditioned Stimulus Examples

What is an unconditioned stimulus?

An unconditioned stimulus (UCS or US) can naturally elicit an automatic response without prior conditioning or learning. It is also called the primary reinforcer. The involuntary response, also known as the unconditioned response, is a reflex triggered by the unconditioned stimulus.

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 because these are natural reactions of our bodies​2​.

Unconditioned Stimulus in Classical Conditioning

Russian physiologist Ivan Pavlov first discovered the classical conditioning process, also known as Pavlovian conditioning, when he was studying the digestive system of dogs.

Pavlov noticed that the dogs would automatically salivate when they smelled the food. This natural response did not require any prior learning.

He then conducted a series of experiments to study the salivary response. In Pavlov's experiment, he sounded a bell whenever he brought food to his dogs.

After multiple repetitions, Pavlov's dogs started salivating when they heard the sound of a bell, even before they got the sight or smell of the food.

In Pavlov's example, the bell's tone was a neutral stimulus. After being paired with the presentation of the food, an unconditioned stimulus​1​, repeatedly, this initially neutral stimulus becomes a conditioned stimulus.

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 unconditioned stimuli in everyday life.

  1. Hot iron: Touching a hot iron makes you withdraw your hand right away.
  2. Food: When you see or smell food, it causes your mouth to water.
  3. Rock hitting: Dropping a rock on your foot makes you scream in pain.
  4. Dust: Dust entering your nose causes you to sneeze.
  5. Loud Noise: A sudden loud noise can cause a startled response, making you jump or feel alarmed.
  6. Bright Light: Shining a bright light suddenly in someone's eyes can cause them to squint or close their eyes.
  7. Extreme Temperatures: Touching something extremely hot or cold can cause an immediate reflex to pull away.
  8. Pain: If you step on something sharp, the immediate pain can cause a reflexive withdrawal of the foot.
  9. Tickling: The sensation of being tickled can cause immediate laughter or squirming.
  10. Suffocating Atmosphere: An environment with little to no oxygen can trigger an immediate sensation of suffocation and a desire to escape to a breathable atmosphere.
  11. Stomach Upset: Consuming spoiled food or something you're allergic to can lead to nausea or vomiting.
  12. Sweet Taste: Tasting something sweet can cause salivation and a desire to consume more.
  13. Falling: The sensation of suddenly falling, like when you miss a step, can cause an immediate jolt of fear and a reflexive action to grab onto something.
  14. Baby's Cry: A baby crying can trigger concern or a desire for comfort, especially in parents or caregivers.
  15. Pungent Odors: The smell of strong odors, like ammonia or rotten eggs, can cause an immediate reaction to cover one's nose or move away from the source.
  16. Physical Balance Disruption: If someone pushes you unexpectedly, the sudden loss of balance can trigger an immediate reflex to stabilize yourself.
  17. Choking: Getting something stuck in your throat can trigger immediate coughing to clear the airway.
  18. Insect Bite or Sting: The sensation of being bitten or stung by an insect can cause an immediate reflex to swat or brush away the insect, followed by a sensation of pain or itching.
  19. Sudden Darkness: If you're in a well-lit room and the lights suddenly go out, it can cause a momentary sense of disorientation or surprise.
  20. Air Puff to the Eye: A sudden puff of air directed towards the eye can cause an immediate blink reflex.

The Difference Between Unconditioned Stimulus and Neutral Stimulus

The difference between an unconditioned stimulus and a neutral stimulus is that an unconditional stimulus (UCS) elicits a natural, reflexive response, called the unconditioned response (UCR), while a neutral stimulus (NS) doesn't naturally elicit the same response.

For example, food was a UCS for Pavlov's dogs, and the salivation response was a UCR.

Before conditioning, ringing a bell alone didn't have any effect on the dog. The bell's sound was hence an NS.

dust enters nose causing woman to sneeze example of unconditioned stimulus

The Difference Between Unconditioned Stimulus and Conditioned Stimulus

The key difference between an unconditioned stimulus and a conditioned stimulus is that an unconditioned stimulus causes a reflexive, automatic response, while a conditioned stimulus can only trigger a response after an association is formed through conditioning.

An unconditioned stimulus (UCS) can naturally trigger an unconditioned response (UCR).

A conditioned stimulus was originally a neutral stimulus (NS) that does not trigger a response.

The classical conditioning theory suggests that a neutral stimulus can be paired with an unconditioned stimulus repeatedly to form associative learning. The previously neutral stimulus then becomes a conditioned stimulus (CS) that can trigger the same response as the unconditioned stimulus. This response is called a conditioned response (CR) or learned response.

Also See: Primary vs. Secondary Reinforcers

child tries to touch white rat unconditioned stimulus psychology definition

Little Albert Experiment

The infamous "Little Albert Experiment" is another example of classical conditioning​3​.

In this research, psychologist John B. Watson exposed an infant, Little Albert, to a white rat. When the infant attempted to touch the rat, Watson would make a loud bang to scare him. Several repetitions later, the child cried upon seeing the rat alone without the noise.

In this case, the loud noise was an unconditioned stimulus that could trigger the unconditioned reflexes of crying. The rat was initially a neutral stimulus that did not scare the child. But after conditioning, the rat became a conditioned stimulus that elicited a conditioned fear response in Little Albert.

This type of learning, called fear conditioning, is important in teaching us about potential dangers.

However, it becomes problematic if benign, neutral stimuli become conditioned to trigger emotional responses.

Psychologists believe this type of aversive conditioning is responsible for psychological disorders such as anxiety disorder and Post-Traumatic Stress Disorder (PTSD)​4​.

In traumatic situations, people may become conditioned to fear benign environmental cues, such as an object or a place. This fear then becomes generalized to similar items that now serve as sources of threat to them.

Stimulus Generalization

Stimulus generalization in psychology refers to the tendency for a person's response to one stimulus to be generalized to other similar stimuli.

An example would be if a person was bitten by a dog and became afraid of it. Whenever he sees a dog again, even if it is not the same breed, he fears it. He now fears all dogs despite their differences because he has generalized his fear of one dog. His fear may even be generalized to all four-legged animals.

In the Little Albert Experiment, after the child was conditioned to fear a white rat, his fear was generalized to a white feather. There was no discrimination between a rat and a feather.

In those suffering from PTSD, the fear may be generalized to something or someone they see often. That means they experience traumatic fear in their presence often, which is debilitating.


  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

Updated on September 28th, 2023 by Pamela Li

Pamela Li is an author, 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). Learn more


    * All information on is for educational purposes only. Parenting For Brain does not provide medical advice. If you suspect medical problems or need professional advice, please consult a physician. *

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