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Pinocchio’s Arm: A Lie Detector Test


Key Concepts
Theory of the Mind, brain states, psychology
Sabine De Brabandere, PhD, Science Buddies
Two children facing each other with one arm sticking out, and one child pushing down on the other's arm


Did you ever think of lying as an interesting social skill? Lying is actually tied to empathy, the ability to see things from another person’s perspective. Most humans begin to develop these skills around the age of three, when they begin to understand that what they know about the world might be different from what other people know about the world. Telling lies might be a skill, but lies can be bothersome. Sometimes, you really like to know how to tell a truth from a lie. Will the ingenious lie detector proposed in this science project really work?

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


Lying is a skill that activates specific parts of the brain.  Lying is also often accompanied by a feeling of guilt, which creates stress. Standard lie-detection techniques look for the body’s reactions to this stress, such as elevated heart rate or blood pressure, faster breathing or sweating. This is difficult. How can you distinguish between stress that is related to a lie and other stress, like the anxiety about a lie detector test? Or what if the person telling a lie does not feel guilty about the lie, or has learned to stop the body’s response to feeling guilty?

Neuroscience provides an alternative method to separate a truth from a lie that uses functional magnetic resonance imaging (or fMRI) of the brain. An fMRI scan shows which parts of the brain receive the most blood – or are the most active – as the brain performs a specific task. These scans reveal that more and different parts of the brain are active during lying, as compared to truth-telling. In short, the brain works harder when it is telling a lie than when it is telling a truth.

Excluding unconscious tasks such as walking and talking, we generally sacrifice efficiency when we do two or more activities at the same time. Do you think it is harder to perform well at a physical task while our brain is busy telling a lie, compared to while our brain is busy telling a truth? Find a friend, and see if you can detect a hard-working brain!


  • Volunteer(s)
  • Paper and pen
  • If your volunteer(s) are much taller than you, you will need a stable chair on which to stand. If the difference is relatively small, the first or second step of a stair will help. The goal is to have your shoulders at the same level as the shoulders of your volunteer.


  1. Inform your volunteer that you would like to test a particular lie detector and need to collect a little information from the volunteer to do so.
  2. Ask your volunteer to complete the following sentences three times, filling in something different each time. “I strongly dislike …”. Write down the answers or draw a picture of it so you remember what it was.
  3. Repeat the previous step with the following sentence: “I love …”.


  1. Have the volunteer stand facing you, a few feet away. If you are much shorter than your volunteer is, then raise yourself up on a stool or stair so your shoulders are approximately level with the shoulders of your volunteer.
  2. Have your volunteer extend his or her arm straight out in front, palm facing down so the whole arm is at shoulder level. Tell the volunteer you are going to have him or her say a few phrases, that you will push the arm down and you would like him or her to try to keep the arm up.
  3. Extend your arm straight out and place your hand, palm down, over the volunteer’s hand and wrist.
  4. Ask the volunteer to say the following sentence three times in a row: “I love …” where you fill in the blank with the first item your volunteer mentioned in the list she or he loved so this is a truth for your volunteer. As the volunteer says the sentences, press down on the volunteer’s arm and apply a steady, constant pressure. You do not need to press it all the way down; you just want to get an idea of how hard you need to push to get the arm to move down.  Is it easy or hard to get the arm down?
  5. Repeat the previous two steps, replacing the sentence with “I love to vomit.” This phrase is a lie since nausea is universally an unpleasant experience. Is it easier, similar or harder to get the arm down compared to when the person was telling the truth (previous step)? In other words, was it easier or harder for your volunteer to perform the task of holding up his or her arm?
  6. Repeat the previous steps, filling in the stated likes or dislikes of your volunteer and rate each time how much resistance you feel, or how difficult it is to get the arm to lower. Can you see a pattern? Is it easier, similar or harder to get the arm down when your volunteer is telling a truth compared to when your volunteer is telling a lie?

Extra: Can you detect specific body language revealing a lie?  Pay attention to facial expressions, the pitch of the voice, hand movements and breathing rate. Do any of these change when the volunteer tells a lie compared to when the volunteer tells a truth? Why would this be so?

Extra: Test several volunteers. Does the lie detector work better on some volunteers than on others?

Extra: If you feel the lie detector works, test if it passes the double-blind test. In this case, do not tell the volunteer you are testing a lie detector. Instead, only ask the volunteer to perform the task of holding his or her arm up while you push on it and ask the volunteer to tell a few truths and lies that they are okay revealing later whether or not they were truths. Were you able to distinguish the truths from the lies?  

Extra: How is this lie detector test setup different from real-life experiences where you try to uncover a mystery? If you feel the lie detector works in the test setup, do you think it would also work in real-life cases? Why or why not?

Extra: In this test, you used a raised arm as the task for the volunteer to do. Can you find other tasks that better display when someone is telling a lie compared to telling a?

Observations and Results

You probably felt that it was easier for you to push the arm down when the volunteer was telling a lie compared to when the volunteer was telling a truth. This is what is expected; the volunteer has a harder time fulfilling a physical task while telling a lie.

Neuroscientists have discovered that the brain works harder when it is telling a lie than when it is telling a truth. They found that just four parts are active during truth-telling, whereas seven parts are active during lying. This difference in brain states makes it harder for volunteers to perform a small physical task while telling a lie. As a result, your volunteers had a harder time performing well on the task to hold their arm up while their brains were busy telling a lie, compared to when their brains were engaged in telling a truth.

Telling a lie is often accompanied by other clues, like a higher-pitch voice, avoidance of eye contact, shrinking (taking up less space) and hands touching the ear or clothing. 

The test situation in this test is very different from a real-life situation. In this test, the subject is asked to tell a lie and no consequences are attached. In real life, lies are self-generated and more might be at stake. These and other factors might influence how well the lie detector works.

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