Science project

How to Make a Lie Detector

Research Questions:

  • What are some examples of early practices that were done to determine who was lying?
  • What were some of the earlier models of lie detectors?
  • What does the polygraph monitor?
  • How can the polygraph user tell if the person is lying?
  • How accurate is the polygraph?   

In old Chinese times, people suspected of a crime held rice in their mouths during their speech; the person whose mouth was dry at the end was said to be the guilty one because the Chinese believed that a lack in saliva is caused by nervousness and anxiety. There were similar practices in West Africa as well as other parts of the world.

The lie detector, or polygraph, was invented in 1921 by John Larson. Today, most law enforcement agencies rely on the polygraph determine whether the suspects or the perpetrator is telling the truth for their alleged crimes. What looks like a very complicated machine with a lot of hanging wires is actually quite simple.

In this experiment, students will make their own lie detector. You can obtain a blood pressure monitor for home use at your local drug store or pharmacy. The rest is pretty basic and should be prepared and gathered by the experimenter. 


  • A blood-pressure monitor
  • A list of premade yes/no questions (about 10) of your choice to ask each of your test subjects (examples that are possible questions will be suggested below)
  • At least 10 test subjects (it is best if it is random people or people you do not know to well; perhaps some kids around school? Just make sure that you DO NOT know the answers to any of the questions you ask them!)
  • Pen and paper
  • A keen mind
  • Optional: Stethoscope
  • Optional: A partner
  • Optional: A watch

Experimental Procedure 

  1. Prepare beforehand a list of about 10 numbered questions that you are going to ask each of your test subjects. Make sure they are 10 simple yes/no questions with no elaborate answers. Suggestions are shown below.
  2. Find test subjects. Remember that it is better not to choose the people who are close to you as you will most likely know the answers to the questions. That defeats the purpose of this experiment. A suggestion is to find students around school! :) You are to question your test subjects separately.
  3. Once your test subject has been seated, follow the instructions provided by your chosen blood pressure monitor. Most likely, it will require you to strap the cuff around the subject's arm. Do this.
  4. Instruct the test subject to purposely pick questions to lie on without revealing to you what they are and that they should remember which ones they lied on for the conclusion of the test.
  5. Start asking your prepared questions. Observe the test subject's traits that seem to be out-of-place on certain questions. Perhaps they are fidgeting? Playing with their thumbs? Both of these are very likely signs of deception. Note your thoughts for each question. A chart is provided below as a guide.
  6. When the subject is answering each of the questions, make sure to use the blood pressure monitor and record the rates. (OPTIONAL: If you decided to have a friend help you out, have them place the stethoscope on the subject's chest or if you do not have a stethoscope then have your partner place their index and middle finger on the subject's wrist to count their heart rate in beats per minute or rate it by speed and record it.)
  7. At the end of the questioning for each subject, hypothesize on which questions you think the test subject is lying on based on your observed physical movements, traits, and blood pressure. The higher the blood pressure is, the more likely they are to have lied. At the same time, ask the test subject to write down on a piece of paper themselves the questions they actually lied on; no peeking! :)
  8. Compare your hypotheses with your test subjects' responses and evaluate how many questions you got right out of 10.
  9. Repeat steps 3-8 for all your test subjects.
  10. Evaluate how well you did by finding number of questions you got right against the ones you got wrong.   

Here is a list of possible questions you may use in the experiment. Don't forget, you are free to make up your own.

  1. Do you own a pet?
  2. Did you grow up here?
  3. Do you have any siblings?
  4. Have you ever gone to *insert location here* ?
  5. Do you know *insert person name here* ?
  6. Have you ever had *insert food/drink item here* ?
  7. Can you play any instruments?
Suggested chart for recording data:
Question #
Blood Pressure
Irregular vocals?

Other strange movements and notes


 ** Optional: Heart Rate can also be included in the chart if you have chosen to test that as well.** 

Terms and Concepts: Polygraph, Pneumographs, Galvanic Skin Resistance (electro-dermal activity), Galvanometers, Heart Rate, Blood Pressure 


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