How does a lie detector test work?

How does a lie detector test work?

You have probably experienced seeing or being in a situation where someone accused another person of something the latter claims they didn’t do. The guilty one has 2 options: to face the consequence by telling the truth or to tell a convincing lie. Not everyone can rely on their instincts and if suspicion is still in the air, the best way to put things to an end is to conduct a lie detector test.

What is a lie detector test?

A lie detector test or polygraph test uses a specifically designed machine to measure certain bodily functions such as heart rate, blood pressure, breathing and skin conductivity. Those factors are not easily controlled by the mind which is why experts believe that measuring these can reveal whether a person is telling the truth or not.

The machine, also known as a polygraph, measures the change of the mentioned physiological variables while asking a series of questions related to the issue. A chart will then be produced showing the readings of the examiner. The machines that we use now are highly developed compared to the ones that people used back then.

The history of lie detector test

Ancient China and West Africa first found out about the idea that lying produces physical side effects. In West Africa, those who are suspected of a crime were tasked to pass a bird’s egg from one place to another. If the accused broke the egg, he will be considered guilty due to his nervousness. 

In Ancient China, the accused is tasked to hold a handful of rice in his mouth during a prosecutor’s investigation. During this time, salivation was believed to be caused by emotional anxiety. If the rice remained in the accused’s mouth after the prosecutor’s speech, he would be considered guilty.

The modern polygraph was developed in 1913 by a Harvard University psychologist named William Moulton Marston. He used systolic blood pressure as a method of detecting lies. However, a more complex device was invented by Dr John A. Larson of the University of California. The device records both blood pressure and galvanic skin response.

Larson’s device was used in law enforcement work by the Berkeley Police Department and was the first machine to be referred to as a polygraph. Marston’s device, which uses only one graph, remains as the polygraph’s primary advocate.

Today, polygraph examiners use analogue and computerized devices depending on the preference of the service provider. Testing procedures will also vary based on the types of instruments used. 

Basic components of a polygraph machine

The polygraph machine has undergone various changes over the years. Polygraphs are the same instruments you used to see in the movies that have little needles and scribbling lines on a strip of scrolling paper and these are called analogue polygraphs. 

However, modern polygraphs are now administered using digital equipment. The previous scrolling papers are now replaced with computer monitors and advanced algorithms. Both analogue and digital polygraphs still measure the same physiological parameters such as the respiratory rate, blood pressure, heart rate and galvanic skin resistance. Read on below for more details:

Respiratory rate

Pneumographs can be seen both in a digital and analogue polygraph machine. In a digital polygram, you’ll see rubber tubes filled with air and placed around the subject’s chest and abdomen. The air inside the tubes is replaced as the abdominal muscles or the chest expand. Transducers are used to convert the energy of the displaced air into electronic signals.

Analog polygraph, on the other hand, uses an accordion-like device called bellows that displace air when the tube expands. The bellows are attached to a mechanical device that is connected to a pen that marks on the scrolling paper as the subject breathes. 

Blood pressure and heart rate

In both analogue and digital lie detector tests, a blood pressure cuff will be worn by the subject on their upper arm. A tubing will run from the cuff to the polygraph. As the subject’s blood pumps, it will produce sounds. That sound will change the pressure and will displace the air inside the tubing. 

The tubing is connected to the bellows of the analogue polygraph machine and moves the pen. In digital polygraphs, these signals are converted into electronic signals through transducers.

Galvanic skin resistance

The galvanic skin resistance is also called electro-dermal activity. In simple words. It measures the sweat on your fingertips. The device is placed on your fingertips since this is the most porous area of the body and is a great place to look for sweat. 

The more sweat the polygraph detects means the subject is placed under stress.  Galvanometers are the finger plates that will be attached to the subject’s fingers. This will measure the skin’s ability to conduct electricity. If the skin has sweat, it will conduct electricity more easily.

Arm and leg movements

Some polygraphs can record arm and leg movements while the examiner asks questions. The signals from the sensors that are connected around the subject’s body will be recorded on a strip of moving paper.

Polygraph examiners

Only two persons should be present during a polygraph exam: the polygraph examiner and the subject. This is because a subject’s behaviour can greatly affect and change if there are other people in the room. Polygraph examiners have various tasks when it comes to performing a lie detector test. Take a look at some of them below:

  • Setting up the polygraph machine
  • Preparing the subject for the exam
  • Asking the polygraph exam questions
  • Analysing and evaluating the polygraph data.

There are several variables that a polygraph examiner has to take into account when asking the questions of the exam. Some of these variables are cultural and religious beliefs as well as medical conditions. 

Some topics can cause specific reactions that could be misinterpreted as deceptive behaviour. Thus, the examiner must know how a specific person processes the information and analyse how he or she responds. This is why a polygraph test must only be conducted by a trained and licensed examiner.

In the United States, there are more than 3,500 polygraph examiners, 2,000 of which belong to the American Polygraph Association. Those who do not belong to a professional organisation should have valid credentials proving that they are qualified for the job. To become a licensed polygraph examiner, you have to complete the following steps:

  1. Enrol in a school and earn a bachelor’s degree in criminal investigation or have at least 5 years of investigative experience.
  2. Attend and pass a 10-week course that includes the following subjects:
    1. Psychology
    2. Ethics
    3. History
    4. Question construction
    5. Physiology
    6. Psychological analysis of speech
    7. Test-data analysis
    8. Chart analysis.
  3. Complete an internship program that should take anywhere from 8 months to a year.
  4. Conduct a minimum of 25 polygraph exams for actual cases.

How to conduct a lie detector test

Even a stoic person can get nervous and intimidated under a lie detector test. Here, the subject will have to sit with tubes and wires wrapped around the body. A polygraph exam is a long process but it is divided into 4 common stages. Take a look at how a typical lie detector test is conducted below:


The pretest is the first phase of a polygraph exam and may last for at least an hour. Here, the examiner and the subject get to learn about each other. This is where the examiner gets the details about the subject’s story concerning the events under investigation. The way the subject processes and expands the information will be observed by the examiner.

Design questions

Specific questions that are related to the issue under investigation will be designed by the examiner. The examiner will go through these with the subject first without revealing the exact questions before the actual exam is given.


During the in-test, the actual polygraph exam will be given. The examiner will ask at least 10 questions but only 3 to 4 of these are relevant to the issue being investigated. The rest are called control questions which are general and broad that almost no one can respond with a ‘no’. Such questions can be like ‘have you ever stolen anything in your whole life?’.


The post-test is where the examiner analyses the gathered data and determines whether the subject is being deceptive or not. If there are significant fluctuations in the physiological responses of the subject, this may signal that the subject is lying.

However, there are still times when the examiner misinterprets the reaction of the subject to the particular question. Two ways that a response can be misinterpreted is when it turned out to be false positive or false negative. The former is a response of a truthful person but was determined to be deceptive. While the latter is the response of a deceptive person but was determined to be the truth.

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