What Does Polygraph aka Lie Detector Measure

A polygraph measures 4 bodily processes, and they are the following: blood pressure, heart rate, respiratory rate and skin conductivity. It looks for changes in the examinee’s body that are commonly associated with deceitfulness. A lie detector works, in essence, because there are certain medical instruments integrated into it.

Attachments that a lie detector machine comes with allow a polygraph examiner to detect involuntary physiological changes in the examinee’s body as he or she responds to questions. An increase in certain bodily processes, such as heart rate and sweat production, commonly indicate lying during polygraphy.

Below is an in-depth discussion of what a lie detector machine measures and what a polygraph expert looks for.

polygraph test

Cardiovascular Activity

Whether the polygraph machine is the analog kind or the more modern computerized type, it’s designed to observe and record the cardiovascular activity of the individual — the examinee — that it’s strapped to.

That’s because various emotions of a person can have an effect on the heart and its behavior.

And one of them is the feeling of guilt, which the vast majority of the population experiences when attempting to look and sound honest when telling a lie. Being guilty can be stressful to the body, and being stressed can cause all kinds of stress hormones to be produced. Many of these hormones can impact cardiovascular activity.

When being non-truthful, as a result, your blood pressure increases. And when your blood pressure increases, your heart rate tends to follow suit — it beats more than usual per minute.

It’s due to this why both the blood pressure and heart rate are monitored during a lie detector examination.

The machine can pick up changes in their readings, which can help the examiner determine whether or not the examinee could be fabricating lies.

But it’s important to note that changes in the blood pressure and heart rate can be due to many things, not just when answering crime-related questions falsely to cover up the truth.

Read Also: 10 Medical Conditions That Affect a Polygraph

1.    Cardiovascular Activity

Notice that cuff that a doctor places around your upper arm when taking your blood pressure? Well, one of the different medical instruments integrated into the lie detector machine looks exactly like that thing.

The name: cardiosphygmograph. The function: to measure cardiovascular activity.

Sometime during the pretest phase of the polygraph examination, the examiner will place the cuff around the examinee’s arm. And during the question and answer stage, the cuff remains inflated. This is necessary in order for it to be able to monitor a couple of important physiologic changes in the individual.

One of the things that a cardiosphygmograph measure is blood pressure. It works by observing two pressures in the brachial artery, which is a major blood vessel supplying blood to your hand, forearm and upper arm:

  • Systolic blood pressure. The measure of the force that the heart exerts on the arterial walls each time the heart beats is called the systolic blood pressure. It’s the top number in any blood pressure reading. In the blood pressure reading of 120/80 mmHg, the systolic blood pressure is 120 mmHg.
  • Diastolic blood pressure. The measure of the amount of pressure in the arteries each time the heart rests between beats is referred to as the diastolic blood pressure. It’s the bottom number in a blood pressure reading — in the blood pressure reading of 120/80 mmHg, 80 mmHg is the diastolic blood pressure.

Other than the blood pressure, the cardiosphygmograph is also capable of measuring the heart rate, too. Also referred to as the pulse rate, the heart rate is the number of times your heart beats per minute.

Both the blood pressure and heart rate are very important polygraph measurements.

2.    Blood pressure

Except for certain people such as those who are psychopaths, sociopaths and pathological liars, telling a lie can cause a feeling of guilt. In turn, this can cause stress in the body. The release of stress hormones in the bloodstream can momentarily increase an individual’s blood pressure.

This is the reason why your blood pressure is one of the physiologic processes that a polygraph monitors.

Hypertension may affect polygraph test results

Because of this, it’s important for the lie detector test examiner to establish the examinee’s baseline blood pressure when telling the truth and the baseline blood pressure when telling a lie. Such is done during the pretest phase by asking irrelevant questions that require no lying and control questions that can usually force someone to lie.

Each time the blood pressure reading spikes, chances are that the examinee is lying.

It’s not just telling a lie, however, that can cause your blood pressure to rise temporarily but many other things, too. The following are some examples of those that can lead to momentary blood pressure spikes:

  • Anger
  • Anxiety
  • Caffeine intake
  • Discomfort
  • Exercise
  • Excitement
  • Fright
  • Happiness
  • Pain
  • Sexual arousal
  • Stress
  • Urge to pee

Various medications can also have an impact on the blood pressure of an individual. Leading the list are antihypertensive or blood pressure medications, the kinds that doctors prescribe to individuals with hypertension, which is characterized by a higher than normal blood pressure.

A hypertensive individual tends to have a systolic blood pressure reading of 130 mmHg or higher and a diastolic blood pressure reading of 80 mmHg or higher most of the time.

Other medications can also affect blood pressure. Some examples include:

  • Antidepressants
  • Anxiolytics (antianxiety)
  • Cough medications
  • Decongestants
  • Pain relievers
  • Weight loss pills

If you are taking any of the above-mentioned drugs or medications and about to undergo a lie detector test, make sure that you inform the examiner about it during the pretest phase.

3.    Heart rate

When stressed, one of the stress hormones the body produces is adrenaline. What the said hormone does is trigger the body’s fight or flight response, which is a primitive physiologic reaction to just about anything that is perceived as frightening or stressful, such as a threat or a hazard.

Polygraph experts evaluating charts
Polygraph experts evaluating charts

To prepare the body for either fighting or fleeing, the air passages dilate to introduce more oxygen to the muscles. The peripheral blood vessels, meanwhile, contract in order to redirect blood to the major muscle groups.

These reactions that adrenaline causes can elevate the heart rate.

Since lying is basically a form of stress, just like what was mentioned earlier, your heart rate increases each time you attempt to deceive someone, such as a polygraph examiner. And this is the reason why the number of times the heart beats per minute is one of the things that a lie detector instrument measures carefully.

However, because the idea alone of undergoing a polygraph test can be nerve-racking, it’s not unlikely for your heart rate to be faster than usual. Does this mean the examiner will find you guilty?

Not necessarily.

When establishing the baseline physiologic readings during the pretest phase of the examination, the examiner will attempt to determine your usual heart rate when in a relaxed state or telling the truth while answering irrelevant questions. If nervous because of the test, the professional will take this into consideration.

Your already high heart rate will increase further when being deceitful anyway, which means that there is very little to no chance for an experienced polygraph examiner to be confused with your rapid heart rate.

But just like your blood pressure, the heart rate can be affected by certain medical conditions and drugs, too. Needless to say, any health issue you have or medications you are taking should be disclosed to the individual who is administering the lie detector test in order to ward off an error or a false positive.

Respiratory Activity

One of the most important physiologic processes a lie detector machine measures during an examination is the breathing rate. It’s no secret that people who are stressed as a result of lying or some other thing tend to inhale and exhale more than someone who is completely calm and collected.

Your heart and lungs are closely linked organs — one can affect the other.

For instance, if you started breathing heavily, your heart would beat faster than usual. Or if your heart started pounding, your lungs would speed up, too.

Certain medical tests consider an individual’s blood oxygen level or saturation. Also referred to as SpO2 by professionals in the medical field, it’s the measure of how much oxygen your blood is carrying (via the oxygen-carrying blood component referred to as hemoglobin) as a percentage of the maximum that it could carry.

It’s not uncommon for those who are suffering from asthma and obstructive sleep apnea to have the amount of oxygen in their blood measured. During a polygraph test, however, your SpO2 is not measured.

What it does consider are the respiratory rate and pattern.

4.    Pneumograph tubes

While undergoing the question and answer portion of a polygraph examination, a couple of tubes are placed around the torso of the examinee — one around the chest and the other around the abdomen.

These tubes are referred to as pneumograph tubes or simply pneumographs.

Filled with air, they are designed to allow the lie detector test to measure respiratory activity. Whether the polygraph machine is analog or computerized, pneumographs work in the same manner. However, they are attached to bellows that contract and expand as you breathe in an analog model, and hooked up to a transducer in a computerized model.

infidelity test

Function-wise, there are 3 groups of respiratory muscles: the diaphragm, chest muscles and abdominal muscles. A pair of pneumographs are used in order to be able to observe the activity of the rib cage and belly.

The stress response can affect an individual’s breathing in a couple of ways. First, it can speed up the respiratory rate, which, simply put, is the number of breaths you take per minute. While at rest or in a relaxed state, the normal respiration rate for an adult person is anywhere from 12 to 16 breaths per minute.

Second, it can cause shallow breaths.

Pneumographs around the chest and abdominal area of the examinee measure both the respiratory rate as well as the extent of the movement of the respiratory muscles.

5.    Respiratory rate

While we were talking about changes in the heart rate when a person is stressed, the fight or flight response was mentioned. Also cited was the fact that this normal body reaction involves bringing more oxygen to the muscles so that you can perform so much better should you decide to fight the threat or flee the scene.

And to make sure that your muscles have plenty of oxygen, you breathe at a much faster rate.

Your heart rate follows suit in order to make sure that all the oxygen you inhale gets distributed quickly to your muscles — it’s through your bloodstream that oxygen molecules get dispersed throughout your body.

In most instances, the respiratory rate and heart rate are connected to each other. So, in other words, increasing the rate of one usually increases the rate of the other. Since lying can activate the body’s stress response, it’s likely for the examinee’s heart and breathing to race at the same time when being deceitful.

Being stressed also causes a person to switch from diaphragmatic breathing to upper chest breathing, which utilizes the muscles of the chest and shoulders instead of the diaphragm and abdominal muscles.

Coupled with muscle tension as a result of stress, the person may feel like he or she can’t breathe.

A polygraph machine can easily pick up changes in the respiratory rate of someone who is undergoing the exam. But it’s important for the examiner to consider the presence of asthma or any other respiratory issue. It’s also a must to establish whether or not the individual has anxiety or panic attacks for a more accurate chart evaluation.

6.    Respiration pattern

The number of times the examinee inhales and exhales per minute is not the only physiologic process that a lie detector machine is able to measure and record but also the quality of the breathing.

If you are stressed or anxious because of answering a question untruthfully in order to deceive the examiner or for any other reason that can easily cause a person to feel the same way, your breathing becomes shallower, too. And this is something that a polygraph machine can observe with the help of its pneumograph tubes.

What’s wrong with shallow breathing is that it can cause stress, and stress can cause you to breathe shallower.

But the respiratory pattern can also pose a challenge to a polygraph examiner. That’s because while some people take shallow breaths when telling a lie, others tend to take deep breaths.

Because of this, an examiner will tell you before or, if necessary, at any given time during the test to breathe normally — not to take shallow or deep breaths and not to hold your breath or deliberately attempt to modify it. Otherwise, you could look like you are desperately trying to appear innocent.

In some instances, changes in the examinee’s breathing quality can be due not because of the stress being guilty brings but as a result of anxiety or discomfort during the exam or because of a respiratory issue such as asthma.

Considering this possibility is an important step that any good polygraph examiner should take.

Electro-Dermal Activity

Don’t let the name intimidate you. Simply put, electro-dermal activity is monitored during a lie detector test in order to determine the perspiration rate of the examinee.

Besides the activity of the cardiovascular and respiratory systems, the stress response also has an effect on how much sweat the body produces. The more stressed you are, generally speaking, the sweatier you get. That’s because your sweat glands are sensitive to hormones like the adrenaline that your body produces more when you’re stressed.

7.    Electrical Activity in Fingertips

Just before the polygraph examiner calibrates the machine and asks you all sorts of questions, small contraptions will be attached to your fingertips. They are referred to as galvanometers.

polygraph measuring electricity in fingertips

Certain parts of your body have more sweat glands than the rest, and your fingertips are some of them.

Technically speaking, it’s not really the amount of sweat that a lie detector test measures by means of galvanometers but the ability of the skin of your fingertips to conduct electricity — the more sweat is produced by the sweat glands in your fingertips, the more conductive to electricity your fingertips become.

So, in other words, the amount of electrical activity in your fingertips climbs the more stressed you become, such as when answering a question falsely to avoid looking guilty.

While sweating or, in a more scientific sense, electro-dermal activity is one of the things that a polygraph machine observes and records, some lie detector test experts agree that what the cardiosphygmograph and pneumograph tubes bring to the table is more informative and somehow more accurate.

A good polygraph examiner, during the pretest phase of the exam, will establish whether or not the examinee has a disorder or condition that can make him or her sweat more than or less than others. Some examples are:

  • Alcoholism
  • Cancer
  • Diabetes
  • Fever
  • Heart failure
  • Hyperhidrosis
  • Malaria
  • Menopause
  • Parkinson’s disease
  • Pregnancy
  • Rheumatoid arthritis
  • Stroke
  • Thyroid problems

Just Before You Have a Lie Detector Test

Especially if you are innocent and have nothing to hide, it’s important to stay calm and collected while undergoing a lie detector exam. Otherwise, the result might end up being a false positive, which means that the examinee finds you being deceitful when in fact you are simply being truthful all along.

Knowing the things that a polygraph measures can help keep you from feeling anxious unnecessarily.

If you have a medical condition or are taking a drug or medication that may impact the result of your lie detector test, make sure that you inform the examiner about it before you are asked a series of questions.

Read Next: How Does a Lie Detector Work

Disclaimer: The content is intended for informational purposes only and does not contain advice on criminal and investigative questions and inquiries. If you need professional help, please check with your state authorities.

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