The idea of a lie detector, or polygraph as it is sometimes known, can be somewhat daunting – the closest most people have come to one is when watching a spy thriller, where the hero (or villain) is strapped up to a million wires and asked questions of national security by the CIA or MI5. All very mysterious and, in a funny sort of a way, glamorous.

However, in reality, the lie detector is quite ordinary and nothing to be afraid of. You will definitely not have a bright light shone in your eyes, or have a couple of ‘heavies’ standing at each shoulder!

So, now that you know what won’t happen, let’s move on to what will happen.

First things first. A lie detector is a highly sophisticated piece of equipment, designed to measure stress responses in the body, responses which are usually invisible to the naked eye, and which show a marked difference in how the body reacts to various questions. They are also responses which, in general, we cannot control. The three areas which are measured for physiological changes are:

  • Respiratory
  • Cardiovascular
  • Sweat (Now, when we talk about sweat, we are again not referring to the close up of a guilty man’s face with a huge drop of perspiration running down his temple – the changes are much more subtle than that.)

The polygraph consists of three main stages

The Pre-test

During the pre-test, the examiner and the subject will get to know each other. The examiner will explain how the polygraph works, show the subject the equipment and how it will be attached, and answer any questions the subject might have about the process. The matter to which the polygraph pertains will also be discussed, with the subject giving his or her side of the story, and the questions to be asked will be brought up and agreed upon. While the pre-test is being carried out, the examiner will also be watching the subject closely, noting his or her responses, body language, speech patterns and so forth. This portion of the process will take around an hour, and is, in actual fact, usually the longest part of the entire procedure.

The polygraph Test

This is the part of the examination during which you will be attached to the instruments. (It’s not as daunting as it sounds!) Two air filled rubber tubes are placed around the subject’s body – one across the top of the chest, and the other around the waist. This is called the Pneumograph and it measures respiration and movement. As the subject breathes, the air pressure in the tubes changes, and it is these changes which are transmitted to the monitor and recorded.

The next apparatus is called the GSR (no, not gunshot residue) which stands for Galvanic Skin Response. These are small plates which are attached to two of the fingers, and they measure changes in the skin’s responses, such as sweat gland activity. The fingertips are rich in sweat glands, which makes them the ideal place for the measurement of perspiration, and I’m sure we have all, at some time, experienced sweaty palms and hands at some moment of anxiety. Well, this is what the GSR is looking for.
The final piece of equipment to be used is the Cardiosphygmograph, which measures blood pressure and heart rate, and is very similar to the blood pressure cuff used by medical professionals. The cuff is inflated and remains so for the duration of the test. The air in the cuff carries the sound of the blood flowing through the veins, which is then transmitted to the monitor.

All of these components are then attached by wires to the monitor on the examiner’s desk or table, which he will watch closely as the examination is carried out. The results will also be produced on a printout.

So, once the subject is sitting comfortable, the main part of the examination can begin. There are generally around ten questions asked during a typical polygraph, some of which will be what we call ‘control questions’, and the remainder will be ‘relevant questions’ – the ones we need the answers to.

So, for instance, once you are attached to the equipment, the examiner might ask if the subject’s name is (insert correct name here). This information will have already been gleaned from the pre-test conversation. The subject will obviously answer ‘yes’, and this will give the examiner a baseline with which to measure the rest of the answers and the subsequent bodily responses. The next question might be ‘Are you (insert actual age here)?’ Again, the answer is going to be yes, but will be followed by a question designed to be contradictory, such as ‘were you born in (insert the wrong year here)?’ Similarly, the examiner might ask if the subject lives at 4 Privet Drive, and unless the subject is, indeed, Harry Potter, he or she is going to answer no.

Interspersed with these control questions will be the relevant questions. Depending on the reason for the polygraph, these will be related to perhaps theft, infidelity, drug usage, background…whatever the subject is being queried for, the questions will be related to that, always needing either a ‘yes’ or a ‘no’ – there are no open questions.

This part of the polygraph will take approximately 30 minutes, although this can vary considerably, and once it is completed, the subject will have the equipment removed.


Following this phase, the examiner will analyse the readings, and the subject will be given the results immediately. If they have failed the test, and deception has been detected, they will be given the opportunity to explain the results.
Each subject will also be asked to sign every sheet of results, which makes it impossible for the results to be confused with another subject’s.

Everybody who undertakes a polygraph test will be nervous, and this anxiety will not affect the outcome. As nervousness cannot be switched on and off during the test, the results, or rather difference in the results of one person’s test will be the same.

The lie detector has come a long way from the methods the Ancient Hindus used to employ. In order to determine whether a subject was being truthful or not, he would be given a mouthful of rice to chew. If he was able to spit it out onto the leaf of a sacred tree, he would have proved his innocence. However, if he was unable to spit the rice out, he would be deemed a liar. We’ve come a long way since then!