A Simplified Explanation of A Polygraph

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Polygraphy 101

A polygraph (popularly referred to as a lie detector) measures and records several physiological indices such as cardio-vascular activity, respiration, and electro-dermal activity while the subject is asked and answers a series of questions. Deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.

The textbook definition is as follows: "...the instrument and techniques used in the psychophysiological detection of deception, though polygraphs are also used in research in other sciences. In PDD the polygraph traditionally records physiologic activity with four sensors: blood pressure cuff, electrodermal sensors, and two respiration sensors. Some instruments also record finger pulse amplitude using a PPG."


Physiological changes to occur in the subject’s body at the point of deception. 
These physiological changes can not be controlled by the subject.
These physiological changes can be scientifically diagnosed by a trained polygraph examiner
A Polygraph Examiner is also known as a “Forensic Psycho-physiologist”


Screening Examinations (Integrity Screening).

This usually entails the screening of existing employees, as well as screening of candidates to be employed. For existing employees the objective is to ascertain if they have been involved in illegal/undesirable activities whilst employed. For candidates considered for employment, the objective is to ascertain if they will be an employment risk for the potential employer in the future.  Questions are formulated as per the client’s scope of interest.

For example, here-under a question set that is used to screen existing – as well as new members for the ZRP Minerals and Border Control Unit: (Existing members are screened at least twice a year, or when a specific incident occurs and suspects/involvement in a specific incident needs to be eliminated)

  1. Have you personally been involved in any illegal Minerals or Metals dealing in the last two years?
  2. Have you been involved in any illegal syndicate in the last two years?
  3. In the last two years, have you had any personal illegal drug involvement? 
  4. Have you rendered any assistance to commit a serious crime in the last 2 years?
  5. In the last two years, were you part of any corrupt activity?
  6. In the last 2 years, did you give out any information to commit a crime?

Event-Specific (Criminal) Examinations.

This type of examination is related to specific criminal and/or undesirable incidents with the aim of confirming or denying involvement in a specific (known) - criminal issue.

Here-under an example of a specific – known issue – question set that was used for a client in determining if an employee stole money from a locked cabinet in his home:

  1. Did you take that money?
  2. Did you take that money on 12 November 2015?
  3. Did you take that money from the cabinet in the office of XXX


Polygraph examinations are currently being used in more than 50 countries (Zimbabwe Included) in the fields of corrections, criminal investigations and intelligence/counter intelligence.

Law enforcement agencies using the polygraph do so predicated on the basis that it is an important investigative aid, but not a substitute for standard investigative techniques.

For decades the law enforcement community has used polygraph examinations as an investigative aid to:

  • verify the statements of victims;
  • establish the credibility of witnesses;
  • evaluate the truthfulness of suspects;
  • and to help exonerate the innocent who is surrounded by circumstantial or uncorroborated evidence.

Polygraph usage has proven successful in:

  • Eliminating suspects.
  • Recognising false complaints.
  • Examining informants to determine the veracity of information provided
  • Providing a new "key" to an investigation when all other standard investigative techniques have been exhausted.
  • Narrowing the focus of enquiry and gathering additional information and evidence.

 In a commercial environment, polygraphy is essentially used in theft and fraud cases.

It can also be used for pre-employment vetting and integrity checks of existing personnel.

It is cheaper than hiring the services of a private investigator.

The investigation is less time consuming and extremely accurate.


There are three categories of polygraph results, namely:

No Deception Indicated / No Significant Response.
Meaning the person examined answered truthfully to relevant questions.

Deception Indicated / Significant Response.
Meaning the person examined lied.

Inconclusive / No Opinion .
Meaning no conclusive result could be obtained from the person examined.


The polygraph instrument has undergone a dramatic change in the last decade. For many years, polygraphs were those instruments that you see in the movies with little needles scribbling lines on a single strip of scrolling paper. These are called analogue polygraphs. Today, most polygraph examinations are administered with digital equipment. The scrolling paper has been replaced with sophisticated algorithms and computer monitors.

When you sit down in the chair for a polygraph exam, several tubes and wires are connected to your body in specific locations to monitor your physiological activities.

Deceptive behaviour will trigger certain physiological changes that can be detected by a polygraph and a trained examiner. The examiner is looking for the amount of fluctuation in certain physiological activities.

Here's a list of physiological activities that are monitored by the polygraph and how they are monitored:

  • Respiratory rate

Two pneumographs, rubber tubes filled with air, are placed around the examinee's chest and abdomen.

When the chest or abdominal muscles expand, the air inside the tubes is displaced.

In an analog polygraph, the displaced air acts on a bellows, an accordion-like device that contracts when the tubes expand.

This bellows is attached to a mechanical arm, which is connected to an ink-filled pen that makes marks on the scrolling paper when the subject takes a breath.

A digital polygraph also uses the pneumographs, but employs transducers to convert the energy of the displaced air into electronic signals.

  • Cardiovasular Activity / Heart Rate

A blood-pressure cuff is placed around the subject's upper arm. Tubing runs from the cuff to the polygraph. As blood pumps through the arm it makes sound; the changes in pressure caused by the sound displace the air in the tubes, which are connected to a bellows, which moves the pen. Again, in digital polygraphs, these signals are converted into electrical signals by transducers.

  • Electro-Dermal Activity

To simplify – it is a measure of the sweat on your fingertips.

The finger tips are one of the most porous areas on the body and so are a good place to look for sweat.

The idea is that we sweat more when we are placed under stress.

Fingerplates, called galvanometers, are attached to two of the subject's fingers.

These plates measure the skin's ability to conduct electricity.

When the skin is hydrated (as with sweat), it conducts electricity much more easily than when it is dry.