The Physics of the Polygraph

Dillon M Mills
Physics 212 F03
Spring 2015

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General

This page gives a general overview of the polygraph.

Basic Information

Polygraphs are popularly referred to as lie detectors because they are designed to determine whether or not a person is being truthful when asked a series of questions. Polygraphs measure and record several physiological indicators such as blood pressure, heart pulse, respiration, and skin conductivity. The belief is that when a person answers a question deceptively, the polygraph will produce different measurements than if the person told the truth (Rosenfeld).

Testing Procedure

The examiner starts the polygraph session with a preliminary interview, gaining information in order to develop diagnostic questions for later. The examiner then explains how the polygraph works, asserting that it can detect lies so the subject should answer truthfully. Then the subject is asked to deliberately lie so the examiner can be sure the polygraph is working properly. Then the examiner begins the main testing process.
Some of the questions are considered irrelevant or IR; the examiner will ask the subject questions that are obvious in order to have polygraph data to compare the other question responses to. An example of an irrelevant question might be "Are you sitting down right now?" Another type of question is called diagnostic questions; the examiner will ask the subject questions that may be uncomfortable for the subject to answer. An example of a diagnostic question might be "Have you ever seriously though about harming another individual?" The rest of the questions are relevant questions or RQ; the examiner will ask the subject questions that the examiner actually wants to know the answer to. An example of a relevant question might be "Did you murder Batman?"
The examiner alternates the type of question he/she asks the subject. The answers are considered to be truthful if the physiological responses to the relevant questions are smaller than those during the diagnostic questions (Lewis).

In Japan, they use an alternative testing technique known as the Quilty Knowledge Test (GKT), or the Concealed Information Test (CIT). For this technique, the tester has no knowledge of the crime or circumstances in question. The tester quizzes the subject's knowledge of the crime that an innocent person would not know. The tester may ask the subject "What type of weapon was used to kill Batman?" then subject is given multiple choices to choose from. They then tell the subject the correct answer and monitor the subjects reaction with a polygraph. IF the subject reacts strongly to the information, then the subject likely knows facts relevant to the case (Polygraph Procedures).

Validity

The validity of polygraph testing is widely debated and criticized. There are a large number of cases that polygraphs were proven to have thrown a false positive, incriminating an innocent who "failed" the polygraph test.
It is argued that polygraphs are being used for measuring human emotions, but there is simply no way, currently, to measure human emotion. It is also argued that polygraphs measure arousal, which can be caused by a great many things such as anxiety, nervousness, fear, confusion, hypoglycemia, psychosis, depression, substance induced, substance withdrawal, or other emotions.
In addition, polygraphs accuracy has been known to be poor when testing sex offenders with inadequate evidence to support improved outcomes for cases in this population (Lewis).

Usage

The United States is the biggest user of the polygraph, specifically law enforcement and intelligence agencies.
Though a person's security clearance may not be revoked based solely on a polygraph test's results, a person's access to sensitive information may be denied if the test goes poorly. Furthermore, some jobs in the government specifically require successful completion of a polygraph test to be considered for employment.
It is difficult to tell whether or not polygraphs are effective at detecting spies, due to its validity issues. However, it is still used as a deterrent to espionage, "scaring straight" anyone that might consider it (Dedman).