Module 5 - Polygraph Testing & Forensic Document Analysis

Image courtesy of the Virginia State Police Department website (www.vsp.state.va.us)

Contrary to popular opinion, polygraph instruments are not able to specifically detect lying.  A polygraph instrument can effectively monitor and record the involuntary sympathetic nervous system responses occurring when a subject is stressed because of lying.  By asking a subject questions about a particular issue and analyzing the physiological reactions to such questions, a polygraph examiner can determine if lying has occurred.

Each person develops a unique way of reacting to stress.  The polygraph examiner pays special attention to a subject"s reactions to all questions throughout the polygraph examination in an effort to develop a sense of the subject"s values, beliefs, and attitudes.

Phases of the Polygraph Examination

A polygraph examination takes from 1 to 3 hours. The length of an exam depends on its purpose. Some polygraph exams may not take long because a subject confesses to a crime; others may take a long time if the polygraph examiner needs to explore an inconsistent or inconclusive set of responses.

The three phases of the polygraph examination include the pre-test interview, the question procedure, and the post-test interview and analysis. 

1.  Pre-test Interview

Before a pre-test interview, the polygraph examiner studies information about the subject (medical background, physiological condition, psychological history) and details regarding the related criminal case. The polygraph examiner then prepares questions that focus on facts related to the criminal case. 

Three basic types of questioning techniques are used by polygraph examiners:  the relevant/irrelevant (R/I) technique, the control question technique (CQT), and the concealed information technique (CIT).  CQT questions are normally used in criminal investigations.

In the CQT, a mixture relevant questions and control questions are given. Inquiries directly related to a criminal case are called relevant questions, but control questions are not directly related to the case. The subject is not told there is a difference between the relevant and control questions. Polygraph examiners do not create questions that ask the subject about opinions or beliefs.
 
During the pre-test interview, the subject is involved in the creation of the control questions. This is done to convince the subject that these questions are just as important as the relevant questions.  The subject is told that the control questions will help determine if the subject is the type of person who could be involved with the crime being investigated. Control questions are used for comparison purposes.

The pre-test interview takes between 20 to 90 minutes. The polygraph examiner tells the subject details about what will occur during the entire polygraph examination and outlines all the questions the subject will be asked.  The goal of this phase of the examination is to impress thoroughly upon the subject that any deceptive answers will be spotted easily.  This places truthful subjects at ease and increases stress in deceptive and/or guilty subjects.

The Stimulation Test

Before or after the pre-test interview, polygraph examiners give a stimulation test to convince a subject further of the polygraph instrument"s accuracy. Stimulation tests are meant to reassure truthful subjects further and to provoke greater anxiety in deceptive subjects. Research suggests that these stimulation tests increase the reliability of polygraph exams.

In a common stimulation test, the subject is asked to select a number from between 1 and 10.  The subject then writes the number and shows it to the polygraph examiner.  The subject is then asked ten times:   Is the number 1? , Is the number 2? , etc.  The subject is told to knowingly lie and is instructed to say No to each question.  The subject is then shown the actual polygraph chart recordings that confirm that the subject is lying when the examiner states the correct number.

Continue to learn more about the polygraph examination...