How a Lie Detector Works?
                     HOW A LIE 
DETECTOR WORKS?
Introduction
The goal of a lie detector is to see if the 
person is telling the truth or lying when 
answering certain questions.
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What is a Lie 
Detector Test?
A lie detector test, also referred to as a 
polygraph test, is a device that is used to 
measure the changes that involuntarily 
occur in a person's body in response to a 
question or a statement. The primary 
purpose of a polygraph test is to detect 
deception.
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Origin and History
The scientific name for a lie detection device 
comes from two Greek roots. “Graph” means 
“writing” and “poly” means “many.”  
Therefore, a polygraph is a measure of 
multiple physiological functions together into 
one written record. Today, however, it is likely 
that the record is on the computer rather 
than on paper.
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How Do Lie 
Detectors Work?
Lie detector tests use a polygraph 
machine that consists of different parts to 
measure the changes in the physiological 
aspects of a person while answering 
questions or reacting to statements.
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What Does a 
Polygraph Measure?
A modern polygraph consists of different 
parts that measure the physiological 
changes that occur in a person's body. 
Attached to the machine are different 
accessories such as pneumography, which 
are wrapped around a person's chest, and 
are used to record changes in the rate and 
depth of respiration.
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Phases of a 
Polygraph Test
1. The examiner shall get information 
relating to the examinee's version of 
facts regarding the issue being 
investigated.
2. The examiner discusses the issue and 
reviews the questions that will be 
asked during the test.
3. The examiner must give a thorough 
explanation of how the polygraph test 
works.
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Phases of a 
Polygraph Test Cont.
1. The examiner should inform the examinee 
of their constitutional rights to legal 
counsel and that the agreement to do a 
polygraph examination is a voluntary 
action.
2. The examiner must also ensure that the 
examinee has a clear understanding of 
the reason why they are asked to do a 
polygraph test. Should the examinee have 
any questions, the examiner shall answer 
all the questions of the examinee. Source: study.com
How Accurate are 
Polygraph Tests?
1. Using lie detector tests such as 
polygraphs has always been controversial. 
The reason for the controversy is that the 
validity of the test results can be 
questionable. 
2. One has to take note that the test only 
measures the physiological responses or 
changes that occur in a person while 
undergoing the polygraph test.
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Stress Response
The theory behind a lie detection machine is 
that deception causes physical stress on the 
body. When a person is under stress, the 
sympathetic nervous system produces 
specific and predictable responses in the 
body.
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Test Process
The examiner will require the subject to sit in 
a chair with feet flat on the ground and arms 
resting on arm rests. 
It is then necessary to apply a number of 
sensors to the subject’s body, usually on the 
arm, one or more fingers, and the upper 
body. 
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