This presentation is regarding the technology involved in the polygraph machine. It describes the procedures involved in carrying out the so called 'Lie detector ' test for criminal investigation and other purposes.
3. INTRODUCTION TO POLYGRAPH
A polygraph measures and records several physiological
indices such as blood pressure, pulse, respiration, and skin
conductivity.
Commonly referred to as “lie detector”.
Invented in 1921 by John Augustus Larson.
US federal government agencies such as the FBI and the
CIA and many police departments such as the LAPD use
polygraph examinations to interrogate suspects and screen
new employees.
Within the US federal government, a polygraph
examination is also referred to as a psychophysiological
detection of deception (PDD) examination
4. General theory behind polygraph
Arousal Increased ANS activity
○Sweating
○Respiration changes
○Vasoconstriction
○Pulse rate
○Blood pressure
○Specific patterns of arousal during questioning
could indicate guilt or lying
5.
6. Polygraph – Relevant/Irrelevant Test
Earliest method of polygraph testing
Two kinds of questions
Relevant
○ Deal with issue at hand
Irrelevant
○ Deal with outside facts or details
Assumption:
A liar or guilty person will be more aroused by relevant
questions than Irrelevant ones, while an innocent person
will show no difference
○ So, if arousal(relevant) > arousal(irrelevant) = lying
7. Polygraph – Comparison Question
Test
Most common method of polygraph interrogation
◦ Developed by John Reid
Begins with extensive pre-test interview
Three kinds of questions:
◦ 1. Irrelevant
Is your name Orenthal James Simpson?
◦ 2. Comparison (aka probable lie)
E.g. “Have you ever physically harmed someone”
◦ 3. Relevant
E.g. “Did you kill Nicole Brown Simpson”
8. • A liar become more aroused by lying to
the relevant questions than the
comparison questions
• An innocent person will be more
aroused by the comparison questions
• Arousal(relevant) > Arousal(comparison) =
guilty