PSYC 2400 Lecture Notes - Lecture 4: Ground Truth, John B. Larson, Lie Detection
Unit 4: Deception & Malingering
Section 1: The Polygraph: Lie Detector?
History of the Polygraph
• Long history of using physiology to detect deception
o Bedouins of Arabia: lick a hot iron, if tongue not burnt considered truthful
o Ancient Chinese: suspects chew rice powder & spit it out, if powder was dry
suspect was guilty
• Common principle: if lying or guilty → feel nervous, anxious → less saliva → dry mouth
o Detects anxiety, nervous, fear – anything that can increase arousal
Modern Day Polygraph
• 1917: William Marston
o Blood pressure
▪ Rises when lying
▪ Wonder woman creator – lasso
• 1921: John Larson
o included heart rate (blood pressure), respiration, & skin conductivity (sweat on
hand) – as anxiety goes up
o helped solve a murder case
Background & Usage
• What is the polygraph?
o Tool to measure physiological activity
• What is a polygraph exam?
o Combination interrogation + polygraph: responses to specific yes/no or MC
questions
• Usage
o Criminal investigations, security pre-employment screening, disclosure tests
▪ 2 types of disclosure tests: (1) treatment of sex offenders (2) condition of
sex offender release
Comparison Question Test
• Phase 1: Pretest interview
• 10 questions developed
o Irrelevant (to the crime): neutral questions (to get a baseline reading)
o Relevant: relating to the specific crime
▪ On Jan 7th did you enter the home of Betty Brown with an axe handle?
o Comparison: related to possible past misbehaviors
▪ Have you ever lied to a person in authority?
▪ Before you were 25 did you ever verbally threaten to hurt someone?
Comparison Question Test Phases 2, 3, & 4
• Phase 2
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o Questions asked while attached to polygraph
• Phase 3
o Score the physiological responses
o Assumptions
▪ Guilty will show greater response to relevant
▪ Innocent will show greater response to comparison
• Phase 4
o You failed the polygraph. You had better tell me what happened. I can help you.
Polygraph Tracing
• Top blue lines = respiration (one for people who breath more with stomach or chest)
• Gree lie = ski odutae he ae goes up, eas they’e respoded
• Red line = blood pressure
• Bottom of the screen = questions (IR, SR1, C3, etc)
CQT: Problems
• Use of extra-polygraph cues
o Not just physiology; pre-test, post-test, how you respond to the directions, &
examiner may be talking to lead detective which may influence the scoring
• Crucial role of examiner in preparing comparison questions
• Do innocent people show a stronger response to comparison vs relevant questions?
Concealed Info Test
• Probes for concealed info – not lying
• 5 questions asked
o Multiple-choice format
o The robber dropped which of the items while escaping:
▪ A knife? A face mask? Car keys? A sack of money? A gun?
• Assumptions
o Guilty suspect will recognize the correct alternative
o Innocent suspect will give a similar physiological response to each alternative
CIT: Problems
• Applicability
o Only if not much pre-trail media - questions only the suspect would know
• Examiner expectancy
o The way it is said may give cues
• Assumes suspect has knowledge of correct choice
• Does not confirm suspect committed offense
Section 2: Research on the Polygraph
Types of Research: Lab Studies
• Advantages
o Ground truth known
o Control over variables
• Disadvantages
o Samples used
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▪ Usually uni students which is different from age, education and
koledge of hat’s happeig
o Generalizability
▪ There’s ot really oittig a rie, & thus their arousal/eotios
most likely o’t e the sae
Types of Research: Field Studies
• Advantages
o Real suspects
o Trained polygraphers
• Disadvantages
o Ground truth not known
▪ Don't know if they are actually guilty - even if they confessed/convicted
in court bc false confessions & sentencing happens
o Extra cues may be used
▪ Physiological & other deception cues
Accuracy of Polygraph Tests
• Comparison Question Test
o 84 – 92% accurate at detecting guilty
o 9 – 24% of innocent falsely identified as guilty
o High false positive rate
• Concealed Info Test
o Around 95% accurate at detecting innocent
o 15 – 24% of guilty falsely identified as innocent
o High false negative rate
Countermeasures
• Aythig the perso does to fool the polygraph
• General state
o Drug or alcohol to diminish responsiveness
o These are easy to detect
• Specific point
o Physical
▪ If guilty, you want to make sure you respond a lot to the comparison
question
o Mental
▪ Distract mid, so do’t respod to either
Admissibility of the Polygraph
• R. v. Beland (1987)
o Polygraph results are not admissible in Canadian courts
o the polygraph has o plae i the judiial proess here it’s eployed as tool to
determine or test the rediility of itesses
Why is the polygraph still being used in police investigations?
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Document Summary
Comparison question test: phase 1: pretest interview, 10 questions developed. Comparison question test phases 2, 3, & 4: phase 2, questions asked while attached to polygraph, phase 3, score the physiological responses, assumptions, guilty will show greater response to relevant. Innocent will show greater response to comparison: phase 4, you failed the polygraph. A gun: assumptions, guilty suspect will recognize the correct alternative. Innocent suspect will give a similar physiological response to each alternative. Countermeasures: a(cid:374)ythi(cid:374)g the perso(cid:374) does to (cid:862)fool(cid:863) the polygraph, general state, drug or alcohol to diminish responsiveness, these are easy to detect, specific point, physical. If guilty, you want to make sure you respond a lot to the comparison question: mental, distract mi(cid:374)d, so do(cid:374)"t respo(cid:374)d to either. Admissibility of the polygraph: r. v. beland (1987, polygraph results are not admissible in canadian courts, (cid:862)the polygraph has (cid:374)o pla(cid:272)e i(cid:374) the judi(cid:272)ial pro(cid:272)ess (cid:449)here it"s e(cid:373)ployed as tool to determine or test the (cid:272)redi(cid:271)ility of (cid:449)it(cid:374)esses(cid:863)