psy203s detecting deception2
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The Polygraph
The ideal: a machine to detectlies No personal bias Reliable, objective, automatic
Since 1890s: the polygraph A physiological measuring device Measures several channels (heart
rate, respiration, GSR, bloodpressure)
In wide use worldwide Popular in South Africa (insurancecompanies, recruiting)
Eg. Pick n Pay, De Beers Marine,
First National Bank, KentuckyFried Chicken, SA RevenueService
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The polygraph
A polygraphexaminationunderway
A paperrecording of
polygraph data(digital versionsimilar)
Actual polygraphoutput
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Technology not changed since1900s Now it records digital
Physiological measurements arevery accurate Some sensitivity to movement, etc
but can be compensated for
Can record for extended periodsof time
Only measures physicalvariables Not lying/innocence! Lying is determined by making
inferences about the physicalmeasurements
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Inferences about lying
How do you determine lyingfrom physiological data? No actual theory!
Basic idea: Lying will lead toincrease in arousal
Increase in arousal has aparticular reaction Increase in blood pressure, heart
rate, respiration Decrease in GSR
Look for this pattern in theprintout
These variables also varynaturally, often a lot
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An arousal increase?
Is it true that an arousalincrease goes with lying? Assumed rather than
demonstrated
Arousal increases can occurdue to a number of situations Not only lying (eg. stress about
the test) The machine cannot differentiate
between these!
A problem: What do you use asyour baseline?
A neutral statement A harmless lie (?)
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The relevant/Irrelevant test(RIT)
One way of using the polygraph The original way
Two types of questions asked Simple statements, short answers
(yes/no, etc) Relevant questions (about the crime,
etc) Did you take the money? Irrelevant questions (used for
baseline/control) Do you live in CapeTown?
If activity is greater in relevantquestions, conclude the subject islying
BUT: Relevant questions will lead to anincrease in arousal anyway! (falsepositive rate is high)
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Control Question Test (CQT)
Most common polygraph test inuse
Compare critical questions withunrelated lies Critical: Did you take the money? Unrelated: Have you ever stolen
anything before this year?
Questions discussed before the
examination
If the critical response is greaterthan the unrelated one,
conclude he was telling a lie
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Problems with the CQT
It is necessary for the subject tobelieve the polygraph works To establish the unrelated lie
baseline stimulus test (eg. fake card trick)
Much of the effect of the testoccurs before you begin! Trick your subject Examiner establishes themselves
in a position of power over thesubject
Great variability on results
depending on the examiner A lot depends on the questionschosen
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Control in the CQT
The control questions (unrelatedlies) are not effective controls They do not show that the
increase in critical questions canonly be due to lying
The content of the critical questionmay greatly increase arousal in an
innocent subject The unrelated lie may not lead to
significant arousal (didnt care)
In legal disputes, criticalquestions will probably lead tohigh arousal, even in innocentsubjects
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External information in theCQT
The polygraph operator has severalroles Operates the machine Interrogates the subject expected to provide the answer to the
mystery
Polygrapher often knows about thecase before the test External information is used to reach a
conclusion
Removes the machine objectivity of thetest Polygraph used as a tool for coercing
confessions
Should use blind examiners only
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So what if the theory sucks?
Even if lying/arousal is notrelated so what? If the machine can detect lies,
theory is irrelevant We are solving a practical
problem! Use empirical studies to measure
the usefulness of each test
The RIT does very badly Correctly identifies lies only 50%
of the time Effective guessing the result
(coin toss would be as good) Most researchers agree the RIT is
useless to detect lies.
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How good is the CQT?
Attracted a lot of research Lab experiments and field studies
Confused results ( 40 studies) Lies accurately detected with 78%
accuracy (53% - 90%) Innocents accurately detected with
84% accuracy (70% - 90%)
Lab experiments have beencriticized Unrealistic (low external validity) Perfect conditions for the machine
(overestimate accuracy)
Big difference between real-worldlying and lab lying
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Field studies of CQT accuracy
Major problem: Was a lie reallytold? Ground truth mostly not available
Confession or externalcorroboration (rare) No clear agreement on what is
acceptable to include
Iacono & Lykken (big critics) Sampling bias in confession cases Innocents who failed the test are
omitted from the sample Guilty cases who got away with it
are not included in the sample Studies are heavy with cases of
successful identification (failuresmissing)
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Field results for the CQT
Raskin & Honts (proponents ofCQT) Guilty correct identifications
average 86% (73% - 100%) Innocent correct identifications
average 50% (30% - 83%)
Iacono & Lykken (oppose theCQT) Find about the same numbers
Numbers are not very good Average at catching liars
Very likely to generate falsepositives (horror!)
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The polygraph andemployment screening
Difference between criminal useand employment use Employers want to know if a
person is honest, truthful Event-free use of the polygraph
Orwellian fantasy People will be honest if the
machine can tell when they lie!
In event free situations, the RITis often used The CQT designed to ask about a
specific thing RIT you can ask about anything
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A big problem: base rate
Types of events managementwants to uncover are very rare But the accuracy of the polygraph
itself is low
This leads to extremely highfalse positive rates (Bayesian
probability calculation) A lot of people being turned
down/fired With 2M screenings, as many as
320 000 in the US each year(estimate mid 1980s)
USA now has a law preventingpolygraph use in the workplace But we still use it (Yay! Yay!)