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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!)