1 The Polygraph The ideal: a machine to detect lies –No personal bias –Reliable, objective, automatic Since 1890’s: the polygraph –A physiological measuring device –Measures several channels (heart rate, respiration, GSR, blood pressure) In wide use worldwide –Popular in South Africa (insurance companies, recruiting) –Eg. Pick ‘n Pay, De Beers Marine, First National Bank, Kentucky Fried Chicken, SA Revenue Service

2 The polygraph A polygraph examination underway A paper recording of polygraph data (digital version similar) Actual polygraph output

3 Technology not changed since 1900s –Now it records digital Physiological measurements are very accurate –Some sensitivity to movement, etc but can be compensated for –Can record for extended periods of time Only measures physical variables –Not lying/innocence! –Lying is determined by making inferences about the physical measurements

4 Inferences about lying How do you determine lying from physiological data? –No actual theory! –Basic idea: Lying will lead to increase in arousal Increase in arousal has a particular reaction –Increase in blood pressure, heart rate, respiration –Decrease in GSR Look for this pattern in the printout –These variables also vary naturally, often a lot

5 How to look for a lie Look at all four channels –Any one of them may tell you –An increase will indicate an increase in arousal and thus a lie How much of an increase indicates a lie? –Depends on each person –Must compare within subjects –Compare a ‘truth’ situation with a ‘lie’ situation Obtain baselines –Ask subject to lie about something unrelated, check levels. –Do the same for truth telling

6 An arousal increase? Is it true that an arousal increase goes with lying? –Assumed rather than demonstrated Arousal increases can occur due 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 as your baseline? –A neutral statement –A harmless lie (?)

7 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 Cape Town?” If activity is greater in relevant questions, conclude the subject is lying –BUT: Relevant questions will lead to an increase in arousal anyway! (false positive rate is high)

8 Control Question Test (CQT) Most common polygraph test in use Compare critical questions with unrelated 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 greater than the unrelated one, conclude he was telling a lie

9 Problems with the CQT It is necessary for the subject to believe the polygraph works –To establish the unrelated lie baseline –“stimulus test” (eg. fake card trick) Much of the ‘effect’ of the test occurs before you begin! –Trick your subject –Examiner establishes themselves in a position of power over the subject Great variability on results depending on the examiner –A lot depends on the questions chosen

10 Control in the CQT The control questions (unrelated lies) are not effective controls –They do not show that the increase in critical questions can only be due to lying –The content of the critical question may greatly increase arousal in an innocent subject –The unrelated lie may not lead to significant arousal (didn’t care) In legal disputes, critical questions will probably lead to high arousal, even in innocent subjects

11 External information in the CQT The polygraph operator has several roles –Operates the machine –Interrogates the subject –‘expected’ to provide the answer to the mystery Polygrapher often knows about the case before the test –External information is used to reach a conclusion –Removes the ‘machine objectivity’ of the test –Polygraph used as a tool for coercing confessions Should use ‘blind’ examiners only

12 Beating the polygraph All polygraph tests work on the basis of an arousal comparison –Base state vs. lying state You will know which questions are control questions and which are relevant –Increase arousal in control state to remove the difference –Confuses the examiner (strange pattern) How to increase arousal –Clench leg muscles, count backwards from 100 in 13s, think of something annoying, etc. Must do it without the examiner knowing –Will prevent non-polygraph information from being emphasized

13 So what if the theory sucks? Even if lying/arousal is not related 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.

14 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 been criticized –Unrealistic (low external validity) –Perfect conditions for the machine (overestimate accuracy) –Big difference between real-world lying and lab lying

15 Field studies of CQT accuracy Major problem: Was a lie really told? –Ground truth mostly not available –Confession or external corroboration (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 (failures missing)

16 Field results for the CQT Raskin & Honts (proponents of CQT) –Guilty correct identifications average 86% (73% - 100%) –Innocent correct identifications average 50% (30% - 83%) Iacono & Lykken (oppose the CQT) –Find about the same numbers Numbers are not very good –Average at catching liars –Very likely to generate false positives (horror!)

17 The polygraph and employment screening Difference between criminal use and 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 RIT is often used –The CQT designed to ask about a specific thing –RIT you can ask about anything

18 A big problem: ‘base rate’ Types of events management wants to uncover are very rare –But the accuracy of the polygraph itself is low This leads to extremely high false positive rates (Bayesian probability calculation) –A lot of people being turned down/fired –With 2M screenings, as many as in the US each year (estimate mid 1980s) USA now has a law preventing polygraph use in the workplace –But we still use it (Yay! Yay!)