Transcript Detecting deception - University of Cape Town

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
1
The polygraph
A polygraph
examination
underway
A paper
recording of
polygraph data
(digital version
similar)
Actual polygraph
output
2
• 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
3
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
4
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
5
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 (?)
6
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)
7
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
8
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
9
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
10
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
11
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
12
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.
13
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
14
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)
15
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!)
16
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
17
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
320 000 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!)
18