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Lecture notes: PS1810: Detecting Deception
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Semester 1 PS1810 notes. Written in 2020/21
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PS1810-WEEK 6 – Detecting DeceptionLying
“A successful or unsuccessful deliberate attempt, without forewarning to create in another a
belief which the communicator considers to be untrue” (Vrij, 2008, p15)
DePaulo et al. (1996) Diary study:
- College students told 2 lies per day
- Community members told 1 per day
Frequency of lying changes depending on relationship, but it does occur in all relationships
(DePaulo & Kashy 1998)
What is polygraphy?
People think ‘Lie detectors’
But just a device that can accurately measure physiological changes:
- Blood pressure
- Respiration rate/skin conductance
- Heart rate
- Etc… Its not actually standardised
And doesn’t detect lies very well at all
How is a lie detector meat to work?
Lying can be an emotionally difficult act and so we may show some physical responses when
we do it
This change in physiological response can be read very accurately by the polygraph
Lots of different methods
- Comparison Question Test (CQT)
- Guilty Knowledge Test (GKT)
Control Question Test (CQT)
Compares physiological responses to ‘control’ questions with physiological responses to
‘relevant’ questions
Control questions explore acts related to crime but not specific to the crime the person is
accused of
- “Have you ever wanted to hurt someone who upset you”
- Essentially designed to force examinee to lie
Relevant questions = address the crime itself. Very specific
E.g. “Did you steal the wallet from the kitchen?”
Control Question Test (CQT)
Guilty person should show a larger physiological response to relevant questions compared
to their physiological responses to control questions
Innocent person should show a larger physiological response to control question compared
to their physiological response to the relevant question
How good is the CQT? (Vrij, 2008)
Field studies Laboratory studies
Data from real Mock crime:
cases - Some guilty/some
CQT used innocent
Ground truth not participants
known Given CQT
Corroboration: Ground truth known
- Confession? Determine accuracy
- DNA? 74-80% lie
- Other 63-84% truth
evidence?
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