CHAPTER 4.docx

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30 Mar 2012
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CHAPTER 4
THE POLYGRAPH TECHNIQUE
- Underlying principle of polygraph: deception is associated with physiological change. The origins
of modern polygraph date from 1917 when William Marston developed a systolic blood
pressure test and attempted to use this physiological response as evidence for a person’s
innocence
- Polygraph: A device for recording an individual’s autonomic nervous system
- Measurement devices are attached to upper chest and abdomen to measure breathing. The
amount of sweat on the skin is measured by attaching electrodes to the fingertips. Sweat
changes the conductance of the skin, which is known as the galvanic skin response. Finally, heart
rate is measured by a partially inflated blood pressure cuff attached to an arm. Each of these
measures is amplified and can be printed out on paper or stored in a computer to be analyzed.
Polygraph is used to measure a person’s physiological responses to questions asked by an
examiner
Applications of the Polygraph Test
- In Canada, the police often use them to help in their criminal investigations
- Although not common, police may ask alleged victims of crimes to take a polygraph test to help
to verify whether a crime has occurred.
- Polygraph disclosure test: Polygraph tests that are used to uncover information about an
offender’s past behaviour
- Polygraph tests are used to determine whether the offender is violating the conditions of
probation or are used to test for evidence of risky behaviour, such as sexual fantasies about
children.
- In United States, polygraph testing was done on employees to identify those who engaged in
theft or those using drugs at work. For the selection process, it was also used to weed out those
with criminal tendencies and substance abuse problems.
Types of Polygraph Tests
- The two main types of polygraph tests are reviewed below:
o The comparison question test:
Comparison Question Test (CQT): Type of polygraph test that includes
irrelevant questions that are unrelated to the crime, relevant questions
concerning the crime being investigated, and comparison questions concerning
the person’s honesty and past history prior to the event being investigated
A critical component of this technique is the pretest interview
There are three possible outcomes of a polygraph test: truthful, deceptive, and
inconclusive
o The Concealed Information Test:
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Concealed Information Test (CIT): Type of polygraph test designed to determine
if the person knows details about a crime
A CIT question in the context of homicide might take the following form:
“Did you kill the person with (a) a knife, (b) an axe, (c) a handgun, (d) a
crowbar, or (e) a rifle?” The guilty suspect is assumed to display a larger
physiological response to correct option than to the incorrect option. An
innocent person, conversely, who does not know the details of the
crime, will show the sae physiological response to all options
Underlying the CIT is the principle that people will react more strongly to
information the recognize as distinctive or important than to unimportant
information
The most common physiological response measured when administrating the
CIT is palmar sweating (i.e. Skin conductance response measured in the palm of
the hand)
Although law enforcement in Canada and the united States does not routinely
use the CIT, it is used regularly in a limited number of other jurisdictions, such as
Israel and Japan
Validity of Polygraph Techniques
Types of Studies:
- Studies of the validity of polygraph techniques can be classified into two types: laboratory and
field studies.
- In laboratory studies, volunteers (often university students) simulate criminal behaviour by
committing a mock crime. Volunteers come to a laboratory and are randomly assigned to one of
two conditions: committing a mock crime or not committing a mock crime. The main advantage
of these studies is that the experimenter knows ground truth (i.e. who is truly guilty or
innocent).
- The results of laboratory studies may have limited application to real life
- Field studies involve real-life situations and actual criminal suspects, together with actual
polygraph examinations. Field studies often compare the accuracy or “original” examiners to
“blind” evaluators
- The largest problem with field studies is establishing ground truth. To deal with this problem,
two additional ways of establishing ground truth have been developed: judicial outcomes and
confessions.
- Thus, reliance on confessions to establish ground truth likely inflates polygraph accuracy rates
Polygraph Tests: Accurate or Not?
- It is controversial
- High false-positive rates indicate that innocent people respond more to relevant than control
questions, suggesting that the premise underlying the CQT does not apply to all suspects
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