The theory underlying a lie detector test -- or a polygraph
test, in more scientific terms -- is that lying is stressful,
and that this stress can be measured and recorded on a
polygraph machine. Lie detectors are called polygraphs because
the test consists of simultaneously monitoring several of the
suspect's physiological functions -- breathing, pulse and
galvanic skin response -- and printing out the results on
graph paper. This printout shows exactly when, during the
questioning period, the biologic responses occurred. If the
period of greatest biologic reaction lines up with the key
questions on the graph paper -- the questions that would
implicate the person as being involved with the crime --
stress is presumed. And along with this presumption of stress
comes a second presumption -- that the stress indicates a lie.
Supporters of lie
detector tests claim that the test is reliable because:
- very few people
can control all three physiological functions at the same
time, and
- polygraph
examiners run preexamination tests on the suspect that
enable the examiners to measure that individual's reaction
to telling a lie.
On the other hand,
critics of polygraph testing argue that:
- many subjects can
indeed conceal stress even when they are aware that they
are lying, and
- there is no
reliable way to distinguish an individual's stress
generated by the test and the stress generated by a
particular lie.
The courts in most
jurisdictions doubt the reliability of lie detector tests and
refuse to admit the results into evidence. Some states do
admit the results of polygraph tests at trial if the
prosecution and defendant agree prior to the test that its
results will be admissible.
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