Why Aren't Lie Detectors Used in Legal Situations?
Lie detectors seem like an easy solution where legal matters are concerned, but the relationship between the courts and lie detectors isn't an easy one. From stress levels to human nature, the legality of lie detectors is a matter of debate.
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Polygraphs
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Lie detectors, or polygraphs, are an instrument used to measure certain physiological responses to determine whether the answers to questions are truthful or not. Since their development in the late 19th century, polygraphs have experienced a tumultuous relationship with law.
How They Work
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The basic concept behind lie detectors is the participant's stress level. This stress is measured through the monitoring of breathing, pulse and galvanic skin response. When the results are analyzed, the moments of most biologic reaction are determined to be a lie.
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Stress
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The inherit problem with lie detectors is in the stress. Though simple test questions determine the base level of the participant's stress, just the process of a lie detector test can be very stressful and can produce untrustworthy results. For some, lying isn't a stressful thing, and polygraphs can be cheated. Others induce more stress upon themselves --through pain, for example--to confuse the polygraph.
Truth
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Another problem of lie detector tests is in the variable nature of truth. Two people can experience the same event and have two completely different ideas of what happened. To each person, their version is the truth. It is also possible for a person to convince themselves that an event happened differently than it actually did--in their mind, they are telling the truth.
Legal History
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In 1923, a U.S. court denied polygraph evidence in a murder case; this set the precedent for polygraph use for the next 70 years. From 1993 on, polygraph evidence was allowed, but only on the judge's discretion. Most courts only allow polygraph evidence if both prosecution and defense agree to the admission.
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References
- Photo Credit US Supreme Court image by dwight9592 from Fotolia.com
