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How do lie detector tests work? And are they accurate?

On Behalf of | Nov 22, 2022 | Criminal law

The polygraph, or “lie detector” test, is a well-known piece of technology that most of us have seen in movies and television shows. But how often are these devices used in the “real world?” And are they reliable, or just another example of junk science?

Three biological responses measured

A polygraph test does not measure the accuracy of anything you say. Instead, it measures three biological processes. They are:

  • Sweat gland activity (trying to detect an increase in sweating)
  • Blood pressure/pulse
  • Respiration rate (how heavily you are breathing)

Why measure these biological activities? They are all considered attributes of the “autonomic nervous system.” In more common terms, they are automatic bodily responses that suggest the person being tested has gone into fight-or-flight mode. If someone fears being caught in a lie, they will theoretically experience increased heart rate and begin to sweat. Their breathing, however, may actually slow down rather than increase.

How accurate are these tests?

The reliability rate of polygraph tests depends on who you ask. The American Polygraph Association claims that they are about 85-90 percent accurate. Studies cited by other sources put the accuracy rate at 75 percent (or less in some cases).

Here is an important consideration: these autonomic responses can be triggered by many things other than the test subject’s fear that they will be caught lying. Health issues could cause us to sweat more or have a higher heart rate. An overly warm room could cause excess sweating. Someone prone to anxiety could exhibit changes in all three metrics simply because they are nervous about being tested.

On the other end of the spectrum, someone who is guilty of a crime could test as completely innocent, either because they are a very practiced liar or because they have an incredibly low fear response. In short, the inaccuracy of the tests and the various other explanations for “failing” mean that polygraphs are not reliable proof of guilt or deception.

Polygraphs are generally not admissible as evidence

Approximately 28 states have prohibited the admission of polygraph tests as evidence in a criminal trial. New Jersey is among approximately 18 states that only allow them as evidence if prosecutors and defense attorneys agree to include them. As you can imagine, this is rare occurrence.

Even if they can’t be used as evidence, some law enforcement agencies use polygraph tests to try to scare a defendant into making a confession. This is one of many reasons why anyone in police custody should have an experienced criminal defense attorney by their side before answering questions or agreeing to cooperate.