Polygraph Testing: What You Should Know

Polygraph testing is just one tool the the police and prosecution may try to use against you if you’re facing criminal charges. Connect with a criminal defense attorney before you answer any questions. Fill out the form on this page or call 877-445-1059 to find a lawyer near you.

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Polygraph tests appear regularly in crime dramas, thriller novels and detective movies – often to offer irrefutable evidence about a suspect’s innocence or guilt. In reality, though, the science behind polygraphs is hotly debated. Here’s what you need to know.

What is a Polygraph?

A polygraph is an instrument that measures various physiological responses of the human body, such as pulse, heart rate, blood pressure, rate of breathing, and rate of sweating. The word comes from the prefix “poly,” meaning “many” and “graph,” meaning something written or drawn. Basically, the polygraph machine got its name because of all the lines it etches while in response to body functions.

The theory behind using polygraphs as lie detector tests is that people under stress (from lying) will have certain physiological responses (like sweating, increased heart beat, raised blood pressure, etc.), which will show up on the gauges.

What Happens during a Polygraph Test?

Because polygraphs as lie detection tools are so controversial, it’s important to understand how they’re used and where mistakes can enter the equation. After hooking a subject up to a polygraph machine, a tester asks a series of questions.

  • Control Questions: Testers ask subjects certain “control questions,” such as “have you ever lied to avoid trouble?” These questions are expected to elicit false responses from subjects since it’s assumed that everyone has lied at some time to stay out of trouble, but wouldn’t want to admit it. Testers may even encourage lying (subtly) by suggesting that anyone who would answer yes would also commit the crime in question.
  • Baseline Responses: Assuming subjects lie on the control questions, testers have an example of a subject’s physiological functions while lying.
  • “Baseline for Truth”: Testers often ask subjects obvious questions like “Are the lights on in this room?” Testers may claim that the results recorded from these questions provide a “baseline for truth,” but in fact, they simply allow for some down time between important questions, so that the results are more easily decipherable.
  • Relevant Questions: During the examination, the tester asks the subject questions relevant to the investigation (e.g. “Did you commit the crime?”). The machine records the subject’s physiological responses to such questions.
  • Follow-up Interview: Following the polygraph test, the tester may try to elicit a confession from the subject, using the results of the polygraph as leverage, regardless of how the subject performed.

“Passing” and “Failing” a Polygraph

Once the results are in, a polygraph test is examined by the tester or another polygraph expert to determine whether the subject’s test indicates a pass, a fail or inconclusive results.

  • Passing: If the recorded responses for relevant questions is less severe than those for the control questions (i.e. the subject was less agitated when asked about the crime than about general subjects), the subject “passes” the test and is assumed not to have lied.
  • Failing: If the recorded responses for relevant questions indicate more activity than those for the control questions (i.e. the subject was more agitated when asked about the crime than about general subjects), the subject “fails” the test and is assumed to have lied.
  • Inconclusive Results: If the responses recorded for the control questions and the relevant questions are roughly equal, the results are deemed inconclusive.

Because of the way results are judged, polygraph tests tend to be biased against the innocent. This happens because those who are more relaxed and honest during control questions have lower baseline responses and therefore require only a slight increase in discomfort during relevant questions to “fail” the test.

Similarly, some researchers suggest that anyone can “beat” a polygraph by skewing the results for baseline questions (by thinking exciting thoughts, performing mental math problems, biting the tongue during questioning, etc.), therefore making their responses to relevant questions less dramatic in comparison.

Controversy over Polygraphs as a Scientific Tool

Because polygraphs measure presumed byproducts of lying, rather than actual truthfulness, scientists have conducted much research to determine their value and reliability as tools for criminal investigations. Briefly, some of the results:

  • In 1997, 421 psychologists reviewed test results and estimated that the average accuracy of polygraph tests is 61%, which is little better than chance.
  • In 1998, the Supreme Court ruled in United States v. Scheffer that there is no consensus that polygraph evidence is conclusive. The decision also noted that, unlike experts on other types of evidence, polygraph experts can offer only opinions to a jury.
  • A 2003 report by the National Academy of Sciences found that polygraphs may have some utility, but none of the evidence reviewed suggested that the tools had an especially high level of accuracy.
  • The admissibility of polygraph evidence is determined at the local level, meaning that the federal government has neither endorsed nor refuted its accuracy.

Other Methods of Lie Detection

Polygraphs may be the best known method of detecting lies to most people, but they are not the only tool available to criminal investigators. Other methods include the following.

  • Voice Stress Analysis (VSA): This relatively new technology bases its results on the principle that stress causes muscular reactions not controllable by the conscious mind. One of these reactions apparently causes a subtle, machine-detectable change in a voice’s pitch.
  • Functional Magnetic Resonance Imaging (fMRI): This is a way of mapping active neurons in the brain, and in theory could identify when a person is lying based on what areas of the brain are active.
  • Electroencephalography/Brain Fingerprinting: This allows testers to measure electrical activity in the brain, and could be used similarly to the fMRI test.
  • Truth Drugs (Sodium Thiopental): Considered illegal in international law, these drugs have various effects on the subject. Some reportedly interfere with judgment and brain function and cause the subject to talk more. But they’re not always accurate, as some subjects apparently mix fact with fiction and/or are affected only if they’re aware they’re under the influence of a “truth drug.”

Obviously, polygraph tests are designed to make you nervous. Fortunately, you don’t have to go through them alone. A criminal defense attorney may be able to help you prepare for the test, which could lower your stress during the questioning.

It’s your right to have an attorney present during questioning. Let Total Criminal Defense connect you with a lawyer in your area. Simply fill out the free case evaluation form or call us at 877-445-1059.

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