Polygraph testing, commonly known as “lie detectors,” are often used in sex crime cases, including investigation, prosecution, defense and for managing convicted sex offenders while they are on parole.

With all the questions about the accuracy of polygraph tests, how much legal weight do these tests carry?

Lie detectors for defendants

An accused sex offender may be compelled to submit to a polygraph test interrogation by law enforcement to help establish guilt. But the offender may choose to take a second test, through an impartial third-party company, with coaching from their attorney, if they believe it will weaken the prosecution’s case.

These test results aren’t a silver bullet for the prosecution or defense, but it has the potential to move the needle in favor of the defendant.

Lie detectors for prosecutors

While this option is available to the prosecution, due to the muddy science of polygraph testing, there are limits as to how this evidence is used. Namely, both the prosecution and defense must agree to the testing and the admissibility of the results in court.

The use of lie detectors by law enforcement

Washington State allows law enforcement to use polygraph testing on alleged sex crime victims when determining probable cause, before an arrest warrant is issued for the alleged offender. However, this is entirely voluntary. Police cannot force a victim to submit to a test.

Lie detectors for convicted sex offenders on parole

After a convicted sex offender is released and placed on a sex offender registry, courts may consider polygraph test results when reviewing the conditions of their parole. This is often done when the offender requests removal from the sex offender registry after a period of good behavior.

Offenders sentenced to mental health treatment facilities may also opt for a test when petitioning for release.

The drawbacks of using lie detectors

Polygraph testing is subjective and questionable, even under the best circumstances. This is particularly true for people defending themselves against serious crimes, who will be anxious and nervous, even if they are innocent.

Polygraph tests measure physical responses, including heart rate, breathing, blood pressure and increased sweating. All of these conditions will certainly be erratic and heightened for someone facing serious charges. Therefore, the possibility for a “false positive” is high. Likewise, an individual with exceptional bodily control can beat the test, even while lying.

Furthermore, a variety of mental and physical conditions can affect test results, such as anxiety, post-traumatic stress, heart conditions, chronic pain or even a mild cold. Medication for any of these conditions can also affect results.

For now, polygraph tests used during criminal cases are, at best, weak evidence for either side. If you feel that a test might help your case, consult an attorney for advice.