Conditions That Can Cause a False Positive on a Polygraph Test

Polygraph tests aren’t psychic tools that can tell beyond any doubt that someone is lying. They sometimes produce false results. A false positive can occur when someone telling the truth triggers the device, which then may indicate he’s lying even if he isn't. Because of this, polygraph results aren’t admissible evidence in any state. Some states prohibit them entirely, while others will admit the results if all parties to the lawsuit or trial agree. Certain conditions may trigger a false positive.

Most polygraphs measure three things: the subject’s sweat, respiration and cardiovascular activities. Some more advanced methods add in other checkpoints, such as motion sensors, but these are mostly to account for any abnormalities in the three primary areas. One thing that isn’t measured is the tone or pitch of your voice.

Read More: How to Read a Polygraph Test

Although blood pressure is monitored during a polygraph, it should not produce a false positive result if you tell the examiner you have a condition before the test begins. You should also disclose if you’re taking medication. If you have high blood pressure, it will be elevated even when you’re asked innocent preliminary questions, such as those concerning your name or address. So while it can affect the test, it probably won’t affect the accuracy, because a baseline blood pressure reading is created by these first questions.

If you’re a basket case about taking the polygraph, this condition is going to be present throughout the entire test. All your monitored activities will be higher from the start – not necessarily because you’re lying but because you’re stressed. This also creates a baseline, or norm, so if your physical responses ratchet up at certain questions, this could – rightly or wrongly – create a positive result. Your nerves might also cause a false positive if they cause you to wriggle in your seat, breathe deeply to steady yourself or shift your weight in your seat.

It’s a myth that illicit drugs or alcohol can help you beat a polygraph test if you’re actually lying, but they can still affect the test, depending on the nature of the medication. The examiner can make allowances for this if you tell him. Otherwise, it could lead to a false positive if he can’t identify the cause of any reactions you may have if you've been drinking or taking drugs (prescription or otherwise).

If you’re pregnant, this shouldn’t skew the test, but you should tell the examiner about your condition if it’s not obvious. Most examiners won’t test you in the first place if you’re carrying a baby, at least not without approval from your doctor. A pregnant woman may have elevated blood pressure or have other vital signs affected that may change the results of a polygraph.

Other factors that can cause a false positive aren’t medical in nature. If you’re extremely tired – because you’re coming off 24 hours or more without sleep, for example – this can affect test results, either positively or negatively. Avoid tight and restrictive clothing, because physical discomfort can alter the results. Don’t squeeze the polygraph appointment in between other must-do activities. If you’re worried that you’re not going to get to your next appointment on time, this could affect the test. Polygraph examinations can take up to six hours or more.