Can a lie detector be used in court?

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Two men sit across from each other in a cramped grey room. The table between them is old and worn. A bare bulb beats down from above. One of the men is confident. He leans across the table. The other man shies away; nervous sweat begins to trickle down his forehead. On the table, a machine scratches away, its needle jumping across the page as accusations fill the air. The polygraph is iconic. It’s a staple of almost every crime show or movie. If TV is to be believed, then lie detectors are as common as phones at police stations. But just because we see it on TV, and just because police use them in real life, doesn’t mean that the results of a lie detector test can be used in court.

Polygraphs record small changes in your body as you answer questions; for example, a change in how fast your heart beats. An operator then reads and interprets those changes to tell if you’re lying.

Lie detectors are based on the assumption that people who lie will be nervous and that people who tell the truth won’t be. This isn’t always the case. A sociopath may be able to lie without breaking a sweat. An innocent person may be nervous about being questioned by the police. What you think you saw may not be what actually happened. If you consider these situations, you begin to see how polygraphs could cause problems in the courtroom.

In fact, that’s exactly what the Supreme Court Canada ruled in a 1987 court case, R. v. Béland. The criminal court system has defined rules about what can be used as evidence, and lie detectors break these rules. Since lie detectors don’t meet the court’s standards, they can’t be used as evidence. You can read the full reasons in the court case above.

In family law, it’s not as clear. Polygraph results have been allowed as evidence in at least one case but rejected in others.

Polygraph fact: William Marston invented a blood pressure test that became one of the key components of the first polygraph. He’s also the creator of Wonder Woman, who had a lie detector of her own: a magic lasso that makes people tell the truth.