Lie Detectors Don’t Work as Advertised and They Never Did

Barred from use in U.S. court, lie detectors are still used today in other parts of the legal system

A man administers a security screening at the Clinton Engineer Works, part of the Manhattan Project. By Ed Westcott, [Public domain], via Wikimedia Commons

“Lie detector.” The name has a promising ring, but in reality the polygraph test that we know under that name is anything but.

Leonarde Keeler administered the first lie detector test in a court on this day in 1935. That’s 82 years ago. They're still used today in a variety of places, but they've never been proven to work. 

In that 1935 case, writes Brandy Zadrozny for The Daily Beast,  the machine’s readout was considered admissible evidence in court and both prosecutor and defense had agreed to its use. “On the stand Keeler was measured in his statements,” she writes. “‘I wouldn’t want to convict a man on the grounds of the records alone,’ he told the judge. But outside the courthouse, Keeler beamed when the jury returned with a guilty verdict. ‘It means the findings of the lie detector are as acceptable in court as fingerprint testimony,’ he told the press.”

But even then, she writes, an earlier Supreme Court case had already said decided that the lie detector, which didn’t have approval from the scientific community, wasn’t able to give admissible evidence. In almost every instance since, the polygraph has been “barred from federal and most state courts.” But elsewhere in the legal system, they still use it—mostly, it seems, to intimidate.

Here’s what a lie detector does, in the words of the American Psychological Association:  “So-called ‘lie detection’ involves inferring deception through analysis of physiological responses to a structured, but unstandardized, series of questions.”

We all know what it looks like when a lie detector is used: the machine provides polygraph readouts of a person’s physical responses to the questions that are asked. It usually measures heart rate/blood pressure, breathing and skin conductivity, writes the APA.

The questioner—in fiction, usually a cop—asks the person hooked up—in fiction, usually a suspect—a series of questions, beginning with simple questions designed to establish a baseline of what readouts are “normal” for the person in the chair. “What is your name,” is a common one. In real life, the APA writes, the most common method of questioning uses broader-based questions about “misdeeds that are similar to those being investigated, but refer to the subject’s past and are usually broad in scope.” An example: “Have you ever betrayed anyone who trusted you?”

The two biggest problems, writes the APA, are these: there’s no way to know if the symptoms of "bodily arousal" (like an elevated pulse) that the machine measures are caused by lies, and there’s no way to know if someone’s results are affected by the fact that they believe in the polygraph machine. If this second view is correct, they write, “the lie detector might be better called a fear detector.”

Of course, “the quest to defeat lying is as old as humanity,” writes Drake Bennett for Bloomberg Businessweek. Bennett goes on:

In Bronze Age China and India, suspects had to chew uncooked rice and spit it out to reveal if their mouths were dry. Medieval Europe had trial by fire or water. In the 1950s and ‘60s, the CIA experimented with LSD as a truth serum. Then there’s torture, formalized in ancient Greece as a method to compel honesty and recast for the 21st century as “enhanced interrogation.”

The lie detector, Bennett writes, is “today’s most widely trusted lie-detection device.” Even though its evidence cannot be used in a court of law, it helps determine how those in positions of trust—the CIA, the FBI, police departments—get hired. “Police detectives use it as an investigative tool, intelligence officers use it to assess the credibility of sources, and exams are commonly required as a condition of parole and probation for sex offenders,” he writes. Lives and livelihoods can hang on its readouts, but it's not a reliable test of any one thing.  

“What distinguishes a culture is how it copes with deceit,” writes historian Ken Alder in Lie Detectors: The History of an American Obsession: “the sort of lies it denounces, the sort of institutions it fashions to expose them.” America, he writes, is the only country that has produced the polygraph test.

We know that lie detectors lie. But we still use them. What does that say about us?

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