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How Do Lie Detectors Work?

They’re notoriously unreliable, but continue to be used in criminal investigations.

how do lie detectors work
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If you turn on your TV and tune into an episode of a courtroom drama or police procedural, there's a good chance that you’ll run across a depiction of a polygraph administration—more popularly known as a lie detector test. These tests often add an interesting twist to fictional crime shows, with seemingly trustworthy characters shown to be hiding something, or someone who looks like a good suspect revealed to be telling the truth.

There’s a lot of fascination around these remarkable machines in real life, too. Vanity Fair even has an entire YouTube series devoted to administering polygraphs, in which viewers at home can watch their favorite celebrities squirm as they’re asked blunt or embarrassing questions. Videos grilling the likes of Will Ferrell, Sarah Paulson, and Jennifer Lawrence have racked up millions of views. But for all of the onscreen portrayals of lie detectors, few of us know how they actually work, or whether they’re even capable of achieving what they set out to do.

Attempts at lie detection in criminal proceedings date at least as far back as the Middle Ages, which usually invoked some form of torture; innocent people were thought to better withstand painful interrogation. Unsurprisingly, this didn’t yield the best results. But the modern polygraph as we know it today has its roots in an early 20th-century research project undertaken by psychologist William Moulton Marston, who proposed that systolic blood pressure changed when an individual was lying. Marston was also a comic book writer who created the character Wonder Woman, whose Lasso of Truth may be a nod to her creator’s legacy.

Today, polygraphs measure a range of physiological responses in addition to systolic blood pressure, including pulse rate, respiration, and skin conductivity. All of these measurements can indicate arousal of the nervous system. The idea underpinning the test is that someone who is being deceptive will have different physical responses than someone who is being truthful, and that the test can pick up on what would otherwise be invisible to the naked eye.

Related: 9 Engrossing Books About the Science of Death

There are a few different techniques to administering polygraph tests, the most popular of which is the Control Question Technique. The tester asks three different types of questions. Irrelevant questions establish mundane facts such as the suspect’s name, while diagnostic questions are somewhat related to crime, but are broad in scope and not accusatory. Relevant questions are more directly pertaining to the crime in question and the suspect’s role in it. The suspect fails the test if they have a larger physiological response to the relevant questions than the other types.

With all that being said, polygraphs aren’t admissible in a court of law, and for good reason. Technically, they don’t measure deception; they measure arousal of the nervous system. Physiological changes can stem from causes other than lying. Innocent people can fail polygraphs for a variety of reasons including: being nervous, emotional, or confused; having an anxiety disorder or depression; hypoglycemia; ingesting nicotine; alcohol withdrawal; and more.

The vast majority of scientific organizations reject polygraphs due to their unreliability, and the National Research Council found in their 2002 review of polygraphs that in a best-case scenario, they “can discriminate lying from truth telling at rates well above chance, though well below perfection”. That’s not good enough to be accepted as expert testimony in court, the standards of which were determined in a 1993 Supreme Court case, Daubert v. Merrell Dow Pharmaceuticals, Inc.

On the other side of the spectrum, guilty people have been known to pass polygraphs, though it’s not entirely clear how. And beware if you try to find out; the federal government prefers to keep that information under wraps. In fact, beginning in 2013 they moved to indict individuals who claimed to be able to teach others how to fool a polygraph.

Related: Death Becomes Her: How Frances Glessner Lee Pioneered Modern Forensics with Her Nutshell Studies of Unexplained Deaths

So why use polygraphs at all? Law enforcement officers take into account a suspect’s willingness to take one in the first place, believing that guilty people will be more reluctant to comply. And in an interrogation setting, a guilty suspect may ultimately be goaded into confessing if they’re told that they failed the polygraph, and can be convinced that continuing to lie is futile. 

However, the fact of the matter is that while polygraphs can occasionally be useful in criminal investigations, they’re ultimately unreliable and have to be taken with a huge grain of salt. The following true crime cases include infamous examples in which lie detector tests played a part in the investigation—whether they helped or hindered it.

Family Annihilator Chris Watts

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When 33-year-old Chris Watts’s pregnant wife and two young daughters were reported missing in August 2018, his pleas for their safe return were broadcast across America, capturing the sympathy of strangers. But early on in the investigation, police thought that Chris was behaving strangely. He was brought in for questioning and a polygraph test, footage of which can be found on YouTube. When the interrogating officer told him that he failed the test and that the truth would come out one way or another, Watts confessed to murdering all three family members and led law enforcement to their remains. He was arrested just two days after they went missing.

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Serial Killer Gary Ridgway

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Better known as the Green River Killer, serial killer Gary Ridgway was convicted of an astounding 49 murders committed in Washington state in the 80s and 90s. All of his victims were female, and many of them were sex workers, runaways, or other people in vulnerable positions. He became a suspect in the investigation in 1983, but police stopped looking at him after he passed a polygraph exam in 1984. He would go on to murder several more women before DNA profiling proved he was the killer in 2001.

Related: Mind of a Monster: Gary Ridgway, the Green River Killer Who Murdered At Least 49 People

Serial Killer Dennis Rader

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Serial killer Dennis Rader styled himself as BTK (Bind, Torture, Kill) in taunting letters he addressed to the media and police. Although Rader himself didn’t take a polygraph test, someone else close to the investigation did: Bill Wegerle, the husband of Vicki Wegerle, one of Rader’s victims. Bill Wegerle failed two polygraphs in the wake of his wife’s death. On top of his grief, he had to endure the suspicion of police and his community for 18 years until Rader’s 2005 arrest.

Related: 7 Chilling Books by People Who Knew Serial Killers

Child Killer Lloyd Lee Welch

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Sisters Katherine and Sheila Lyon disappeared on a March day in 1975 after a trip to the mall. They were just 10 and 12 years old, respectively. There were multiple confirmed sightings of them at the mall, and one witness even came forward to say he had witnessed their abduction by a man who was demonstrating cassette players. That witness was Lloyd Lee Welch. When he failed a polygraph exam, he admitted he lied and was dismissed by the authorities as an attention-seeking oddball. The case went cold for nearly 40 years. 

Related: Justice for the Lyon Sisters: How a 40-Year Cold Case Was Finally Solved

When it was reopened, investigators resurfaced a curious police sketch. It was drawn up after one girl said she caught a man staring inappropriately at the Lyon sisters that day at the mall. The sketch bore a startling resemblance to Welch. Additional evidence led the police to conclude that Welch was responsible for abducting, sexually assaulting, and killing the two children, and he pled guilty to their murders in 2017.