How Polygraphs Work—And Why They Aren't Admissible in Most Courts

iStock/Sproetniek
iStock/Sproetniek

The truth about lie detectors is that we all really want them to work. It would be much easier if, when police were faced with two contradictory versions of a single event, there was a machine that could identify which party was telling the truth. That’s what the innovators behind the modern-day polygraph set out to do—but the scientific community has its doubts about the polygraph, and all over the world, it remains controversial. Even its inventor was worried about calling it a "lie detector."

AN OFF-DUTY INVENTION

In 1921, John Larson was working as a part-time cop in Berkeley, California. A budding criminologist with a Ph.D. in physiology, Larson wanted to make police investigations more scientific and less reliant on gut instinct and information obtained from "third degree" interrogations.

Building on the work of William Moulton Marston, Larson believed that the act of deception was accompanied by physical tells. Lying, he thought, makes people nervous, and this could be identified by changes in breathing and blood pressure. Measuring these changes in real-time might serve as a reliable proxy for spotting lies.

Improving upon previously developed technologies, Larson created a device that simultaneously recorded changes in breathing patterns, blood pressure, and pulse. The device was further refined by his younger colleague, Leonarde Keeler, who made it faster, more reliable, and portable and added a perspiration test.

Within a few months, a local newspaper ​convinced Larson to publicly test his invention on a man suspected of killing a priest. Larson's machine, which he called a cardio-pneumo psychogram, indicated the suspect’s guilt; the press dubbed the invention a lie detector.

Despite the plaudits, Larson would become skeptical about his machine’s ability to reliably detect deception—especially in regards to Keeler’s methods which amounted to “a psychological third-degree." He was concerned that the polygraph had never matured into anything beyond a glorified stress-detector, and believed that American society had put too much faith in his device. Toward the end of his life, he would refer to it as “a Frankenstein’s monster, which I have spent over 40 years in combating.”

But Keeler, who patented the machine, was much more committed to the lie-detection project, and was eager to see the machine implemented widely to fight crime. In 1935, results of Keeler’s polygraph test were admitted for the first time as evidence in a jury trial—and secured a conviction.

HOW IT WORKS

In its current form, the polygraph test measures changes in respiration, perspiration, and heart rate. Sensors are strapped to the subject's fingers, arm, and chest to report on real-time reactions during interrogation. A spike on these parameters indicates nervousness, and potentially points to lying.

To try to eliminate false-positives, the test ​relies on "control questions."

In a murder investigation, for instance, a suspect may be asked relevant questions such as, "Did you know the victim?" or "Did you see her on the night of the murder?" But the suspect will also be asked broad, stress-inducing control questions about general wrongdoing: "Did you ever take something that doesn't belong to you?" or "Did you ever lie to a friend?" The purpose of the control questions is to be vague enough to make every innocent subject anxious (who hasn't ever lied to a friend?). Meanwhile, a guilty subject is likely to be more worried about answering the relevant questions.

This difference is what the polygraph test is about. According to the American Psychological Association, “A pattern of greater physiological response to relevant questions than to control questions leads to a diagnosis of ‘deception.’” They proclaim that, "Most psychologists agree that there is little evidence that polygraph tests can accurately detect lies."

But a diagnosis of deception doesn’t necessarily mean that someone has actually lied. A polygraph test doesn’t actually detect deception directly; it only shows stress, which was why Larson fought so hard against it being categorized as a "lie detector." Testers have a variety of ways to infer deception (like by using control questions), but, according to the American Psychological Association, the inference process is “structured, but unstandardized” and should not be referred to as “lie detection.”

And so, the validity of the results remains a subject of debate. Depending on whom you ask, the reliability of the test ranges from near-certainty to a coin toss. The American Polygraph Association claims the test has an almost 90 percent accuracy rate. But many psychologists—and even some ​police officers—contend that the test is ​biased toward finding liars and has a 50 percent chance of hitting a false-positive for honest people.

NOT QUITE THE SAME AS FINGERPRINTS

Most countries have traditionally been skeptical about the polygraph test and only a handful have incorporated it into their legal system. The test remains most popular in the United States, where many police departments rely on it to extract confessions from suspects. (In 1978, former CIA director Richard Helms argued that that's because "Americans are not very good at" lying.)

Over the years, the U.S. Supreme Court has issued numerous rulings on the question of whether polygraph tests should be admitted as evidence in criminal trials. Before Larson’s invention, courts treated lie-detection tests with suspicion. In a 1922 case, a judge prohibited the results of a pre-polygraph lie detector from being presented at trial, worrying that the test, despite its unreliability, could have an unwarranted sway on a jury’s opinion.

Then, after his polygraph results secured a conviction in a 1935 murder trial (through prior agreement between the defense and prosecution), Keeler—Larson’s protégé—asserted that “the findings of the lie detector are as acceptable in court as fingerprint testimony.”

But numerous court rulings have ensured that this won’t be the case. Though the technology of the polygraph has continued to improve and the questioning process has become more systematic and standardized, scientists and legal experts remained divided on the device's efficacy.

A 1998 Supreme Court ruling ​concluded that as long as that’s the case, the risk of false positives is too high. The polygraph test, the court concluded, enjoys a scientific “aura of infallibility,” despite the fact “there is simply no consensus that polygraph evidence is reliable,” and ruled that passing the test cannot be seen as proof of innocence. Accordingly, taking the test must remain voluntary, and its results must never be presented as conclusive.

Most importantly: The court left it up to the states to decide whether the test can be presented in court at all. Today, 23 states allow polygraph tests to be admitted as evidence in a trial, and many of those states require the agreement of both parties.

Critics of the polygraph test claim that even in states where the test can't be used as evidence, law enforcers often use it as a tool to ​bully suspects into giving confessions that then can be admitted.

“It does tend to make people frightened, and it does make people confess, even though it cannot detect a lie,” Geoff Bunn, a psychology professor at Manchester Metropolitan University, told The Daily Beast.

But despite criticism—and despite an entire ​industry of former investigators offering to teach individuals how to beat the test—the polygraph is still used ​widely in the United States, mostly in the process of job applications and security checks.

10 Killer Gifts for True Crime Fans

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Ulysses Press/Little A

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Humans have a strange and lasting fascination with the dark and macabre. We’re hooked on stories about crime and murder, and if you know one of those obsessives who eagerly binges every true crime documentary and podcast that crosses their path, you’re in luck—we’ve compiled a list of gifts that will appeal to any murder mystery lover.

1. Donner Dinner Party: A Rowdy Game of Frontier Cannibalism!; $15

Chronicle Books/Amazon

The infamous story of the Donner party gets a new twist in this social deduction party game that challenges players to survive and eliminate the cannibals hiding within their group of friends. It’s “lots of fun accusing your friends of eating human flesh and poisoning your food,” one reviewer says.

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2. A Year of True Crime Page-a-Day Calendar; $16

Workman Calendars/Amazon

With this page-a-day calendar, every morning is an opportunity to build your loved one's true crime chops. Feed their morbid curiosity by reading about unsolved cases and horrifying killers while testing their knowledge with the occasional quizzes sprinkled throughout the 313-page calendar (weekends are combined onto one page).

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3. Bloody America: The Serial Killers Coloring Book; $10

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Some people use coloring books to relax, while others use them to dive into the grisly murders of American serial killers. Just make sure to also gift some red colored pencils before you wrap this up for your bestie.

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4. The Serial Killer Cookbook: True Crime Trivia and Disturbingly Delicious Last Meals from Death Row's Most Infamous Killers and Murderers; $15

Ulysses Press/Amazon

This macabre cookbook contains recipes for the last meals of some of the world’s most famous serial killers, including Ted Bundy, Aileen Wuornos, and John Wayne Gacy. This cookbook covers everything from breakfast (seared steak with eggs and toast, courtesy of Ted Bundy) to dessert (chocolate cake, the last request of Bobby Wayne Woods). Each recipe includes a short description of the killer who requested the meal.

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5. Ripped from the Headlines!: The Shocking True Stories Behind the Movies’ Most Memorable Crimes; $15

Little A/Amazon

In this book, true crime historian Harold Schechter sorts out the truth and fiction that inspired some of Hollywood’s best-known murder movies—including Psycho (1960), Scream (1996), Arsenic and Old Lace (1944), and The Hills Have Eyes (1977). As Schechter makes clear, sometimes reality is even a little more sick and twisted than the movies show.

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6. The Deadbolt Mystery Society Monthly Box; $22/month

CrateJoy

Give the murder mystery lover in your life the opportunity to solve a brand-new case every single month. Each box includes the documents and files for a standalone mystery story that can be solved alone or with up to three friends. To crack the case, you’ll also need a laptop, tablet, or smartphone connected to the internet—each mystery includes interactive content that requires scanning QR codes or watching videos.

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7. In Cold Blood; $10

Vintage/Amazon

Truman Capote’s 1965 classic about the murder of a Kansas family is considered by many to be the first true-crime nonfiction novel ever published. Capote’s book—still compulsively readable despite being written more than 50 years ago—follows the mysterious case from beginning to end, helping readers understand the perspectives of the victims, investigators, and suspects in equal time.

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8. Stay Sexy & Don’t Get Murdered: The Definitive How-To Guide; $13

Forge Books/Amazon

Any avid true crime fan has at least heard of My Favorite Murder, the popular podcast that premiered in 2016. This book is a combination of practical wisdom, true crime tales, and personal stories from the podcast’s comedic hosts. Reviewers say it’s “poignant” and “worth every penny.”

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9. I Like to Party Mug; $12

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This cheeky coffee mug says it all. Plus, it’s both dishwasher- and microwave-safe, making it a sturdy gift for the true crime lover in your life.

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10. Latent Fingerprint Kit; $60

Crime Scene Store/Amazon

Try your hand (get it?!) at being an amateur detective with this kit that lets you collect fingerprints left on most surfaces. It may not be glamorous, but it could help you solve the mystery of who put that practically empty carton back in the refrigerator when it barely contained enough milk for a cup of coffee.

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The Reusable Rocketbook Smart Notebook Is the Ultimate Recycling Move

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StackCommerce/Rocketbook

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You may be conscientious of your environmental footprint, but some habits are tough to change, like transitioning from writing notes on paper to typing notes and saving trees. There’s just something about physically writing with a pen and paper that’s fulfilling, engages your memory, and just feels right. You can keep jotting without the guilt on this smart reusable notebook from Rocketbook that translates all of your written text to your favorite cloud services so you can recycle the pages to use again and again.

Once you start using cloud storage for notes, it’ll be hard to go back. You’ll have every song lyric, appointment, and list of directions you jot down anywhere you have internet access (which is basically everywhere these days). The set comes with 42 pages for various uses, like calendars and to-do lists that wipe clean with a damp cloth to reuse after you save your notes to the cloud.

One of the reasons it’s hard to ditch traditional pen and paper is the feeling of your favorite writing utensil gliding on the page. This set comes with a FriXion pen that writes smoothly on the synthetic paper just like the old-fashioned way—except you can’t misplace or ruin these notes in a shuffle or coffee spill since everything will be stored digitally. It’s better for the environment and kind of like an insurance plan on your personal thoughts and notes.

For a limited time, you can replace all of your archaic and environmentally wasteful notebooks with one easy set. Score the Rocketbook Fusion Executive book in black, the Rocketbook Mini in black, two FriXion pens, and two microfiber cloths for wiping with a 12 percent discount. You’ll get unlimited paper from the sleek large and small black notebooks for just $44.

Prices subject to change.

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