Is the polygraph a reliable lie detector? | Kiowa County Press


Does your body betray whether you lie or not? AP Photo / Edward Kitch

Jessica Gabel Cino, Georgia State University

Lawyers for Christine Blasey Ford, the woman who charged Supreme Court candidate Brett Kavanaugh with sexual assault, have released the results of a polygraph test focused on the decades-old incident. They suggest that Ford’s answers to two questions about his claims were “not indicative of deception.”

How reliable is this assessment and the polygraph technology on which it is based?

People have long yearned for a way to separate truth from lies, whether in high stakes court cases or in family disputes. Over the years, inventors have developed an evolving set of tools and instruments aimed at determining whether someone is lying. They have tried to incorporate more and more science, but with varying degrees of success. Society has often turned to instruments like the polygraph to inject some objectivity into the detection of deception.

As a defense attorney, many clients have told me that they did not commit the alleged crime. But I’ve never asked a client to undergo a polygraph exam: it’s high risk, low reward, and the results – though inadmissible in a criminal case – are unpredictable. How reliable is a polygraph at identifying who is lying and who is telling the truth?

Looking for signs of lies

Lie detection methods have progressed from their torture-centric roots. The first techniques involved subjecting someone to a water test: those who sank were considered innocent, while floating indicated guilt, lies and witchcraft. Neither result was good news for the accused. In medieval Europe, it was believed that an honest man was able to immerse his arm in boiling water longer than a liar.

Eventually, people developed more humane methods, focusing on physiological factors that could be used as arbiters of truth. At the turn of the 20th century, William Moulton Marston – self-proclaimed “father of the polygraph” – showed a strong link between systolic blood pressure and lying. Basically tell a story and your blood pressure goes up. Martson also created the comic book character Wonder Woman, whose golden lasso can extract truth from those he traps.

In 1921, physiologist John Larson, of the University of California at Berkeley, was the first to couple measurements of blood pressure and respiration, examining the rises and falls in respiration. The Berkeley Police Department adopted his device and used it to assess the reliability of witnesses.

In 1939, Larson’s protege Leonarde Keeler updated the system. He made it compact for travel and added a component to assess the skin’s galvanic response, which measures sweat gland activity that might reflect the intensity of an emotional state. His device, purchased by the FBI, was the precursor of the modern polygraph. Later versions were variations on this original.

Sensors detect changes in the way the subject’s body responds to the examiner’s questions. AP Photo / Fernando Vergara

Lie detectors today

“Lie detector” is a broad term. It most often refers to a polygraph, but also applies to a certified vocal stress analysis, fMRI brain scanner, or even software used to analyze the word choice and variation a subject uses when he recounts an event.

What today’s polygraph does is encapsulated in the word itself. “Poly” means several or multiple, and “-graph” means write. The system records several physiological responses – most often sweating, heart rate, respiratory rate, and blood pressure – and represents them visually so that an examiner can interpret them.

There are two most common approaches to administering a polygraph. In what is called the controlled question technique, an examiner will ask irrelevant questions, control questions and relevant questions. Then, based on what he sees in the graphical representation of the subject’s physiological responses, he will identify if they change significantly in response to the relevant questions. The underlying assumption is that deception, due to the stress induced by lying, will result in a measurable response in the form of increased sweating, heart rate, etc.

The second approach is known as the Guilty Knowledge Test, which is really a misnomer. It tests all knowledge of events, not just guilty knowledge. The examiner measures a subject’s response to specific questions in an attempt to discern whether the subject in fact has personal knowledge of an event. This can range from the number of times a victim has been stabbed to the color of the getaway car.

Presumably, a person who is unaware of an event would not react significantly differently to the exact answer because they would not know what is right and what is not. Meanwhile, logically, a person with first-hand knowledge would demonstrate a physiological response. Of course, this method also has inherent limitations regarding, among other things, the types of questions that can be presented.

Can polygraphs really distinguish truth from lies?

An examiner keeps an eye on how various metrics change in response to particular topics or questions. DENker / Wikimedia Commons

The effectiveness of polygraphs is hotly debated in the scientific and legal communities. In 2002, a National Research Council study found that in populations “untrained in countermeasures, incident specific polygraph testing (GKT) can discriminate lie from truth at rates far greater than chance, although much inferior to perfection ”. Better than flipping a coin to see if someone is telling the truth, but nowhere near consistent and reliable results.

The NRC cautioned against using polygraphs in employment screening, but noted that polygraph tests of specific incidents in the field give more accurate results. It seems focused and relevant questions – for example, “Was the theft committed with a firearm?” – intended to unmask a subject who may have a strong motive for lying or withholding information seems to work better.

Polygraphs can give false positives: claiming that someone is lying and telling the truth. The consequences of “failing” a polygraph can be serious – from not finding a job to being labeled a serial killer.

In the 1998 Supreme Court case United States v. Scheffer, the majority said that “there is simply no consensus that polygraph evidence is reliable” and “[u]Like other expert witnesses who testify on facts beyond the knowledge of the jurors, such as analysis of fingerprints, ballistics or DNA found at a crime scene, a polygraph expert cannot provide the jury that another opinion.

Notably, the litigation over the precursor of the modern polygraph gave rise to the DC Circuit’s seminal Frye Opinion in 1923, which ruled that the polygraph evidence was inadmissible in court. In 2005, the 11th Circuit Court of Appeal reiterated that “polygraphy has not enjoyed general acceptance in the scientific community”.

The reality is that multiple factors – including nervousness in a high-stakes situation – can affect the readings detected by a polygraph and make the subject appear to be lying. For this reason, polygraphs are generally not admissible in criminal cases, although police interrogators sometimes trick a suspect into submitting to them. Polygraphs may be admissible in civil cases, depending on the state, and some states allow the use of polygraph tests in criminal cases if everyone agrees.

Better than nothing?

In short, polygraphs can offer some confidence – albeit slight – that a person is telling the truth about a particular incident. Studies have shown that when a well-trained examiner uses a polygraph, he can detect a lie with relative accuracy.

But a polygraph is not perfect: an examiner’s interpretation is subjective and the results are unique to the person being tested. Under the right circumstances, the polygraph can be fooled by a trained person. Even some of my forensic evidence students “passed the test” when I brought in a polygraph examiner for a classroom demonstration.

Perhaps the 11th Circuit summed it up best: there is no Pinocchio factor associated with polygraphs. As much as we would like a sign as obvious as a growing nose, there is no such thing as a 100 percent reliable physical sign of telling a lie.

A polygraph exam shows “that the person being examined believes his or her own story.” And that may be enough. A subject’s willingness to submit even to an examination often reveals a level of truthfulness and can fill a void when the other party has not submitted to an examination in the same way.

The conversation

Jessica Gabel Cino, Associate Dean of Academic Affairs and Professor of Law, Georgia State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.


About Florence M. Sorensen

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