How can scientists determine if a person is lying?

We’ll forgive you if you assume that scientists could just use polygraph machines—commonly called “lie detectors”— to detect a lie, but those devices aren’t exactly foolproof.

“Proponents will say the test is about 90 percent accurate. Critics will say it’s about 70 percent accurate,” said Frank Horvath of the American Polygraph Association to ABC. “Many people refer to polygraph tests as lie detector tests, and that’s a bit of a misnomer.”

“Since the process is not perfect, that could lead to the possibility of error, and that’s why there’s problems when trying to get them in the courts.”

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Polygraph machines work by measuring heart rate and other physical cues. Ultimately, however, they’re subject to deception; an appropriately skilled test subject can control his or her breathing to throw off the results.

According to a new study in Psychology, Crime & Law, scientists might have a new weapon in separating fact from fiction: accuracy in travel plans.

Researchers from the University of Portsmouth recruited 399 travelers at an international airport, then split them into two groups, telling half of the participants to lie about their travel plans.

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Interviewers then asked the participants questions about their plans, paying special attention to verifiable details. As website PsyPost wrote:

“Participants in the truth telling group provided a higher ratio of potentially verifiable details to total details. A statement such as ‘I am attending the Biennial Conference on Chemical Education in Singapore’ was considered verifiable, while one such as ‘I am going to visit some different shops in Verona to look for a dress’ was considered a non-verifiable detail.”

The researchers found that liars used significantly fewer verifiable details, regardless of their cultural backgrounds.

“The [Verifiability Approach] is not only less susceptible to individual biases, but is also less exhaustive on the scarce resources available within airport security,” said corresponding author Louise M. Jupe.

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“Building upon the current findings has the potential to allow us to work towards increasing positive hits on suspicious individuals, whilst reducing the number of individuals who are wrongfully flagged as suspicious.”

In other words, the study could change the way that airport security personnel assess threats.

Another recent study showed a counterintuitive way to improve lie detection.

Researchers from the University of Ontario Institute of Technology in Canada and the University of Amsterdam in the Netherlands filmed two videos showing a woman watching a stranger’s bag. In one of the videos, the woman stole items from the bag.

The researchers then played one of the videos to female study volunteers. The women were then told to testify in a mock court about what they’d seen—but half were told to lie. 

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Additionally, while testifying, a third of the volunteers wore a black niqab, which hides every part of the face other than the eyes, while a third wore a black hijab, which surrounds the face without covering it. The remaining third of the volunteers didn’t wear any sort of face covering.

Meanwhile, another group of participants listened to the testimonies and attempted to determine whether the women were lying. The study’s results concluded that participants could detect lies much more easily when the women were wearing hijabs or niqabs.

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Scientists believe that this is because the viewers didn’t rely on appearance, so they could focus on other details, such as body language and verifiable information in the story.

So, how can you tell if a person’s lying?

You can’t—not with 100 percent certainty. By removing some of the human factors from the situation, however, your accuracy can greatly improve. For the time being, scientists will continue to work on a perfect truth detection technology (preferably a machine capable of detecting pants that have been set on fire).