Scientists found a brilliantly simple hack for spotting a liar

Detecting a lie often feels like a gut instinct, relying on vague cues like eye contact or a shaky voice. But for truly skilled liars, these mannerisms are often controlled and managed. Fortunately, scientists have discovered a simple, surprisingly effective method that exploits the one thing even the best liars can’t hide: mental effort, per a report from Indy100.

The new method is based on the principle that deceit requires significant mental energy, or "cognitive load." According to recent research, asking someone to perform an additional cognitive task during questioning significantly increases the chances of detecting the lie. Experts from the University of Portsmouth found that overloading a liar makes it nearly impossible to sustain a convincing, plausible story.

The research, published in the International Journal of Psychology & Behavior Analysis, examined 164 participants. They were asked to either tell the truth or lie about their level of support for sensitive societal issues. The core condition involved two-thirds of the participants being given a secondary task: they had to remember and recall a car registration number during the interview. The experimenters found that liars' stories became far less plausible when they were forced to multitask, as per IFL Science.

The success was most evident when the participants were told the secondary task was important, proving that liars tend to prioritize maintaining the lie over completing a distraction. One of the study’s authors, Professor Aldert Vrij, noted that careful execution is key for practical use.

"The pattern of results suggests that the introduction of secondary tasks in an interview could facilitate lie detection, but such tasks need to be introduced carefully," Vrij commented.

Gif of Pinocchio via Giphy

He also provided concrete advice on how to use this principle in real life: "It seems that a secondary task will only be effective if lie tellers do not neglect it. This can be achieved by either telling interviewees that the secondary task is important, as demonstrated in this experiment, or by introducing a secondary task that cannot be neglected (such as gripping an object, holding an object in the air, or driving a car simulator). Secondary tasks that do not fulfill these criteria are unlikely to facilitate lie detection."

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This research fundamentally shifts the focus of lie detection from trying to read subtle body language to simply introducing cognitive friction. By giving the subject too many things to juggle—a compelling lie, plus a critical, separate task—the truth will inevitably be the first thing to slip out.

This article originally appeared last year.