Psychologists Expose Liars by Dividing Their Attention

Psychologists have discovered a new way to unmask liars by making them multitask during conversations. A recent study by researchers from the University of Portsmouth in the UK and the University of Florida in the US examined how performing a secondary task affects the arguments made by people telling the truth versus those who are lying. The results were published in the International Journal of Psychology & Behavior Analysis.

Why Lying Is More Mentally Demanding

Lying requires significantly more cognitive effort than telling the truth. According to Professor Aldert Vrij from the University of Portsmouth, “Truth and lies can sound equally convincing if liars have time to carefully plan what they’re going to say. But when that opportunity is taken away, the truth often sounds more plausible than a lie.”

The researchers note several reasons why lying is so mentally exhausting:

• Unplanned lies require more mental effort.
• Liars must remember everything they say to keep their story consistent.
• Liars are less likely to take the credibility of their words for granted and tend to monitor their own behavior more closely to appear honest.
• They have to watch the listener’s reactions to see if they seem convincing.
• Liars must constantly remind themselves to keep lying.
• Lying requires justification: for each point, the liar must decide whether to lie or tell the truth.
• Before lying, they must first suppress the truth.

The Experiment: Multitasking Makes Lying Harder

One way to increase cognitive load is to ask someone to perform a secondary task while telling their story. This forces the person to split their attention, making it harder to maintain a lie. The researchers tested this method in their study.

The experiment involved 164 participants (49 men and 115 women, average age nearly 27, mostly British). First, participants completed a 20-item questionnaire about their opinions on various social and current topics, rating their agreement on a scale from one to seven. Statements with the highest and lowest agreement were selected for each person.

Participants were then randomly assigned to either a “truth” or “lie” group. Within each group, some participants performed a secondary task during the interview, some did so without any incentive, and others with an incentive. Truth-tellers were asked to honestly express their opinions on three topics, while liars pretended to hold the opposite views. All participants were told it was important to appear convincing, as those who succeeded would be entered into a prize drawing for £50, £100, or £150 (in reality, everyone was entered).

The secondary task involved memorizing a seven-digit car registration number displayed on a screen for five seconds. As an incentive, some participants who failed to recall the number during the interview were allowed to write their opinions on the topics afterward.

Key Findings

The study found that liars’ stories sounded less plausible, direct, and clear than those of truth-tellers—but only when liars were told the secondary task was important. Interestingly, both groups performed equally well on the secondary task, suggesting that liars did not pay less attention to it. Therefore, the researchers caution that such multitasking conditions should be used carefully in interviews or interrogations, as they are only effective if liars do not ignore the secondary task.