One of the best ways to catch someone in a lie is to try and distract them while they're not telling you the whole truth, a new study reveals.

It seems that the extra cognitive effort required to construct a lie and do something else at the same time means the falsehood doesn't stand up quite so well.

There are some caveats though – for example, the secondary task needs to be seen as important by the lying person or they'll be able to prioritize the lie over whatever else it is that they're supposed to be doing.

The researchers behind the new study suggest that interviews could be structured in a certain way to involve secondary tasks and therefore spot lies. However, they emphasize this isn't a foolproof system, and that further work is required to fully understand how multitasking hampers liars.

"Our research has shown that truths and lies can sound equally plausible as long as lie tellers are given a good opportunity to think what to say," says psychologist Aldert Vrij, from the University of Portsmouth in the UK.

"When the opportunity to think becomes less, truths often sound more plausible than lies."

"Lies sounded less plausible than truths in our experiment, particularly when the interviewees also had to carry out a secondary task and were told that this task was important."

Here's how the experiment worked: 164 volunteers gave their opinions on societal topics in the news, then split up into two groups for mock interviews. They were instructed to either tell the truth about their views or lie about them.

Those two groups were further subdivided into three. One-third were given a second task to do and told it was important in order to pass the interview, one-third also had a second task but no information about how important it was, and the final third didn't have any second task to worry about.

Said task involved writing down a seven-digit car registration number that had previously been shown to the participant. At the end of each interview, the interviewer ranked what they had heard from the study participant in several different areas, including the plausibility of what they were saying.

"The most diagnostic differences between truth tellers and lie tellers occurred in plausibility, immediacy, directness, and clarity," write the researchers in their published paper.

The results matched what the researchers were expecting, though the differences between the groups weren't huge. Telling lies involves making up details, trying to not get caught out by those fictitious details, and keeping the falsehoods sounding as truthful as possible. All this can require a lot of brainpower.

For best results in rooting out liars, the team suggests that the secondary task needs to be seen as important, or should be something that absolutely has to be done – holding on to something, for example, or working a simulator perhaps.

There will always be plenty of other variables to factor in as well of course, not least that some people are much better at lying than others. Still, this is an intriguing way of trying to show up lies when they're being told – one that doesn't require any special setup and which you could even try out yourself.

"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," says Vrij.

The research has been published in the International Journal of Psychology & Behavior Analysis.