How unexpected questions can catch out liars


Aldert Vrij and Matthew Francis write about how an interview technique – asking unexpected questions – can help catch out liars.

One of the rituals when you go on holiday is the time spent answering questions at border control. ‘What is your reason for visiting this country?’, ‘Which tourist attractions are you going to see?’ We know we’re going to get asked these kinds of questions, but so do people who are lying about their intentions, so it’s easy for them to prepare answers. One technique to catch out liars at this stage is to ask unexpected questions.

CREST Researcher Aldert Vrij, at Portsmouth University, has been researching what kind of unexpected questions catch out liars, and how. Of course this kind of approach isn’t just useful for border staff, it is also helpful for anyone interviewing people where they need to know whether they are telling the truth or not.

How does this process work? Well, whilst a liar can say “I don’t know” or “I can’t remember” in response to a question, doing so tends to raise suspicion if it is about something the liar is supposed to know, so they are forced to make up a plausible answer on the spot. This has the problem that it may contain inaccuracies which are easy to uncover (unlike pre-prepared lies), but also the strain of thinking up an answer on the spot leads to verbal leakage in how they answer. For example, liars will tend to give less detailed and/or less plausible answers to unexpected questions than they do to expected questions. This is in contrast to truth-tellers, whose answers to both kinds of questions tend to be comparable.

A study showed that liars give more detail than truth tellers about the outcomes questions but less detail than truth tellers about the process questions.

Why does this happen? Well, it happens because the liar experiences a lot more cognitive load when answering the unexpected question. Cognitive load is a term used to refer to the mental effort needed to carry out multiple tasks at once – we can see a dangerous example of increased cognitive load at play when trying to text and drive at the same time, or a frustrating example when trying to do anything with your computer whilst the virus scanner is running. In the case of liars, they are trying to construct a lie which sounds plausible and doesn’t contradict the facts all whilst trying to look believable as they deliver it.

Professor Vrij’s research has shown that spatial and process questions are good unexpected questions to catch out liars. For example, instead of asking what a person ate in a restaurant, ask them where they sat, or how they choose that restaurant or how they got there.

In a CREST guide on unexpected questions, Professor Vrij gives examples on which kind of questions work (and which don’t) as well as how this technique can be applied to single and multiple interviewees.

the unexpected questions techniqueYou can download the guide for free here. The Unexpected Questions Technique guide will take fifteen minutes to read. It is one of a number of CREST guides on information elicitation which are available to download from this website here.

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