Over 100 hundred students and university staff were allocated to either tell the truth in answering detailed questions about a real job they really had, or they were asked to lie and answer questions about a fictional job. After having three days to prepare, the participants were invited to a psychology lab for questioning. A female interviewer with a neutral style asked the questions (e.g. “If you were training me to do your job for a day, what things would I need to know about it?”) while a second male interviewer took notes. Crucially, this male interviewer either struck a supportive demeanour (smiling and nodding his head), a neutral demeanour, or acted as if he had suspicions (frowning and shaking his head). The participants were incentivised with the promise of a £5 reward if they fooled the interviewers.

Here’s the headline result – the truth-telling participants gave more detailed answers than the liars, but only when the second interviewer provided a supportive presence. This runs entirely counter to the aggressive questioning styles so often portrayed in fiction. By creating a reassuring atmosphere, the second interviewer encouraged the honest interviewees to open up more, which made the the lack of detail given by liars stand out.

Another sign of deception was the amount of negative comments made by liars about their (fictional) boss. But again, this difference only appeared when the second note-taking interviewer acted supportive. [Researcher Samantha] Mann and her team said this was the first time a study had shown the beneficial lie-detecting effect of having a supportive second interviewer.