Legal, criminological and forensic

Forget good cop, bad cop

Here’s the real psychology of two-person interrogation.

17 September 2013

We're all familiar with the good cop, bad cop interrogation technique so often portrayed in TV and film. In reality, at least in the UK, when two officers perform a joint suspect interview, one of them asks the questions and the other simply takes notes. That doesn't mean the double-interviewer set-up can't be exploited to make it easier to spot whether a suspect is lying.

In a new study Samantha Mann and her colleagues tested the effect of the demeanour of the note-taking interviewer. 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. 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.

The findings weren't all as the researchers expected. They thought that liars would look more at the second interviewer than the truth-tellers did, but this didn't happen, perhaps because he seemed unimportant.

A final cue to deceit was that liars engaged in more "deliberate eye contact" – moments when they held the gaze of the first interviewer for slightly longer than seems normal. This contradicts the myth that liars avoid eye-contact, but it's not clear how useful this finding is because what counts as "longer than normal" is subjective.

Like all studies of this kind, it's important to remember the dangers of extrapolating too readily to real-life scenarios. These were low-stakes lies and no real criminals or police officers were involved.

Forget good cop, bad cop

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