Two heads are better than one? How to effectively use two interviewers to elicit cues to deception

Background.
 We examined the effect of a second interviewer's demeanour on cues to deception. We predicted that a supportive demeanour would be the most beneficial for eliciting verbal cues to deceit, as it would encourage truth tellers, but not liars, to say more. In addition, we examined the extent to which interviewees deliberately made eye contact with the interviewers. Liars take their credibility less for granted than truth tellers, and therefore have a greater drive to be convincing. Liars are thus more likely to monitor the interviewer to determine if the interviewer appears to believe them.

Method.
 Participants appeared before two interviewers: the first asked all the questions and the second remained silent. The second interviewer exhibited either a supportive, neutral, or a suspicious demeanour.

Results.
 Truth tellers provided significantly more detail than liars, but only in the supportive second interviewer condition. The effect of a second interviewer's demeanour on detail was perhaps remarkable given that the interviewees hardly looked at the second interviewer (less than 10% of the time). Liars displayed more deliberate eye contact (with the first interviewer) than truth tellers did.

Conclusions.
 A supportive second interviewer has a positive effect on interviewing. We discuss this finding in the wider contexts of investigative interviewing and interviewing to detect deception.

Experimental laboratory study with over 100 university students and staff randomly assigned to truth-telling or lying conditions about employment. Participants faced two interviewers: one asked questions while the second remained silent and displayed either supportive (smiling, nodding), neutral (bland expression), or suspicious (frowning, head shaking) demeanour.

This laboratory study investigated how dual-interviewer dynamics affect deception detection by manipulating the nonverbal demeanour of a silent, note-taking second interviewer. Participants truthfully or deceptively answered questions about employment in a controlled setting. Results showed that supportive nonverbal behaviour from the second interviewer significantly enhanced verbal cue differentiation between truth tellers and liars, challenging traditional 'good cop, bad cop' assumptions. The effect emerged despite interviewees rarely looking at the second interviewer (<10% of time), suggesting that peripheral awareness of supportive behaviour influences disclosure and deceptive strategies. The findings suggest that a supportive interview environment constrains liars' verbal output while encouraging truth tellers to elaborate, likely through differential cognitive and social mechanisms.

These findings challenge traditional interrogation strategies and suggest that interview protocol should prioritize supportive nonverbal behaviour from secondary interviewers to maximize detection of verbal deception cues. The results have practical applications for training investigators and designing interview best practices that leverage dyadic dynamics to improve accuracy in deception detection.

Background & Context

This study addresses a fundamental question about investigative interviewing practice: how can the presence and demeanour of a second interviewer affect an interviewee's behavior and the elicitation of deception cues? Over 100 students and university staff participated in a paradigm where they either told the truth about a real job or lied about a fictional job, creating an ecologically valid test of dual-interviewer dynamics.

The research builds on cognitive approaches to deception detection that focus on magnifying differences between truth tellers and liars through strategic interviewing techniques. The hypothesis predicted that a supportive demeanour would be most beneficial for eliciting verbal cues to deceit, as it would encourage truth tellers, but not liars, to say more. This prediction stems from theoretical models suggesting that truth tellers do not initially say all they know and can be encouraged to provide more information in the right interview environment.

Research Design & Methodology

Participants were allocated to either tell the truth about a real job they had, or lie and answer questions about a fictional job. After having three days to prepare, participants were invited to a psychology lab for questioning. Participants were incentivised with the promise of a £5 reward if they fooled the interviewers, adding motivation to their deception attempts.

The experimental design manipulated the second interviewer's demeanour across three conditions:

• Supportive condition: The second interviewer sat with an open posture, nodding, and smiling whilst participants gave their accounts
• Neutral condition: The interviewer sat with an open posture but kept a bland expression
• Suspicious condition: The male interviewer acted as if he had suspicions (frowning and shaking his head)

A female interviewer with a neutral style asked the questions while a second male interviewer took notes. Critically, the first interviewer asked all the questions and the second remained silent, isolating the effect of the second interviewer's nonverbal demeanour alone.

Results & Key Findings

The supportive second interviewer significantly enhanced verbal cue detection. Truth tellers provided significantly more detail than liars, but only in the supportive second interviewer condition. This effect did not emerge in the neutral or suspicious conditions, demonstrating the specific power of supportive nonverbal behavior.

Key findings included:

• The effect of a second interviewer's demeanour on detail was perhaps remarkable given that interviewees hardly looked at the second interviewer (less than 10% of the time)
• Liars displayed more deliberate eye contact (with the first interviewer) than truth tellers did
• Liars made more negative comments about their (fictional) boss, but this difference only appeared when the second note-taking interviewer acted supportive
• Liars engaged in more "deliberate eye contact" – moments when they held the gaze of the first interviewer for slightly longer than seems normal

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, representing a novel contribution to interview protocol research.

Discussion & Significance

The findings challenge traditional "good cop, bad cop" assumptions about dual-interviewer strategies. Rather than suspicion or neutrality, a supportive second interviewer has a positive effect on interviewing, creating an environment where truth tellers feel comfortable elaborating while liars remain constrained by their cognitive burden and deceptive strategies.

The mechanism appears rooted in differential response to social support: truth tellers, having genuine experiences to draw from, respond to encouragement by providing richer narratives. Liars, managing the cognitive demands of fabrication and monitoring interviewer belief, cannot easily expand their accounts even in supportive environments. Liars take their credibility less for granted than truth tellers, and therefore have a greater drive to be convincing, making them more likely to monitor the interviewer to determine if the interviewer appears to believe them.

The deliberate eye contact finding contradicts the popular myth that liars avoid eye contact. Instead, liars were more inclined than truth tellers to report that they had displayed deliberate eye contact to convince the interviewer and to check whether the interviewer believed them, revealing strategic impression management rather than behavioral leakage.

Limitations & Considerations

The study used a university sample discussing employment rather than criminal suspects, limiting generalizability to high-stakes forensic contexts. The researchers expected that liars would look more at the second interviewer than truth tellers did, but this didn't happen, perhaps because he seemed unimportant, suggesting participants primarily focused on the active questioner.

What counts as "longer than normal" eye contact is subjective, potentially limiting the practical utility of the deliberate eye contact cue without clear operational definitions. Additionally, the three-day preparation period for liars may not reflect spontaneous deception scenarios common in field settings.

Practical Applications

For law enforcement and intelligence interviews, this research suggests that supportive nonverbal behavior from an observing interviewer can enhance information elicitation. Rather than projecting skepticism or neutrality, a note-taking or observing second interviewer should display warmth through nodding and smiling. This approach aligns with information-gathering interview philosophies that prioritize rapport and cooperation over confrontation.

Interviewers should also be alert to instances of deliberate, prolonged eye contact as potential indicators of deception rather than assuming gaze aversion signals lying. The finding that supportive demeanour differentiates truth tellers from liars provides investigators with a practical, easily implemented technique to enhance the diagnostic value of interviews without extensive training in complex questioning protocols.