Mann et al., (2004)

Mann et al., (2004) – ‘Detecting True Lies: Police Officers’ Ability to Detect Suspects’ Lies’




This is the second study we look at from the ‘Interviewing Suspects’ section of ‘Making a case’. As part of your OCR A2 Psychology Exam. It is further categorised into ‘Detecting Lies.

Can you tell when someone is lying?

If you answered yes, what cues to do you use to detect lies? Eye movements? Gestures?

Could you detect lies at a rate better that chance (50%)?  This would mean that you would have to be able to accurately ascertain the truthfulness of at least 6 out 10 statements.

Previous research only used students for their samples and was therefore ethnocentric and lacking in ecological validity. Mann et al,  used the first sample consisting of real Police Officers to research lie detection.

Inbau cues: gaze aversion, displaying unnatural posture changes, exhibiting self
manipulations and placing the hand over the mouth or eyes when speaking.

None of these behaviours have been found to be reliably related to lying in deception research.


To test Police Officers’ ability to distinguish truths and lies during Police interviews with suspects.

There were six hypotheses for this experiment:

  1.  “We expected truth and lie accuracy rates to be significantly above the level of chance (which is 50%), and, as a consequence of this, expected lie accuracy rates to be significantly higher than typically found in previous research (44%).”
  2. “We also expected individual differences, with some police officers being more skilled at detecting truths and lies than others. We predicted that the reported experience in interviewing suspects would be positively correlated with truth and lie accuracy.”
  3. “We expected good lie detectors to mention speech related cues significantly more often than poor lie detectors.”
  4. “We expected negative correlations between mentioning such cues and accuracy rates, in other words, the more of such cues the officers reported to look at, the lower their accuracy rates would become.”
  5. “In other words, the more of these ‘Inbau cues’ that police officers mention that they use to detect deceit, the worse at distinguishing between truths and lies we expected them to be.”
  6. ” It was predicted that poor lie detectors would be significantly more guided by invalid cues, such as ‘gaze aversion’, than good lie detectors.”

Field Experiment and correlation.


99 British Police Officers from the county of Kent.

24 were female and 75 male. Their age ranged from 22 years to 52 years.

78 participants were from CID (Criminal Investigation Department), 8 were Police Trainers, 4 were Traffic Officers, and the remaining 9 were Uniform Response Officers.

The average time served with the Police was 11.2 years and the range of time served was 1 to 30 years.


Independent Variable:

The independent variable was if the clips were of truthful or deceitful statements.

Dependent Variables:

Accuracy scores. (Either 1 = correct or 0 = false)

Self-reported behaviours, which each participant associated with deception before and after the task.

Self-reported cues used to detect deception.

Self-reported confidence scores during and after the task.

The participants were asked to judged the truthfulness of real suspects in videos of 14 real Police interviews. The Police officers all completed this task individually. Before they completed the task they filled out a questionnaire about their age, gender, length of service, division, perceived level of experience in interviewing suspects (1 = totally
inexperienced and 5 = highly experienced, the mean score was M = 3.75, SD = .85)), and the
verbal or non-verbal cues they use to decide whether another person is lying or telling the

After completing the questionnaire, the participants were given the following instructions:

“You are about to see a selection of clips of suspects who are either lying or telling the truth. The clips vary considerably in length, and the suspects may appear on several occasions. This is irrelevant. They will be either lying the whole length of the clip or truth-telling for the length of the clip. After viewing each clip I would like you to indicate whether you think the suspect is lying or telling the truth (measured with a dichotomous
scale), and how confident you are of your decision, on a seven-point scale. If you recognise any of the suspects please bring it to my attention.”

Upon completing the task, the participants answered more questions on the questionnaire. These were questions about how they had distinguished truth from lies, and questions about their confidence in their judgements.

The cameras were high enough quality to show eye blinks, but not to show subtler facial movements. In some cases the suspects legs could not been seen, thus leg movements were not analysed. The audio was of good quality.

Videos were only shown if the truthfulness of each word was known to the researchers.

The length of each video clip varied from 6 to 145 seconds: in total there were 54 clips, 23 truthful and 31 deceptive. The number of clips for each suspect varied between a minimum of 2 to a maximum of 8. This ensured that each participant viewed at least one truth and one lie. The clips were presented in a random order to ensure the same suspect was not viewed consecutively.

Permission to approach and use Police Officers was granted by the Chief Constable. The Police Officers were told that they would be participating in a study ‘about Police officers’ ability to detect deception,’ there were also informed that their participation would be anonymous.



The difference between the mean lie accuracy and truth accuracy was not significant: 66.2% and 63.6% respectively. However, these results were significantly above chance, which was 50%.

For the 99 participants, in total, 677 behaviours mentioned on the
questionnaires before and after completing the task were coded.

Participants mentioned a mean of 6.84 behaviours.

The most frequently mentioned cues to detect lying were gaze, movement, vagueness, contradictions and fidgeting.

Participants were significantly more confident after they had viewed truthful clip, than after they had viewed a deceitful clip. Those two confidence measures were significantly correlated with each other.

Experience in interviewing was weakly positively correlated with truth accuracy and lie accuracy.

Neither the truth accuracy – truth confidence correlation, nor the lie accuracy – lie
confidence correlation were significant.  Neither was the post-task estimated
accuracy significantly correlated with the actual lie accuracy or actual truth accuracy.

Age, length of service and experience in interviewing suspects were not significantly correlated with truth confidence, lie confidence or post-task estimated accuracy.


The levels of accuracy found in this study exceed those found in other studies and are the highest for a group of ordinary Police officers. The more experience a Police officer has the better they are at distinguishing truths from lies.

Higher accuracy is found in people who do not use stereotypical Inbau cues when detecting lies, instead rely more on story cues, for example contradictions.

Mann et al., Evaluation

– No Control group – therefore comparisons cannot be made between Police officers and lay people, to understand if there is something fundamentally different about Police officers that makes them better at detecting lies than lay people.

+ Ethics – the reason there was no control group is because ordinary people could not be allowed access to real case materials.

+ High Standardisation, the experiment is highly standardised, each participant was subject to the same videos and suspects.

+ Ecological Validity, as experiment one used real police officer, we can say that the experiment is high in ecological validity. However, we can also argue the opposite as the participants only watched videos of suspects.

+ Ethics, as the participants were informed about the interview beforehand, we can say that the study was ethical.

– Ethnocentrism, the study was only focused on British police officers, and therefore cannot be generalised to other populations.

+ Experimenter bias, as the results of both experiments were analysed by laboratory assistants who were blind to the conditions, we can say that the study is lacking in experimenter bias, which is a strength.

+ Construct Validity, the results of the study showed support for the effectiveness of the cognitive interview.


Audio Podcast

  1. Mann et al., (2004) – ‘Detecting True Lies: Police Officers’ Ability to Detect Suspects’ Lies’
Further Reading

OCR A2 Psychology Student Unit Guide: Unit G543: Forensic Psychology (Student Unit Guides)

Article Name
Mann et al (2004)
Mann et al., (2004) - 'Detecting True Lies: Police Officers' Ability to Detect Suspects' Lies' Can you tell when someone is lying?