Suspect, Lies And Videotape


What do you think are the signs of a liar? Rubbing your nose, avoiding eye contact, coughing, biting your lips, scratching your neck, fidgeting nervously; what are the tell-tale signs? Most people believe that liars avoid eye contact and behave nervously, but there is very little scientific evidence to support this.

Most experiments on lying take place in laboratories under artificial settings and with unrealistic activities. For example, if an experimenter is asking the participant to lie, the participant would not feel any guilt in doing so. One way to address this is to give the participant a choice between lying or being honest, but that still leaves a very low level of guilt because the participant knows it is just an experiment. Also, if a participant knows someone is analysing their behaviour or will be analysing their behaviour later, this automatically influences them and results in strong bias. Furthermore, lying as part of an experiment will not result in a jail sentence for obstruction of justice. This means that, again, participants will not actually feel realistic emotions or reactions that are linked with lying.

Mann, Vrij and Bull decided to use video footage of real liars in police custody. They were all in high-stake, real-life situations. All the suspects had lied and later admitted to the truth; this was supported by reliable witness statements and concrete evidence.

It’s So Frustrating


  • To determine if there are certain behaviours which differentiate a liar from a truth-teller.
  • To determine if cognitive load increases deceptive behaviour.


  • The sample was made up of 16 police suspects (3 females and 13 males).
  • Four were juveniles (three were 13 years old, one was 15 years old).
  • The remaining 12 suspects were adults under the age of 65.
  • Fifteen were Caucasian who spoke English as a first language.
  • One was Asian whose first language was Punjabi but was fluent in English.

All interviews were conducted in English. Nine were suspected of theft, two were suspected of arson, one was suspected of attempted rape and one was suspected of murder. Over half of the participants were familiar to the police and had previously been questioned for criminal offences.


Police detectives at Kent County Constabulary (UK) were asked to recollect videotapes in which a suspect had lied at one point and told the truth in another, for example: first denying that they were at the scene of a crime but later admitting they were present when shown evidence. The appropriate clips were found and compiled together, resulting in an hour-long videotape of parts from the 16 suspects’ interviews. Out of the total 65 different clips, 27 were of them being honest and 38 were of them lying. The clips varied from five second snippets to two minute parts.

Two independent observers were asked to observe and ‘code’ the behaviour; basically, they had to organise their observations. They found eight behaviours:

  • Gaze aversion (duration of suspect looking away from interviewer).
  • Blinking (frequency of blinks).
  • Head movements (frequency of head nods, shakes, tilts, turns, etc.).
  • Self-manipulations (frequency of scratching head, wrist, etc.).
  • Illustrators (frequency of arm and hand gestures alongside talking).
  • Hand/finger movements (touching hands, etc. without moving the arms).
  • Speech disturbances (frequency of ahhs and mmms, word/sentence repetition, sentence change, sentence incompletion, stuttering, etc.).
  • Pauses (duration of noticeable pauses).

An inter-rater reliability test was also conducted to check the similarities in both coder’s judgements. To avoid all bias, the coder’s were not aware which parts of the clips were showing truth or lies, and they were not informed about the hypotheses behind the study. Their only instruction was to “code the video footage”. There was a strong consistently between the coders.

It was decided that self-manipulations, illustrators and hand/finger movements could be clustered together. Thus, the final organisation consisted of six grouped behaviours:

  • Gaze aversion
  • Blinking
  • Head movements
  • Hand and arm movements
  • Speech disturbances
  • Pauses


The results are summarised as follows:

  • Lies were accompanied by decreased blinking and increased pauses (shown in 81% of the sample).
  • Other than the above, no significant differences of behaviour in lying and honesty was observed.
  • There were many individual differences.
  • 50% of liars showed more head movements and speech disturbances, the other 50% showed a decrease.
  • 56% of liars shown more gaze aversion, 44% of liars showed a decrease in gaze aversion.
  • Most suspects showed a decrease in hand and arm movements, compared to 31% who showed an increase.

The experimenters did not measure cognitive load or nervousness, just the behaviour above. However, the longer pauses and decreased blinking may have some link to a cognitive load. Previous research linked more blinking to nervousness, which suggests that the blinking behaviour shown in this study is an indication of the cognitive load being linked to deception.

Compulsive Liars. Type of experiment

This was a quasi (or “natural”) experiment because no variable was manipulated by the researchers.

Independent variable

Telling the truth or telling a lie is the IV.

Dependent variable

The DV is the behaviour that is shown when the person lies or is honest.


Very high ecological validity: The ecological validity of this study was very high because nothing was manipulated or changed by the experimenters; it was all natural behaviour in a natural setting.

Quantitative data: Most of the data was quantitative, in the form of frequencies and statistics. This means it is objective and free of personal interpretations, and it is also easier to analyse (how many times someone did something is easier to conclude than ‘why’ someone did something).

Reliability of coders: The coders who organised the material were unaware of the nature and processes behind the study, making them less likely to bias the findings. Because they were unaware, their observations were objective and based on factual evidence and solid observations rather than their personal thoughts. The reliability of both coders’ observations was also checked and found to be very consistent, meaning their ratings were highly similar.


Less control: There was absolutely no control over extraneous variables, meaning that many other factors may have been distorting the findings. For example: the suspects were interviewed by different people, they were interviewed about different things, some suspects had more than one interviewer in the room.

Small sample size: The sample consisted of 16 participants only, all from a similar background, ethnicity and area. This means the results cannot be generalised to a wider population as the sample is unrepresentative.

Ethical issues

Informed consent: The suspects had given consent to be videotaped, although it is unclear if they consented to have their tapes analysed. The local police did give consent for the experimenters to use the tapes.

Deception: There was no deception.

Confidentiality: All personal details were kept private and care was taken to ensure that only a small amount of necessary people saw the actual clips.

Emotional or physical harm: No physical or emotional harm was inflicted.

The right to withdraw: This is unclear, again, whether the suspects were asked about the use of their videotapes.

Debriefing: There was not much need for debriefing and none took place.

References: Mann, S., Vrij, A. and Bull, R. (2002). Suspects, Lies, and Videotape: An Analysis of Authentic High-Stake Liars. Law and Human Behaviour. 26