1

Detecting deception

• A lie: a deliberate attempt by one person to mislead another– No prior warning of this intent

• To detect a lie, we need to understand why lies fail– Speech content, mannerisms– Will tell us what to look for

• Problem: How do we know when we have caught a liar?– Error 1: He was lying and we

missed it– Error 2: He wasn’t lying and we

said he was!

2

Misses & False accusations

• “we can detect lies with 96% accuracy” what does it mean?– We can only work out this number

if we know base truth

• Base truth is when we know the actual truth about the event– Normally happens in experiments– Not normally available in the field

unless the person confesses to lying

• We must trade off one type of error against the other– A problem of all hit/miss decisions

on a yes/no event

3

Limits of decisions

• Your decision will be balanced on two poles– Being too lenient, and increasing

your chance of him getting away with it if he actually lied

– Being too conservative and increasing your chance of falsely accusing him if he is actually innocent

• So why not just be extra conservative, if we think he is actually lying?– Because we don’t know the base

truth! So all decisions are compromises between these

4

Why do lies fail?

• Some external reasons– Someone rats them out, physical

evidence is found

• Sometimes the liar exposes the lie– Some behaviour or statement may

reveal the lie

• But beware! Contradictions are tricky!– A contradiction can be a sign of a

lie– But truth can contain

contradictions too (bad memory, etc)

5

Lying on the fly

• To effectively lie, you need time to prepare– Get the story straight– That way, it will flow naturally

when telling it

• When being questioned, a non-prepared lie may become apparent– Show signs of thinking about

answer– Pause, averting gaze, speech

mannerisms– Can only be taken as a sign of

lying depending on the context

6

Emotions and lying

• Faking an emotion is hard– Either showing one you don’t feel

or suppressing one you do

• Some signs of emotion are ‘reliable’ (extremely hard to fake)– Narrowing red edge of lips (anger)– Eye muscle movement in

Duchenne’s smiles (happiness)– Could be faked by Stanislovskian

method

• Concealing emotions in harder than faking them

7

Concealing an emotion

• One emotion is generally concealed by trying to express another– Eg. Hide sadness by attempting to

smile

• This can fail in two ways– Leakage (part of the masked

emotion escapes) – eg. Brow remains raised even when smiling

– Produce a deception cue (behaviour which doesn’t fit in with the rest of the lie) – eg. Smile may not be held for long enough

8

Further role of emotions

• Even if the lie is not about emotion, emotions probably play a role– Fear, guilt, happiness (“dupe

delight”), excitement– Not in every lie

• Whether emotion is felt depends on various factors– Characteristics of the liar– Characteristics of the target– Content of the lie

• Each type of emotion has two effects– Increase in level of arousal– Specific behavioural changes

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Examples of emotion in lying

• Fear– Chance of punishment is high– Lie is not practiced– No experience of success with the

target– Known that target is suspicious

• Guilt– Values shared with target or target

respected– No personal benefit from the lie– Lie not authorized by an institution

• Duping Delight– Allies of the liar are watching

10

Detecting lies by behaviour

• Several channels to consider– Face, body, voice, paralinguistics

• No one channel provides more information than the others– Each can provide some,

combinations can provide more

• Showing these behaviours does not guarantee lying– “Othellos’s error”– Need to consider the context of

the behaviour– Would a truthful person show

those emotions in that circumstance?

11

Experiments in lying

• Experiments would be useful– Allow us to tell which cues are

linked to lying

• How does one experiment on lying?– Get people to lie/not lie– Measure various cues and see if

they are useful predictors

• What about high stakes?– Lying about something silly will not

give a level of arousal matching a real world situation

– But creating a high stakes situation increases the motivation to not be caught (ruins the experiment!)

12

Example:Riggio & Friedman (1983)

• Undergraduate volunteers

• Subject sits alone in front of video camera

• Given a folder with pictures– Each had instructions on whether

to describe the picture or lie

• No punishment/reward for lying

• Extremely well controlled experiment– But very unlike a real lying

situation

13

Example: Eckman et al (1989)

• Student nurse subjects– The study was part of their course

• Had to describe, as they watched, a gory video and lie– Spoke to a person in the room who

could not see the video

• Control group who described a pleasant video without lying

• Well controlled, realistic experiment– Base truth is known– High stakes situation– More generalizable to real

situations

14

Getting cues from Eckman’s study

• Looked for cues in various domains– Facial expression (using FACS)– Voice (stress, pitch, volume, etc)– Body movement (mannerisms,

suppression, etc)

• Asked judges (observers) to look at videos– Asked if the subjects were lying or

not– Asked them to infer about

personality and affect

• Analyze these to find if there were reliable cues to predict lying

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Ekman’s results

• Indicators in facial expressions– Duchenne’s smiles in rue

enjoyment– Leakage smiles (micro-traces) in

lying

• Indicators in voice / text– Pitch increases in lying– Number or self-references

(‘I’/’me’) decreased in lying

• Best predictor is in combining both expression/voice data– Accurate assignment rate of 96%– Comparable with best published

polygraph results (see later)

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Why the changes when lying?

• Duchenne’s smiles are automatic– Difficult to fake

• Leakage smile (ie. other emotions leaking through)– Emotions of lying, or about the

nasty film?– Probably lying; smiles different to

miserable smiles and compliance smiles

– In another experiment (no lying) leakage smiles did not occur

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Why the changes when lying?

• Changes in self-reference– Could be due to simultaneous

planning of the lie

• Changes in voice pitch– Fear of being caught? Arousal

from the film? Both?– Probably lying (also been found in

lying studies without nasty films)

• Note: the indicators are clear, but the reasons why they occur are not!

18

Looking at videos of lying

• Note: subjects only told they were seeing a conversation!

• Only text cues (what was said) and mannerisms made a difference– Duchenne’s smiles, Leakage

smiles, voice pitch etc not used

• The most important predictors of lying were ignored!– Most useless behaviours were

focussed on– In social world, people act to

maintain lies (?)

19

Ekman’s conclusions

• Some lying cues can be found– They cut across channels (not

simple)– Face and voice together provide a

high hit rate

• BUT: observers who are not privy to the lie do badly at spotting it– Observers ignore these cues– Focus on content of the

conversation– This is a terrible predictor of lying– Question: Can observers be

trained to ignore useless cues and focus on reliable ones?