Forensic Psychology For Dummies (42 page)

Summarising why detecting deception is so difficult

Any lie-detection procedure takes into account the intellectual and emotional demands that lying requires (as I describe in the preceding section ‘Discovering the difficulties of successful deception’). The nitty-gritty of detecting a lie is in testing the plausibility of the claims the suspect is making. But if a liar truly believes he’s telling you the truth none of the intellectual or emotional aspects of lying exist and the usual procedures for detecting deception are unlikely to work.

Hardened liars are experts in using strategies to reduce the likelihood of being detected:

The lie is rehearsed so that there’s no need to invent a lie on the spot with all the associated risks of getting details wrong or saying something implausible.

The lie is built upon something that actually happened, so that most of the details are true and don’t need to be invented and are plausible. Only some crucial aspects of the lie are untruthful.

The liar avoids giving any details to cut out the risk of tripping himself up.

You can make things much harder for a suspect you think is lying by asking for as much detail as possible. The more the liar has to invent, the more chance he’s going to reveal inconsistencies in his story.

Detecting Lies: Some Attempted Procedures

Lie-detection procedures have been developed for interviewing suspects in a criminal investigation, helping to weed out the lies, and leave the truth exposed.

You have to bear in mind that none of the lie-detection procedures that I describe in this section are completely accurate or foolproof. Indeed, some procedures can mislead you into thinking that you’ve detected lying when the opposite is the case.

There are four general approaches you can take in detecting lying or deceit:

Physiological approach,
which records the physiological changes in a person’s body when answering specific questions.

Behavioural approach,
looks at the way a person is behaving to see whether the person is showing any of the emotional or stressful aspects of lying (see the earlier section ‘Discovering the difficulties of successful deception’).

Semantic assessment,
which carefully examines the words the person is using and the possible meanings in the answers the person is giving under questioning.

Legal approach,
where a person is being questioned in a court of law and being put through a detailed examination of the plausibility of his statement.

Lie-detector procedures like the physiological, behavioural, semantic and legal approaches are concerned with testing the truthfulness of what the person is saying while he’s giving his account of the event. These procedures don’t include a careful investigation that can show that a person’s
alibi
– the claim not to have been at the crime scene – is false. Nor do these procedures have the means of examining the impossibility of the person doing what he claims to have done or not done – for example, because that person has a physical disability, or the journey he claims to have taken could not have been completed in the time available. You need to test the credibility of what the person is claiming independently to find out if the person’s story is true.

Testing the validity of a lie-detector procedure can be problematical. For example, you set up an experiment in which you get a person to simulate committing a crime. Then the person is questioned using a lie-detection procedure. But in an artificial situation, the reactions of the suspect and the truthfulness or otherwise of the suspect’s statement doesn’t carry the same high stakes as in real life where not being believed could mean a long prison sentence.

Getting true-life examples of the validity of lie-detector procedures can throw up many ethical and legal problems. In the real world, getting adequate comparisons with what is likely to have happened in a criminal investigation if the lie-detector procedure hadn’t been used is often impossible. Companies selling lie-detection equipment or software typically avoid providing important comparison data. So although the company claims their product can show that a lot of deceptions were uncovered using their (often expensive) system, no-one can tell you if the lie-detector system really did add value. I discuss this anomaly in more detail, later in this chapter, in the section ‘Combating insurance fraud’.