LIE detection is crucial for most people who come into contact with the public. That includes police, social workers, customs officers, judges, lawyers, immigration officials and many working in Government offices such as benefit agencies.

But the latest psychological research suggests we are not as good at spotting deception as we think we are. The average rate of lie detection is just 44 per cent, which is a worse result than we would get if we based decisions on the toss of a coin.

In an experiment, officers were shown TV broadcasts of people asking for help in finding missing relatives, or the murderers of their relatives. In fact, they were lying and were subsequently convicted of killing their relatives themselves. The officers, who did not know the background to the videotapes, were asked whether they could spot any deceit. They did not perform better than could be expected by chance.

Psychologist Professor Aldert Vrij, from the University of Portsmouth, who conducted this experiment, found that only one profession did better than the public in spotting lies - officers who work for the US Secret Service and the FBI. But this could be partly because these agents seem to adopt a powerful but simple strategy: trust no one.

This approach might be good at spotting deception but it runs into trouble when it comes to identifying the truth, which is equally important.

Another study, from the Department of Psychology at Williams College in Massachusetts, shows there are dangers in assuming you know whether someone is lying or not before interviewing them. Psychologist Dr Saul Kassim created a situation where suspects "guilty" or "innocent" of a mock theft were interrogated to establish the truth. The interrogators had been led to have a prior belief about the suspects' guilt or innocence, even though that bore no relation to their actual guilty status.

Those who already believed they had a guilty suspect exerted more pressure and used more guilt-presumptive questions, resulting in a 23 per cent increase in guilty judgments by questioners.

Even more fascinating was the finding that neutral observers were influenced by how defensive the suspect appeared. They were much more likely to conclude the defendant was guilty if they had seen the kind of interrogation carried out by those interrogators who were already convinced of the suspect's guilt.

So interrogators trying to work out whether someone is lying or not need to take more account of the effect of their own behaviour on the subject and try to keep an open mind.

This has vital implications for how a team of social workers on a suspected child abuse case might develop a conviction that someone is lying.

If they rely on observing one team member conducting an interview where there is already a presumption of guilt, it is easy to see how a whole organisation can become convinced that someone is deceiving when they may not be.

One problem is that many professionals do not understand how liars behave.

The majority (75 per cent, says a survey) assume that liars avert their eyes.

Research shows this is not usually true.

Liars are having to manufacture reality, which requires a lot more intellectual effort than simply reporting the truth. They need to monitor how their story is coming across so as to know whether to modify their strategy or not: so they are likely to look closely at their interrogator.

Truth-tellers are not so involved in monitoring the listener, so are more likely to look away.

Also liars take longer to answer questions, pause more, are not as fluent in the flow of their answers and - if they are good liars - tend to make fewer gestures to try to ensure they don't "leak" body language clues to their lying.

It is this body language which holds the real key to detecting lies. A change in body language as the sophisticated interviewer switches from a topic where it is known the interviewee is telling the truth to one where lying is possible can be a real giveaway.

Dr Raj Persaud's book From The Edge Of The Couch is published by Bantam Press (12.99).

Aaron Barschak: if you'd been on the gate would you have turned him away?

(C) 2003 Evening Standard - London. via ProQuest Information and Learning Company; All Rights Reserved

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Last updated: 07/20/2003 - 07:56 AM