Humans are the best lie detectors

September 12, 1997

Agricultural policy, smart salmon and sex-change chickens at the British Association's annual festival of science in Leeds

Human beings could be better at detecting lies than polygraphs, but husbands and wives can use knowledge of their partners to avoid detection, new research has found, writes Cath Cotton.

According to Richard Wiseman of the University of Hertfordshire, "those in favour of the polygraph will be keen to tell you that research shows the machine correctly detects about 83 per cent of liars. This is very true. But they are reluctant to admit that it also incorrectly classifies around 40 per cent of innocent people as liars."

The problem is that polygraphs detect stress, based on pulse rate, blood pressure, respiration rate and sweating - all of which is taken as evidence of guilt. What it fails to consider is that truth-tellers can become stressed and liars are often aware of counter measures, such as biting your tongue, which can fool the polygraph.

Like the polygraph, human beings also make false judgements about body language, often associating low levels of eye contact or high levels of hand-waving as evidence of guilt. In fact, Dr Wiseman's research has found that engaging in more eye contact than usual is often indicative of lying.

But the research has also shown that when potentially misleading visual cues are removed, human beings become significantly better at detecting when an individual is telling the truth. Also, unlike the polygraph, humans can learn. If they are warned of behavioural characteristics that appear to be associated with deceit, they can improve their ability to detect lies. But he warns that the method has some important limitations.

"People did get better - by 5 to 10 per cent. And this improvement was significant. But the relationship between two people can matter. We are better at detecting lies in strangers than in our partners, probably because our partners know us. They know how to lie to us," he said.

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