By Jonathan Amos BBC News Online science staff in Leicester

People are bad at spotting liars because they look for the wrong signals, according to research presented to the British Association's annual science festival in Leicester.

In a lecture on the psychology of lying, polygraph machines were also criticised as being poor at detecting falsehoods.

The work was explained to the festival by psychologist Dr Richard Wiseman, of the University of Hertfordshire, an expert in the science of deception.

"People are really dreadful at detecting when someone is lying," he told the BBC.

"They think that liars avoid eye contact and fidget a lot. In fact, liars maintain more eye contact and they don't fidget."

Lots of mistakes

Liars do this, Dr Wiseman says, to present a truthful demeanour.

Lie detectors are said to often give false results

"What you should do is look to see if there are long pauses between the questions you ask and the answers people give," he said.

Other indicators of lying include the use of short sentences, and any fluffs or errors in a person's speech. Lack of movement can also be a clue to untruthfulness.

Dr Wiseman told the festival that the UK Government should not rely on polygraph machines to determine if sex offenders are likely re-offend on release.

Poor guide

He said such machines were prone to throwing up false positives.

"They detect when someone is stressed, not necessarily when they are lying."

Dr Wiseman is a regular at the festival and has an eye for research that catches popular interest.

Last year, he used the event to launch research into what makes us all laugh. He set about trying to find the world's funniest joke.

He created a website through which people could submit their favourite gag and vote on others using a laughometer.