You would think securing a conviction against a suspected criminal would have to be helped if all the witnesses to the crime independently identify the same culprit.

Key points: Statistical modelling shows probability of a large group of people all agreeing is small

Statistical modelling shows probability of a large group of people all agreeing is small Previous research shows error rate of witnesses is 47 per cent

Previous research shows error rate of witnesses is 47 per cent Modelling shows with just a one per cent error rate, confidence would decrease after three unanimous identifications

But new Australian research suggests that unanimity of witnesses should trigger a warning that perhaps police have the wrong person.

In a paper published today in the Proceedings of the Royal Society A, University of Adelaide researcher Professor Derek Abbott and colleagues reveal that rather than being a sure thing, unanimity is actually unreliable.

Dr Abbott, from the School of Electrical and Electronic Engineering, said if a large number of independent witnesses unanimously identified a suspect to a crime, it was assumed they were right.

However statistical modelling by his team showed the probability of a large number of people all agreeing in these circumstances was small, casting doubt on the veracity of unanimity.

"Paradoxically if many witnesses confirm a certain suspect your doubts should increase rather than decrease," he said.

Dr Abbott pointed to the example of a line-up consisting of bananas and one apple.

"A thousand people are all going to agree it's an apple so a unanimous response is entirely expected," he says.

"But if you have a guy fleeing from a bank with a sack of loot and 20 witnesses have all had a five-second glimpse of him, that is not a sure thing any more."

Abbott said research by psychologists had previously shown that the error rate among witnesses in that type of situation was around 47 per cent.

"So if the error rate is as big as 47 per cent and 20 people all agree it is the same guy you should be now more suspicious of their agreement," he said.

Their modelling showed even with just a 1 per cent error rate, confidence in a police line-up result would decrease after three unanimous identifications.

'Paradox of unanimity' applies across many areas

Dr Abbott said this "paradox of unanimity" is applicable across many fields.

In engineering and computing the paradox suggests where measurements are overwhelmingly consistent it may indicate a systemic failure.

Likewise in archaeological profiling, consistent results over the origin of an ancient artefact could also indicate a specific manufacturing process for that artefact.

Dr Abbott said recently the paradox was key to uncovering the recent Volkswagen scandal where the company was found to have rigged the carbon emission readings of diesel cars.

"The team that busted Volkswagen said the reason they were suspicious was the results of the emission tests were too consistent," he said.