Too much evidence may be a bad thing: study

Melbourne: Overwhelming evidence without a dissenting opinion can in fact weaken the credibility of a case or point to a failure of the system, according to a new study.

The research led by the University of Adelaide mathematically put to the test the old adage that says "If it sounds too good to be true, it probably is."

One of the examples cited by the research team is an ancient Jewish law that said a suspect could not be convicted of a capital crime if all judges unanimously handed down a guilty verdict.

"It might sound counterintuitive to say that a unanimous verdict could be wrong, but this ancient law indicated that the system may be in error if there was complete agreement among the judiciary," said Derek Abbott, a probability expert from the University of Adelaide in Australia.

The researchers put three different scenarios to the test based on mathematical probability: the use of witnesses to confirm the identity of a criminal suspect; the accurate identification of an archaeological find; and the reliability of a cryptographic system.

They found in each case that there was a point at which "too much of a good thing" actually weakened confidence in the result. "In our first example, we imagine there are 13 witnesses who all confidently identify a criminal suspect after seeing the suspect briefly," said Abbott.

"But getting a large group of unanimous witnesses in these circumstances is unlikely, according to the laws of probability. It's more likely the system itself is unreliable," he said. "In our scenario, the probability that a suspect is guilty is strong after three positive identifications by witnesses," Abbott said.

"But our tests showed that the more positive confirmations you have beyond those three, the more it erodes our confidence that this is the right person being identified," he said.

"The ancient Jewish legal practise referred to in our work indicates a surprising level of intuitive sophistication for the time, when such statistical tools would not have been at their disposal. They knew that it was rare for everyone to agree," Abbott said.

The study was published in the journal Proceedings of the Royal Society A.