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Is Eyewitness Testimony Reliable When All Witnesses Agree?

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By Ephrat Livni, Esq. on April 04, 2016 12:14 PM

If you've ever been a juror or watched a court case on television, you know that we rely on witness testimony to decide whether people are guilty. The more witnesses there are, and the more their stories agree with each other, the more likely it is that a jury will convict the defendant. Makes sense, right?

Well, not quite. A recent study has found that too much agreement amongst many witnesses might be proof that witnesses are lying, or at least recalling incorrectly, reports Vocativ. The magic number -- when it comes to witness agreement -- is three. Let's review the study.

The Magic Number

The study -- published in The Proceedings of the Royal Society A -- examined reactions to police lineups. The researchers in France and Australia found that when three witnesses agreed, a point of reliability was reached, but the more people agreed subsequently, the less likely it was that they were really recalling what they had seen.

According to a statement explaining its methodology, the team 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 consistently found a point at which "too much of a good thing" actually weakened confidence in the result.

Agree to Disagree

The reliability of eye witness testimony has long been a subject of debate in the law. Overly-zealous attorneys can and do coach witnesses in an effort to present a story that makes sense to a judge or jury. But what the study shows -- and apparently has been known for some time -- is that people are inclined to be influenced by one another.

Rather than revealing perfection, total unanimity may be an indication of a systemic failure then. "Getting a large group of unanimous witnesses in these circumstances is unlikely, according to the laws of probability," said Professor Derek Abbott, a probability expert at the University of Adelaide in Australia and coauthor on the study. "It's more likely the system itself is unreliable."

The study's authors point out that in the ancient Jewish legal tradition, for example, when 23 judges all agreed to an execution, the defendant was set free. The reasoning is that total agreement is unnatural for humans. In other words, the more people are involved in a matter, the more likely it is that we will have to agree to disagree.

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