Why witnesses agreeing too much could be a bad sign

If too many eyewitnesses agree about what they saw, then a new study says they should be distrusted.

Most people would logically conclude that if 12 eyewitnesses to a crime identify the same person in a police lineup as the culprit then it is safe to conclude that that person is likely the culprit. However, a new study published in the Proceedings of the Royal Society A says that if too many witnesses give the same testimony in a criminal case, it is more likely to mean that their testimony is being influenced by systemic bias. As Vocativ reports, the interesting study utilized statistical analysis and even ancient history to support its conclusions.

Ancient courts and unanimity

Unanimity is something that the modern justice system and society places a lot of value in. When all eyewitnesses agree about what they saw, for example, then that testimony usually carries greater weight. Likewise, when the media reports that the Supreme Court delivered a unanimous decision as opposed to a split decision the implication is often that there can be little controversy about the unanimous decision.

However, it was not always this way. The ancient Jewish court, for example, had an odd system for trying capital cases. For those cases, a panel of 23 judges would hear the case and each would deliver a verdict. The defendant would be convicted and executed if a reasonable majority of the judges voted that he or she was guilty. However, if all 23 judges voted that the defendant was guilty then the defendant would be freed.

Unanimity should be distrusted

That loophole in the ancient justice system may sound illogical, but a recent study shows that the ancients may have been on to something. The odds of a large group of people agreeing unanimously on one thing are so unlikely that when it happens it should be treated with considerable caution.

As Lawyers Weekly reports, this was shown in a recent study that used statistical analysis, while citing the customs of the ancient Jewish court, to show the problem with unanimity among eyewitnesses. That study concluded that if up to three eyewitnesses identified the same suspect in a police lineup as the culprit then it was safe to say that their testimony was reliable. However, if more than three eyewitnesses identified the same suspect then it was more likely that the witnesses were being influenced more by systemic bias rather than by what they actually remembered. The researchers point out that human memory is so fallible that the likelihood of a large number of eyewitnesses identifying the same suspect is extremely unlikely.

Criminal defense

The study is a reminder that in the criminal justice system no outcome is certain. For those who have been charged with a crime, it is important to reach out to a criminal defense attorney right away. An experienced attorney can help clients understand what avenues they have open to them, including how to challenge any evidence the prosecution may present and how to mitigate the fallout of a charge or conviction.