When Everyone Agrees, Something Isn't Right

When everyone agrees on something, you just might have a problem. That's what a new study to be published in Proceedings of the Royal Society A found, according to a press release. Researchers from the University of Adelaide in Australia used mathematical probability to test three scenarios. Each time, they found that too much agreement weakened confidence in the result. In one scenario, for instance, 13 witnesses confirm the identity of a criminal suspect. "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," researcher Derek Abbott says in the release. Other scenarios that were tested included witnesses confirming the identification of an archaeological find and the reliability of a cryptographic system.

In each case, there was a tipping point at which too much agreement weakened confidence in the result. "It seems counter-intuitive," Abbott tells Lawyers Weekly. "It seems that [it] defies logic, but it's also saying that perhaps if everybody is in total agreement there's been a collusion or a bias.” There is a precedent for the researchers' conclusions: Abbott references an ancient Jewish law under which a person could not be convicted of a capital crime if the guilty verdict was unanimous. "This ancient law indicated that the system may be in error if there was complete agreement among the judiciary," he says in the release. In the case of witnesses identifying a suspect, the rate of misidentification can be 48%—"almost like tossing a coin"—Abbot tells Lawyers Weekly. "If 20 witnesses are all agreeing, then that can't be right," he says, "because you are already expecting half of them to be wrong.” (More study stories.)