Unanimous opinions and decisions should be looked upon suspiciously, because they might reveal common cause of mistake. “Unanimity is often assumed to be reliable. However, it turns out that the probability of a large number of people all agreeing is small, so our confidence in unanimity is ill-founded. This ‘paradox of unanimity’ shows that often we are far less certain than we think.” The idea is developed in this excellent post on phys.org ‘Why too much evidence can be a bad thing‘.
“Under ancient Jewish law, if a suspect on trial was unanimously found guilty by all judges, then the suspect was acquitted. This reasoning sounds counterintuitive, but the legislators of the time had noticed that unanimous agreement often indicates the presence of systemic error in the judicial process, even if the exact nature of the error is yet to be discovered. They intuitively reasoned that when something seems too good to be true, most likely a mistake was made.”
In any case the paper shows that when results of a process or experiment are too consistent to be true we should search for a common cause that might explain this consistency. An example in the paper is particularly vivid: “Police in Europe found the same female DNA in about 15 crime scenes across France, Germany, and Austria. This mysterious killer was dubbed the Phantom of Heilbronn and the police never found her. The DNA evidence was consistent and overwhelming, yet it was wrong. It turned out to be a systemic error. The cotton swabs used to collect the DNA samples were accidentally contaminated, by the same lady, in the factory that made the swabs.”
So the next time that you are faced with an overwhelming unanimity, look further for a possible cause that have nothing to do with what is being decided or sought. It might save the day!
