How Many State Witnesses Are Needed to Prove a Case? The Paradox of Consensus

Sometimes, too much agreement makes me wonder.

If you’ve seen as many witnesses testify as I have, when you see their stories miraculously line up so neatly, it makes you wonder.

Do you believe them more because there are 9 witnesses, versus 5? Two witnesses versus one?

Now, just because witness stories line up perfectly doesn’t mean these witnesses are lying. I’m just questioning their accuracy. Every time the prosecutor calls another witness that agrees with the one before, surely the case for my client is getting worse, right?

Well, agreement isn’t always a good thing.

Back in the day, under Jewish law, a capital crime required 23 judges. If every judge in a case found the defendant guilty, the suspect was deemed innocent. Free to go.

This legal concept is known as the Paradox of Consensus. Basically, too much agreement is a red flag that there may be a problem with the process or system generating the result.

I’ve been defending criminal cases for over 27 years, so I’ve had to stand up and say “Objection Your Honor, Cummulative” when the state calls the hundredth witness to say the same thing the other 99 witnesses said. Prosecutors want to call twenty witnesses to say the exact same thing, figuring that with each additional conforming bit of testimony–the chances of them all being wrong would be pretty slim, right?

Or, maybe they’re all wrong.

Scientists once thought that, after seeing a thousand white swans, and after documenting the 142,832nd white swan–surely no black swans exist, scientifically speaking. They had proven that all swans are white through repeated observation. When some explorer reported spotting the 142,833rd white swan, those scientists just figured they had a smidge more evidence that all swans are white.

And, we all know how that turned out.

Sure, it used to be common sense that the earth was flat, or that the earth wasn’t moving through the solar system at tens of thousands miles per hour. But your common sense can trick you.

An article by Lisa Zyga entitled “Why Too Much Evidence Can be a Bad Thing” explores what is called the Paradox of Unanimity. (Phys.Org, January 4, 2016). Zyga explores scientific research that “demonstrated the paradox in the case of a modern-day police line-up, in which witnesses try to identify the suspect out of a line-up of several people. The researchers showed that, as the group of unanimously agreeing witnesses increases, the chance of them being correct decreases until it is no better than a random guess.” id.

Our guts have given us similar warnings, though admittedly not as scientific as the study cited by Zyga. The warning here is that highly consistent observations may be too good to be true–there may be an underlying problem.

The article by Zyga notes that, according to physicist Derek Abbott, “if many independent witnesses unanimously testify to the identity of a suspect of a crime, we assume they cannot all be wrong . . . 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.” id.

Zyga cites a great example in a serial killer case: “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.” id.

In a seminar given by Lachlan Gunn of the University of Adelaide on the paradox of unanimity, he notes that Sanhedrin, in Sanhedrin 17a, that the old Jewish court must find a defendant innocent when there is total agreement as to his guilt, in part because such a result must mean that the defendant was not adequately defended (in other words, as indicated above, there is some systemic problem causing the outcome). id.

It’s counter-intuitive to think that, the more agreeing witnesses the State has, the less confident we should be in the accuracy of their observations. But hey, far be it for me to argue with the scientists. I’m all for a bit more reasonable doubt. Of course, if you couldn’t sell folks hundreds of years ago on the concept that the earth is moving at a rate of 67,000 of miles per hour, I’m not sure how this research on unanimity is going to be accepted.