Thursday, February 24, 2011

Wrongful Convictions From Civil Behavior

In a previous post, I described "Proof Beyond a Reasonable Doubt" as a convenient judicial myth. At the end of that post, I presented the following summary plot comparing the idealized concept against the reality of jury and judge verdicts.  I include that plot below for easy reference. Click to enlarge and clarify.
Judges and scholars, if forced to quantify the reasonable doubt threshold, tend place the threshold near 90%. In Proof Kinda Beyond a Reasonable Doubt, I reported that our jury pool self-reports a threshold of 85%. In Proof Beyond a Flip of the Coin, I report that more-subtle, less-biased surveys indicate that our jury pool places the threshold closer to 66%.

In this post, I regret to inform you that some studies place the actual threshold closer to 50%. The 50% limit is incompatible with a "proof beyond a reasonable doubt" standard. Rather, a 50% threshold is by definition a "preponderance of the evidence" standard. It's the standard jurors are instructed to use in civil cases, when John Doe accuses Joe Blow of destroying his topiary garden with a runaway snowmobile. A preponderance-of-evidence standard absolutely should not be applied when deciding matters of criminal guilty or innocence, when deciding matters of life and death.

But I'm ahead of myself. First, I should explain the studies.

The third group of studies we consider herein is based on decision theory. Decision theorists argue that jurors vote to maximize their personal satisfaction. (This is also known as minimizing their personal dissatisfaction.) Decision theorists describe such satisfaction (or dissatisfaction) as "utility." According to decision theorists, jurors intuitively weigh four utilities:
  1. Utility of acquitting the innocent
  2. Utility of convicting the innocent
  3. Utility of acquitting the guilty
  4. Utility of convicting the guilty
Decision theorists simply ask potential jurors to rate the relative utilities on a common scale, say from 1 to 100, then calculate the person's decision threshold based on a logical and relatively straightforward equation. I refer the reader to the source for the equation rather than complicate this post further by trying to describe it.

Though decision-theory surveys may sound more convoluted than the direct and parallel surveys, my experience tells me there is some merit in the concept. I have independently arrived at a hypothesis that jurors most fear putting bad people back on the street. In decision-theory speak, they assign a substantial disutility to acquitting the guilty, and apply a high utility to convicting the guilty. They show less concern about acquitting or convicting the innocent, since they tend to assume the defendant must be guilty. Certainly the prosecuting attorney and the police and good honest citizens wouldn't attempt to convict an innocent person.

If you simply ask a potential juror to state the reasonable doubt threshold they would use, they will give you number somewhere near 100%, because they know that's what the answer is supposed to be. If you conduct more subtle parallel studies of guilty / not guilty versus probability of guilt, you get closer to the actual threshold jurors use. Even in the parallel studies, however, the jurors don't really feel the gut-wrenching fear of putting a murderer, rapist, armed robber, or child molester back on the street.

Decision theory surveys attempt to penetrate into those inner feelings. While I'm confident the decision-theory surveys provide a more reasonable result than merely asking people what threshold they would use, I'm not sure if decision-theory surveys are superior to parallel surveys. My guess is that the best answer lies somewhere between the two.

In any case, I present the summary of decision theory surveys below. As usual, click on the image to enlarge and clarify it.

So after three posts on reasonable doubt threshold surveys, what have we learned? I fear we have learned why we have so many innocent people are in prison.

If we vote guilty when we are only 85% certain of guilt, or 68% certain, or possibly only 55% certain (depending on which study is more accurate), we should not be surprised that we frequently get it wrong. We should not be surprised when some of the convicted protest their innocence as we slam the prison gate shut behind them.

Nor should we be surprised as some protest their innocence while we inject lethal fluids into them through a sterilized needle.