Class, 11/14

Today, after some special announcements, we discussed the O. J. Simpson case. See Calculated Risks, Chapter 8. The basic thing here is that Alan Dershowitz, the Harvard lawyer who contributed to Simpson's defense, made a mistake when he argued that the fact that Simpson had battered his wife Nicole was not material, since the probability that a batterer would go on to murder his partner is only 1/2500 per year. This reasoning is wrong, because the probability that a woman would be murdered by a random person (not the batterer) in any year is 1/20000. This means that of any group of 100000 battered women, 40 of them would be killed by their batterer in any year, whereas only 5 would be murdered by a random person who is not the batterer. So, when we look at the fact that Nicole was murdered and battered, the probability is 40/45 that Simpson did it (on only this evidence). So the battery is relevant and should be admitted in evidence.

We then discussed the death penalty, which although it does not exist in Vermont state courts, does exist in Federal courts and at least one Vermont jury recently gave the death penalty to a person in Vermont who was tried in Federal court.

We used a decision tree approach. A decision box at the root of the tree has two branches corresponding to our two actions: Convict or Acquit. We put another decision box on the Convict branch, that is, Death or Life Imprisonment. Then each of these three branches has a probability circle with two branches: Guilty (probability p) and Innocent (probability 1-p).

For the losses, we put 0 on each of the two correct outcomes, Acquit Innocent and Convict Guilty with Life. We decided for various reasons that Convict Guilty and Death had a slight loss (reasons being things like moral hazard). It turns out not to matter, we could pick 0 and the result would be the same. We decided that the loss of giving an innocent person the death penalty was huge (we picked 1000, I think), much larger than the loss of sending an innocent person to prison for life. We didn't assign an actual number to acquitting a guilty person as we were running out of time. But the point was, we found that no matter what p is, so long as the loss for Innocent and Death is larger than the loss for Innocent and Life in Prison, we will never choose the death penalty.