In the current day US, the jury for a criminal trial is asked to deliberate and vote on their decision of “guilty” or “not guilty”. How they accomplish this is really up to them. There is no particular guard against a good salesman getting his way, nor of someone particularly stubborn (and hanging the jury) getting his.
Personally, I would argue that justice would be better and more reliably served if jurors were asked simply to state odds for each item of evidence.
For example, the prosecution brings forward a witness. The witness says that he saw the defendant running away from the scene of the crime, soon after the time that the crime is believed to have been committed. The defense points out that this might have been before the crime had taken place, that it was dark at that time, that eye witness testimony is unreliable, that it could have been someone else who looks similar, and that the police might have paid off the witness.
We shouldn’t, now, ask the jury to tell us whether the defense’s arguments open reasonable doubt. Of course they do. It’s always reasonable to doubt. We should instead ask, “What are the odds that the witness’s testimony is correct, versus that one of the defense’s possibilities is the real explanation? Would you bet with the witness 90% of the time? 50% of the time? What?” Each juror would then submit what he thinks is the most reasonable set of odds, and the trial moves to the next set of evidence.
There is accepted math for combining odds. For the one piece of evidence, the juries’ answered would be averaged together. At the completion of the trial, the probabilities would be multiplied. The sentencing would be a factor of the crime and the strength of the evidence.
If the one piece of evidence - the above witness testimony - was the only item of evidence in the trial and the jury’s response averaged out to 50%, then they’d be saying that so far as they were concerned, it is a toss of the coin either way as to whether the defendant is guilty; probably he should just be sent home. If there were 20 items of evidence and the product of their results is that there’s a one in a billion shot that the defendant is innocent; probably he should get the maximum sentence.
The results that I would expect from switching to such a system are:
- More people would be proclaimed guilty (to some degree) than are at the moment.
- While a larger percent of those people would be innocent than are today, they would be receiving minimal sentences.
- Sentencing would become more even across judges.
- Judgements would more fully be a result of the honest feeling of all of the jurors, rather than a select, outspoken few.
The only argument which I foresee against the method is that people will say that “a significant number of people can’t even figure out how to fill out a voter sheet or to perform basic algebra, how are you to expect them to figure out probabilities?” I don’t expect that most people would be able to understand nor perform probability multiplication, but that’s necessary for the jurors in this method. The court can do that. And I haven’t met the person who couldn’t understand “a flip of the coin”, “one in a million”, or “one in four”. If they can at least do that much, once you’re averaging over 12 people, you’re going to arrive at a pretty firm number, even though each individual in the jury might have to fudge their gut feeling by 12.5% to the closest probability they understand. Overall, I don’t think it’s really a big worry.