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mushroom comments on Amanda Knox Guilty Again - Less Wrong
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Has any government ever investigated the rate of false positives in a jury system by faking trials?
I don't mean real-world corrupt trials, but something like totally fabricating crimes against a hired defendant, and seeing what effect different levels of evidence (or things like race, or the attractiveness of the lawyers) have on the outcome. Ideally, the judge, jury, and lawyers wouldn't know the trial was fake.
Would this be a worthwhile thing to do?
Would the natural experiment of DNA evidence furnish the false positive rate you are interested in?
Yes.
I was concerned about sample bias. However, I found this paper.
They found DNA evidence that was "supportive of exoneration" (versus "inculpative" or "exonerative but insufficient") in 8% of sexual assault convictions (33 in 422) . However, many of the cases provided no determinate evidence, (which means something like too little DNA or no reference DNA). Among cases that provided determinate evidence, 15% (33 in 227) seemed supportive of exoneration.
They present quite a bit of other statistical analysis, but this isn't quite my skill set and I'll need to set aside time to go through it later. I didn't find any other data sources of similar quality, but I looked for <30min. There was a more recent project in Arizona that tried to apply DNA testing more broadly, but that was aimed at doing a cost/benefit for current testing.
Are those two sets of 33 the same 33? If so, then that deserves calling out, because at first reading (at least to me) it sounds like some of the 8% are only "supportive of exoneration" because of a lack of DNA evidence.
It's probably worthwhile, but the purpose of a jury was not to improve accuracy. After all, you could just have one or more judges, who would have more experience and thus be less prone to error, determine guilt instead of a jury. The purpose of a jury was to make corruption more difficult (harder to secretly bribe a dozen randomly selected citizens), and to provide a way for the people to negate laws they don't like (can't punish juror for declaring someone innocent).
So even if there were a more accurate way to produce verdicts, there are other factors to consider.
The incentives of a government are more likely to be in suppressing such information than uncovering it. (Crudely speaking) the government wants people to perceive that just is done. Actually doing justice is just one way of achieving that perception.
The point of jury trial is NOT to maximize the chances of a correct verdict, just as the point of democracy is NOT to generate optimal policies.
There should be experiments on different ways of deciding guilt to see which are most accurate. My understanding is that most of the current system was created centuries ago, and wasn't the result of experimentation.
We should determine the optimal number of people on a jury, and whether juries get better results with various selections of jurors (paid professional jurors or drafted jurors, high-IQ jurors or low-IQ jurors, old or young, diverse along various dimensions or not, etc.).