This claim popped up in a discussion on Hacker News where it was claimed that if a legal system has a jury it's more likely it will be just. As one commenter put it:
[..] jury is not as easily bought, harder to threaten, and better at ironing out the moral ambiguities that can be the failings of a single person.
Has there been any evaluations in different legal systems to support or refute that claim?
Empirically, I wouldn't know.
But theoretically, the reasoning behind a jury system has been formalized mathematically.
What does it mean for justice to be served?
There is a dilemma because of what statisticians call "type 1" and "type 2" error.
With a justice system these types of error are:
Condorcet's jury theorem [1785] argues that a traditional jury operating under unanimity for conviction is less likely to commit the second error than picking a single juror and making him the judge of the facts. Suppose that the probability that juror i will wrongfully convict the innocent is p[i]. Then, if we assume independence of jurors (which can be problematic), the probability that an innocent defendant is convicted by the jury under unanimity is product(p[i]; 1<=i<=N). Since p[i]<=1.0 for all i, this product is necessarily smaller than any individual p[i].
This does not answer whether letting a judge decide guilt might do better or worse either because of his training or potential for acquiescing to government demands or falling into corruption. However, it does show that at least on avoiding the conviction of innocents, a larger jury should be fairer than a smaller one.
Type 1 error is increased. The proof, I suspect, is a bit more tedious but basically the same principal with "and" replaced by "or". Only one juror's false belief in innocence (or corruption) is required for type 1 error.
These are different injustices, and trying to mold the system to reduce one error often increases the frequency of the other error. Designing for a balance between these errors involves some serious moral and ethical issues, among them the problem that citizens may have different preferences for them. While Blackstone suggested "better that ten guilty persons escape than that one innocent suffer", any balancing invites both the questions "how many innocents should suffer a false conviction?" and "What casualties do the freed guilty create among other innocents?"
This doesn't answer your question directly. But I agree with user unknown that you would need a meta system to ascertain this. I also think jwenting's anecdotal evidence and Paul's theoretical ideas (I would attack jury independence most) don't give you the whole picture.
I was actually surprised (or is there a more fitting word when I just didn't bother finding out?) to find there is a field devoted to researching and mitigating jury biases, though their research could arguably be easily used to exploit jury biases.
So if you are representing an African American client—you may want to avoid those religious similarity references because they could activate biases against your client. You want to play up similarities between your client and the jury that do not include religious activities (which would draw attention to your clients ‘differentness’). If you are inclined to make use of religious allusion or rhetoric, include a discussion of the risk of racism very overtly, to avoid the priming affect.
Recently a study in Israel caused a stir. They found that having a snack / lunch predicted clemency in parole hearings.
Of course they don't cancel each other out. You could meta-analyze the findings to find where the biases are worse and choose the lesser evil or you could think forward (clear decision rules, lengthy expert essays on non-objective evidence, ...?)
