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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?

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But how are juries not as easily bought, harder to threaten? From n people, one/few person(s) may easily be from economically weak backgrounds/easily threatened. Even if you say you need a majority to achieve a judgement(and thus influencing one or two pesons does not matter), those few influenced people may together be persuasive enough to convince the rest of the jury (Since AFAIK the jury discusses their position before the verdict). I would imagine that judges would anyways be less easily bought and harder to threaten than non-judges. –  apoorv020 Apr 21 '11 at 14:23
    
There is also the system where you have multiple judges. For example in Germany where there is no jury system, in juvenile court there is one main, professional judge plus two Schöffen (~lay judges) without full professional training. They all have one vote and the Schöffen can outvote the judge. A difference to jury members is that these people do this for multiple court cases not just one and aren't randomly selected. –  Martin Scharrer May 29 '11 at 21:45
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2 Answers 2

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:

  1. Letting the guilty go free
  2. Convicting the innocent

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?"

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As far as I understand, not every jury on every trial has to reach a unanimous agreement for conviction. Sometimes a simple majority is enough. –  Konrad Rudolph Mar 21 '11 at 10:25
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@Konrad True -- Condorcet's theorem actually was about majority rule, though. That is, you get a probability boost just with majority rule. I gave the tighter case for a more obvious argument, but the majority rule case can be seen in the wikipedia article and refs. In the USA civil and criminal have different standards, a higher standard (usually unanimity in guilty beyond a reasonable doubt -- though jurors can be removed or relieved) being necessary in criminal cases because someone's freedom is at stake. –  Paul Mar 21 '11 at 10:36
    
One issue is that juror decisions aren't independent. They are based on discussion among the jurors, and they have an interest in coming to a decision fairly soon so they can stop being the jury. –  David Thornley Mar 21 '11 at 13:54
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Type 1 = Rejecting a true null hypothesis. Type 2 = Not rejecting a false null hypothesis. Under an "innocent until proven guilty" system, your list should be the other way around. –  dan04 Mar 22 '11 at 2:07
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"begs the question" - it does not mean what you think it means. –  Andrew Grimm Mar 22 '11 at 6:57
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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, ...?)

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