Why do juries have 12 people?

Because, after many centuries of common law in England,  that’s the number that’s been found to give reasonably fair results in most cases.  But that’s begging the question !  Why 12?

Because that’s the average number of mature adults in a typical hunter-gatherer tribe.  When this number comes to more 12 — that is, when the tribe is around 120 total individuals or more — the dissention inevitable occurs and the tribes invariably splits into two around an existing leader and a forceful potential new one.

A group of about five or six individuals is probably the maximum size group where a totally unified decision can be quickly arrived it.  Irrespective of whether all such decisions are good or bad, this is roughly the size of group that can takes firm decisions as in boards of directors of large firms or government cabinets.

Back to juries, a group of 12 is probably about the best number in which a dissenting voice is encouraged to speak up and gather supporters if the verdict was too easily going one way too soon —  but yet not cause deep problems.

2 thoughts on “Why do juries have 12 people?

  1. Hi Keith,

    Hope you’re well.

    In my view, the 12 jury members would be more efficient if they were made up of 3 groups of 4 rather than one group of 12. The reason being, you want the conformity levels to be at a minimum – and there is a lot more conformity with one large group. In a group of 12 you’ll almost always find a few more prominent members are able to influence the less prominent and less confident members. In three groups of 4 this would diminish greatly, and each member would by and large be less pliable and more confident and competent in his or her involvement. So, if you’re innocent, on trial, and desperate that the decisions made by the jury are consistent with the true facts, you’d be much better off with three groups of 4 than one group of 12.

    The other thing is, I think it is unbelievably inefficient to shield the jury members from knowledge of relevant information, because it impairs their ability to judge the case with optimum effectiveness. This issue should be redressed immediately – it is the biggest thing wrong with jurisprudence. What I find ironic is that the argument against giving the jury all the background information (that such knowledge will potentially bias the jury) is precisely the reason they should be doing it, because any reasonable person should want all relevant information to be brought to bear in the courtroom. At the very least, jurors should have the chance to decide whether this background information is relevant or not.

    If a man has been accused of threatening a neighbour with a shotgun, I’d want the jury to know whether he has a history of similar behaviour, what sort of person he is, what sort of record his lawyer has, and information of that kind. The concept of shielding inquirers from information is alien to every other formal evidence-based system; the work of scientists, political groups, police officers and building surveyors would suffer immeasurably if they had part of their investigative data withheld from them, so why on earth should we do it in a court of law when justice and people’s futures are at stake? At election time in politics we want the electorate to be as well informed as possible; in the biology lab we want the researchers to be apprised of as much information as possible. It is truly unsatisfactory to expect (and wish for) members of the jury trying to get justice in the courts to remain ignorant of the important details, when many of those details are so relevant to the probability of the defendant’s guilt or innocence..

    The compromised admissibility of evidence hinders in every way – and with some irony, the reason people give in support of it is an inversion of understanding the very thing we are trying to enhance – decisions based on explicit and accurate information. Just like in science and police investigations, the information being omitted is important in building a clear probability perspective of the situation. A man on trial accused of threatening behaviour with a shotgun is much more likely to be guilty if he has previously threatened 3 other people with a shot gun. Yet the courts would rather you didn’t know this if you’re on that jury – which means the courts must favour a serial shotgun offender having a greater chance of being acquitted. Apparently if you’re the sort of person who thinks this way it is frowned upon by the courts, because they don’t want juries to come into the courtroom with any biases. I presume the courts must think that the police arrest civilians in a completely random fashion.



    1. James, I agree with your second point, but have reservation on you first one. The smaller the group the more likely it is that it will come to a firm decision all the sooner under a court-appointed foreman or a forceful natural leader. If a case being tried is not open-and-shut then you are more likely to have one 4-person group disagreeing with the other two. This raises dissention to a higher level — given how we bond ourselves so readily to whatever group we find ourselves in. Keith

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