Jury

[porth]

Why 12 on a jury?

[Whickerman]

from Wikipedia

Trial jury size
Historical analysis indicates twelve was not just a "magic number", but a number arrived at through experience trying other sizes. The use of the number 15 for Scottish juries appears to be a legacy of that experimentation. And the ancient Athenians tried juries of 201 and sometimes 401.

Twentieth century "law office history" seems to hold that the size of the jury is to provide a "cross-section" of the public. In Williams v. Florida,399 U.S. 78 (1970) the U.S. Supreme court sustained a state jury of less than twelve persons, finding that six was sufficient to provide a "cross-section". But from the experience of lawyers in jurisdictions where they have a choice of jury size, defendants usually choose twelve, which suggests a different dynamic, and that a jury of twelve provides a higher level of protection of rights, not just representation of diverse views.

[Whickerman]

...cont'd

Jury systems were established because the public could not trust the presiding officers of courts, the "bench", any more than they could trust the parties and their lawyers. Juries were thought to be less susceptible to being corrupted because they are assembled at random and serve for too brief a period of time to be easily compromised. On the other hand, juries composed of randomly selected citizens were also not as expert in deciding legal questions, and if they are not called to jury duty often enough to gain experience, the system has to rely on the attributes they bring with them from their ordinary lives. Justice systems tend to hold visions of things like a "reasonable man", an "ideal judge", or an "ideal juror". If any random selection of jurors drew nothing but ideal jurors, willing and able to exercise sound judgment and perceive the attempts to deceive them, so that they never render a verdict that would be a miscarriage of justice, then there would be little controversy about either using juries to bring verdicts, or the size of them.

[Whickerman]

...cont'd
The system does not and can not depend on every citizen in the jury pool being ideal jurors, but it can work if a sufficiently large proportion of that pool are "adequate" in their judicial attributes, sufficiently resistant to miscarriages of justice, and the jury size is large enough to make it sufficiently likely that at least one such adequate juror will be selected, and, in a criminal case, the verdict is required to be unanimous.
It can be seen, mathematically, that if the prosecution wants at least a 50% chance or conviction to prosecute a case, then for a jury size of 12 no more than about 6% of the population can hold that the standard of proof has not been met or that the offense is not really a "crime". This can be seen by trying various values in the equation nj = r, where n is the proportion of the population from which the jury is drawn who are not "adequate" jurors, j is the jury size, and r is the conviction rate. Then 1- n is the proportion of "adIf one of the objectives of civic education is to train citizens to be "adequate" jurors, than that education would need to produce them at a rate that they would comprise at least 6% of the population if the jury size were 12, but at least 11% of the population for a jury size of 6. If civic education is not sufficiently productive of adequate jurors, then the remedy might be to increase the size of juries, perhaps to a number well beyond 12.

The historical background for the 12-person jury lies in English common law, where crimes were not in general defined by statutes, but by custom and tradition. Therefore, a jury was not just deciding whether the accused actually did the deed, but whether the deed itself was a crime. Without doing a mathematical analysis, their experience would tend toward a system in which there was at least 94% community support for a deed of a certain kind being a crime, if only to avoid public protests from those who did not support that.