A unanimous verdict of twelve makes it more probable than not that there will be at least one juror who does not think the law makes the alleged act an offense if there is not at least a 94% level of support in the community for acts of that kind being offenses.The key idea is that the jury system is not based on the assumption that the public from whom the jury is selected will be uniformly wise and skilled at jury duty. If it were, there would be no need for a jury of more than one.
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.
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. 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.
Consider the formula
nj = rwhere n is the proportion of the population from which the jury is drawn who don't know or care how to be good jurors, j is the number of jurors in a jury, and r is the conviction rate.
Let's look at an array of several values of n, j, and r:
If the values of n can be taken as the proportion of the jury pool who does not take its jury duty seriously, then 1 - n are the proportion of those who do. That means for the jury system to render justice, for a jury of 6 we need to educate at least 11% (1 - .89) of the population to be good jurors, whereas for a jury of 12, we only have to educate 6% (1 -.94). In other words, the number of jurors required to make a jury can make all the difference between whether our efforts at public education are feasible. Reaching 11% is not just twice as difficult as 6%. It is likely to be 100 times as difficult. 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.
The remaining challenge, then, is to prevent voir dire from being conducted in a way that strikes the few good jurors. To the extent the prosecution can identify them, the number of strikes it gets is sufficient to get juries that would convict a ham sandwich.
Going further back historically, the model for the English grand jury that developed in the 12th and 13th centuries was the Hebrew סַנְהֶדְרִין sanhedrin, which was itself modeled during the time of Greek domination on the Greek συνέδριον, synedrion, "sitting together," hence "assembly" or "council". It consisted of twenty-three men appointed from every major city in the land of Israel. The Mishnah arrived at the number twenty-three based on an exegetical derivation: It must be possible for a "community" to vote for both conviction and exoneration (Numbers 35:24-5). The minimum size of a "community" is 10 men (Numbers 14:27). One more is required to achieve a majority (11–10), but a simple majority cannot convict (Exodus 23:2), and so an additional judge is required (12–10). Finally, a court should not have an even number of judges to prevent deadlocks; thus 23, which became the number for a grand jury. The English reduced this to a petit (small) jury of a minimum of 12 that had to swear they believed the defendant's oath, in a system called compurgation, and later to render a verdict (from Latin veredictum, "to say the truth"). That led to the requirement for a 12-person trial jury that had to be unanimous, at least in criminal cases.