The Supreme Court Perspective
The US Supreme Court, in Williams v. Florida (399 U.S. 78, 1970), decided that the Constitutional guarantee of a jury trial did not require a twelve-member jury. The historical analysis seems in keeping with other 6th Amendment jurisprudence of the time. The Court, however, gave rather short shrift to whether it was a good idea to use such small juries. Their complete attention to the question is captured is the following passage from Justice White's opinion:
To be sure, the number should probably be large enough to promote group deliberation, free from outside attempts at intimidation, and to provide a fair possibility for obtaining a representatives cross-section of the
community. But we find little reason to think that these goals are in any meaningful sense less likely to be achieved when the jury numbers six, than when it numbers 12--particularly if the requirement of unanimity is retained. And, certainly the reliability of the jury as a factfinder hardly seems likely to be a function of its size.
So, the Court does identify some of the Constitutionally pertinent features of a jury trial affected by its size, but the Court gets all of its conjectures about jury behavior completely wrong.
A jury as small as six is much more likely to be dominated by a strong personality than is one with more members. Often, the people who try to control deliberations prove to be "false experts" and to have extreme, inflexible views. Such personality traits work contrary to open and complete deliberations, but they typically go unchecked in such a small group.
When juries handle difficult cases, the kind that reveal fundamental disagreement among the jurors, camps tend to form (hopefully not right away). Each camp usually has a de facto spokesperson and the ongoing debate becomes largely a dialogue between the spokespersons, with occasional input from others. In a six-person jury, it is much more likely that no one will want to assume the spokesperson role for one of the sides.
Jurors get facts wrong. These errors can lead deliberations in bad directions. Sometimes, a jury will reach a verdict that it would not have reached had it correctly recalled the evidence and testimony. We would, presumably, want to minimize the chance that a verdict would be based on an erroneous factual foundation. Studies have shown, without fail, that larger juries tend to raise a larger number of probative case facts during deliberation, allow factually mistaken statements to go uncorrected less often and have greater confidence in their verdicts than do smaller juries.
The Court speaks of "outside intimidation," by which is presumably means jury tampering and/or pre-trial publicity. Suppose, for a moment, that a defendant in a criminal trial had successfully bribed or intimidated a juror. The chance that the juror could convince all 11 other jurors to vote with him, contrary to the evidence, is pretty slim. On the other hand, if he could convince only a few others on a six-person jury, he could probably secure an acquittal (Empirical evidence shows a majority "tipping-point" for acquittals. If half the jury favors an acquittal, the rest typically falls in line quickly.) If a defendant is committed to getting to all the jurors on a jury, his job is certainly made easier by cutting the number in half. So, I would think that jury tampering is a bigger concern with smaller juries.
Trial courts typically do an awful job of dealing with the pretrial publicity problem. Jurors routinely fail to volunteer when they have seen, heard or read about a case in advance. When they do admit to being exposed to pretrial publicity, the judge typically asks them exactly the wrong question: "Notwithstanding what you have seen, can you remain fair and impartial in this trial?" The ones who say "yes" get seated. All the empirical studies say that jurors who admit that it might be difficult to ignore pretrial publicity tend to be fairer to defendants than those who swear that they can be impartial. So, it is a given that some jurors who are affected by pretrial publicity will make it onto the jury. Each such juror will be more influential on a six-person jury than on a larger one. Should it become clear that a juror is making arguments based on "extra-legal" material, another juror is more likely to cry "foul" when the jury is larger.
A similar dynamic is at play when it comes to unauthorized juror research, such as consulting dictionaries, googling witnesses, or visiting crime scenes. The influence of one juror who breaks the rules is larger on a smaller jury.
One need not be a statistical genius to realize that, in expectation, a larger random sample will be more representative of the general population than will be a smaller random sample. This problem is only exacerbated by the exercise of peremptory challenges. Such challenges, to the extent that the attorneys are successful at evaluating the tendencies of potential jurors, eliminate people whose views lie at either end of the distribution of opinion. In Dr. Tiller's case, for instance, each side had 5 peremptory challenges. So, of the 18 people who were deemed qualified to serve on the jury, the lawyers could strike more than half. To the extent that the population is at all heterogeneous with respect to attitudes pertinent to the case at hand, using six-person juries substantially hinders the likelihood of obtaining a representative jury.
The Constitution is silent about the identifying the dimensions on which we'd like juries to be representative. If we would like a good distribution of talents and life experiences, it will be hard to achieve this goal with a six-person jury.
Dr. Tiller's case has incited strong passions in the Wichita community. It remains to be seen how the use of a six-person jury will affect the ability of such passions to influence the verdict.