Traditionally, in the United States, a criminal trial jury—the petit jury—has been composed of twelve persons. Early Supreme Court opinions assumed that in federal criminal cases juries of that size were required by the Constitution. In patton v. united states (1930) the Court ruled that during the course of a federal trial a criminal defendant could, with the consent of the prosecutor and judge, waive the participation of one or two jurors and agree to have the verdict rendered by less than twelve.
In duncan v. louisiana (1968) the Supreme Court held that under the fourteenth amendment a person accused of a serious crime in a state court is guaranteed the right to trial by jury according to the same standards applied under the Sixth Amendment in the federal courts. Later, in baldwin v. new york (1970), the Court held that a serious, nonpetty crime for purposes of the jury trial guarantee is one where imprisonment for more than six months is authorized. In the wake of Duncan, the Court in williams v. florida (1970) decided that trial of a serious crime by a jury of six persons did not violate the constitutional right to trial by jury. Eight years later, the Court in ballew v. georgia (1978) ruled that six was the constitutional minimum—that a jury of five persons did not meet the constitutional standard. In Colgrove v. Battin (1973) the Court had also ruled that a six-person jury in a civil case in the federal courts did not violate the seventh amendment right to jury trial.
In early England, the number of jurors on a petit jury came to be firmly fixed at twelve some time in the fourteenth century. The reasons for choosing the number twelve for the jury at common law are shrouded in obscurity; the same number was also in wide use in other countries of Europe from early times. Some writers ascribe this number to mystical and religious considerations, for example, the twelve tribes and the twelve apostles. At the time of the adoption of the Constitution and the bill of rights, the idea of the twelve-person jury was entrenched in the English common law system and practice of the colonial society.
In Williams, the Court rejected the idea that the history of the drafting of the Sixth Amendment jury trial provision enshrined the twelve-person jury in the Constitution. Instead, the Court adopted a functional approach, relating jury size to the purposes of jury trial. The goals of the jury system were seen as interposing the common-sense judgment of laypersons, permitting community participation in the decision-making process, and making the group large enough to promote group deliberation and obtain a fair cross-section of the community. With respect to these various goals, the court majority found "little reason to think" that there is a significant difference between six and twelve, citing in support "the few experiments" and asserting that neither currently available evidence nor theory suggested contrary conclusions.
The interval between Williams and Ballew saw the publication of a significant body of social science research examining the effects of changes in jury size. In Ballew, although the Court was unanimous on the jury size issue, only two Justices relied on these social science studies in concluding that five-person juries did not adequately fulfill the functions of jury trial outlined in Williams. Three Justices had "reservations as to the wisdom—as well as the necessity—of … heavy reliance on numerology derived from statistical studies." The same three Justices suggested that the Constitution does not require every feature of the jury to be the same in both federal and state courts, implying that a different, presumably higher, minimum size standard might be applied in the federal courts.
The studies done since Williams, through experiment, use of statistical analysis, and theorizing, have inquired whether the size of the jury affects: the likelihood of representation on a jury of ethnic and racial minorities and minority viewpoints that might influence results or the incidence of hung juries; the propensity of juries to reach compromise verdicts; the consistency of verdicts; the likelihood that verdicts reflect community sentiment; and the overall quality of group decision making. A few researchers have also studied the cost savings that might be achieved by reductions in jury size.
In the main, the social scientists have criticized the Court's conclusion in Williams, and have argued that decreasing jury size has undesirable effects. Some of these studies have been subjected to methodological criticism, such as the objections to their reliance on small group research. Definitive research on the subject remains to be done. On the issue of the jury's representative character, however, social science has already contributed fairly definitive conclusions. Although it is not possible for a single jury to be representative of the community, six-person juries are less likely than twelve-person juries to contain individuals from minority groups or those who have minority viewpoints. Richard Lempert has suggested that "there may be a positive value in minimizing the number of situations in which minority group members are judged by groups lacking minority representation.…"
In other constitutional contexts, judges often rely on intuition and common sense to reach judgments on functional issues, or they take into account constitutional values that transcend a functional approach. The jury size issue, however, involves specific numbers, and intuition and other constitutional values do not provide an adequate basis for drawing the required fine distinctions. One who is not persuaded by the social science studies is therefore relegated to the type of statement made by Justice Powell in Ballew, defending the line between five and six: "[A] line has to be drawn somewhere." Under such an approach, the constitutional line could as easily have been drawn between twelve and eleven, and with more historical justification.
Because of the Court's reluctance to overrule recent precedents and because of uncertainty whether social science research can ever demonstrate a sufficient basis for drawing a different line, it seems probable that, for a long time to come, six will remain the constitutional minimum for a criminal jury in the state courts under the Fourteenth Amendment. (Whether the Court will some day adopt Justice Powell's view and apply a different minimum size standard for juries in federal criminal trials is problematic.) Perhaps in some future century when legal historians try to deduce the reasons for choosing six as the constitutionally significant number, they, like their predecessors, may speculate about the possible mystical value of the number. In the end, they are likely to conclude that its origins, like those of the number twelve, are shrouded in obscurity.
Lempert, Richard 1975 Uncovering "Nondiscernible Differences: Empirical Research and the Jury Size Cases. Michigan Law Review 73:644–708.