In a series of opinions in the 1970s, the US Supreme Court concluded that juries smaller than 12 persons would be constitutional if they performed no differently than traditional 12-person juries. This study includes a meta-analysis of the effects of jury size on the criteria the court specified as the basis for making such comparisons. A search for all relevant empirical studies identified 17 that examined differences between 6- and 12-member juries. The total sample for the 17 studies was 2,061 juries involving some 15,000 individual jurors. Findings disclosed that large juries appear to be more likely to contain members of minority groups, deliberate longer, hang more often, and possibly recall trial testimony more accurately. Having framed the issue of jury functioning to call for an empirical inquiry, the Supreme Court reached conclusions that are not supported by the data. A careful examination of the relevant studies finds significant differences in jury behavior as a function of jury size. In light of these data and the judgments of other authorities, the Supreme Court might profitably revisit this issue. Notes, tables, references