Valenti, A. C., & Downing, L. L. (1975). Differential effects of jury size on verdicts following deliberation as a function of the apparent guilt of a defendant. Journal of Personality and Social Psychology, 32, 655-663.
Evidence strength interacted with jury size to affect jury verdicts. Larger juries deliberated longer and hung more often, especially when apparent guilt was high. Smaller juries convicted more often only when apparent guilt was high.
The United States Supreme Court ruled in the case of Williams vs Florida (1970) that a Florida statute providing for 6-member juries was constitutional. Allowing the relevance of empirical studies concerning differential effects of jury size, the Court concluded that existing evidence was insufficient to demonstrate that the verdicts of 6-member juries would be different from those of 12-member juries or would operate to the disadvantage of a defendant. Given the paucity of relevant data supportive of such differences, the Court decided in favor of allowing the smaller juries. The present study was designed to determine experimentally whether the 6-member jury was more or less advantageous to the defendant than a 12-member jury at each of 2 levels of apparent guilt of the defendant. Ss were 360 undergraduates. Consistent with predictions based upon logical considerations and upon small group research, jury size had no effect upon conviction when apparent guilt was low (only 2 out of 10 juries of each size reached a guilty verdict), but when apparent guilt was high, 6-member juries were substantially more likely to convict (9 out of 10) than were 12-member juries (2 out of 10; ( p < .001). http://www.researchgate.net/publication/232497986_Differential_effects_of_jury_size_on_verdicts_following_deliberation_as_a_function_of_the_apparent_guilt_of_a_defendant