Based on Research by Michael J. Saks. Psychology Science Minute written by American Psychological Association, adapted by Juanita N. Baker, Ph.D.
When it comes to group decision-making, does jury size matter? Psychologist and law professor Michael Saks tested this question. Research participants placed in group sizes of six or twelve viewed a videotaped trial, and then reached a verdict.
Members of smaller groups participated more equally, and rated the deliberations more satisfying and their groups more cohesive. Although larger groups were more contentious and debated more vigorously, they recalled more evidence from the trial and made more consistent and predictable decisions. Smaller juries will likely make more errors; so in criminal cases they more likely than larger juries would acquit the guilty and convict the innocent, while in civil cases, they may render damage awards that are more unpredictable.
Larger groups increase the likelihood that a dissenter will have an ally and thus be better able to resist yielding to group pressure. Additional studies concluded that larger juries are more likely to contain racial minority members, more accurately recall trial testimony, and spend more time deliberating. They are less able to reach a decision, but reach fewer erroneous decisions.
According to psychological research, a wise choice for the judicial system is to use larger juries that reach fairer decisions, although they may cost more.
For more details see:
American Psychological Association, May 28, 2004
Saks, M.J. (1977). Jury verdicts: The role of group size and social decision rule. Lexington, MA: D.C. Heath.
Saks, M.J. & Marti, M.W. (1997). A meta-analysis of the effects of jury size. Law & Human Behavior, Vol. 21 pp. 451-467.
The empirical studies are cited in Saks & Marti (above) and many of them