I recently completed some weeks as a member of the jury in a criminal trial. It was a strange experience. Without wishing to comment on the case itself (at least, not in a way that identifies any of the people involved), I want to talk about it. Jury trials are a signature of our democratic way of life — that is a commonplace opinion in Australia, Canada, the United States and other countries with a British imperial heritage. But juries are rare or non-existent in many other democracies, and the rationale for their existence is something more believed than proven. Indeed, in a justice system that is emphatic about the value of evidence, the evidence for the effectiveness of juries is remarkably thin. Because the privacy of jury members is protected and juries do not give reasons for their decisions, gathering evidence about their effectiveness is difficult. At the heart of our criminal justice system, we have instituted a mystery.