Why do some countries let ordinary citizens decide guilt?

The answer depends on the legal tradition. Where common law rules, an ordinary group of citizens, sworn in as a jury, hears the evidence, makes findings of fact and gives an impartial verdict. The jury system developed in England during the Middle Ages and is a hallmark of the English common law system. Most countries with civil law or sharia law systems do not use juries: there, guilt is typically determined by a single person, usually a professional judge, although jury use has been spreading.

The idea is old. Athens had jury courts, with records dating back to 500 BCE, whose juries voted by secret ballot. The modern jury trial evolved in the mid-twelfth century under Henry II, and the right to a jury trial has been part of English law since Magna Carta in 1215.

Even where juries exist, the details are local. The English tradition is twelve jurors; Scotland uses fifteen in criminal trials, and in 1970 the US Supreme Court called the twelve-person rule a "historical accident" and upheld six-person juries under state law.

Many civil law countries chose a middle path. In Germany and many other continental European countries, professional judges and lay judges deliberate together at both trial and sentencing. Japan reached a similar answer in 2009: lay judges now form the majority of the panel in certain criminal trials, taking part as judges alongside the professionals rather than as a separate jury.