Why is a suspect treated as innocent until proven guilty?

A courtroom begins from a rule that looks generous but is really a piece of design: the accused is treated as innocent, and the state must prove otherwise. The principle says that anyone accused of a crime is considered innocent until proven guilty, which puts the burden of proof on the prosecution. If the prosecution cannot prove the charges, the accused is acquitted, and in most cases the standard is proof beyond a reasonable doubt, a higher bar than the balance of probabilities commonly used in civil cases.

History shows what came before. Medieval trials by ordeal were common from the sixth century to the early thirteenth, and survived into the seventeenth in witch-hunts. A feudal accused could clear himself by having twelve people swear to his innocence, which in practice favoured nobles over poorer people, whose witnesses were less likely to be believed.

The idea behind the presumption is old, too. The sixth-century Digest of Justinian records a Roman rule of evidence: proof lies on the one who asserts, not the one who denies. Many civil law countries, including Brazil, China, France, Italy and Spain, adopted that maxim. The English phrase "presumed innocent until proven guilty" is often credited to the barrister Sir William Garrow, who in a 1791 Old Bailey trial insisted that accusers be tested robustly.

Today the principle is written into article 11 of the Universal Declaration of Human Rights. It is a common belief that French law presumes the accused guilty; that is a popular misconception. Even the United States, whose Constitution never states the rule outright, is widely held to derive it from the Fifth, Sixth and Fourteenth Amendments.