A Durham rule, product test, or product defect rule is a rule in a criminal case by which a jury may determine a defendant is not guilty by reason of insanity because a criminal act was the product of a mental disease. Examples in which such rules were articulated in common law include State v. Pike (1869) and Durham v. United States (1954). In Pike, the court wrote, "An accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect."