Criminal law, that part of the law that defines criminal offenses, establishes procedures for trying accused persons, and fixes penalties for those convicted of criminal offenses. Many offenses once punished as crimes now come under the civil law. These offenses, which usually grow out of carelessness or accidents, are called torts. The penalty is likely to be the payment of damages to the party that has been injured. Generally, crimes are willful acts considered dangerous to society. The modern criminal law of England, the United States, and Canada originated in the English common law of crimes, which grew over hundreds of years from judicial decisions applied throughout the realm and therefore “common.” Legislation has supplemented these decisions, particularly in the past two centuries, when types of crime such as embezzlement that were almost unknown to the common law began to occur with frequency. When it achieved independence, the United States incorporated English common law into its legal systems but made it subject to the state and federal constitutions. Many states repealed the common law of crimes. In other states, offenders can be charged under both common law and statutory law.
The common law system has established some general principles of criminal law. One of these is that nothing can be treated as a crime unless there is a law against the conduct in question (in Latin, nulla crimen sine lege). A corollary of this is that no punishment can be imposed unless it is provided for by law (nulla poena sine lege). Another essential principle is that the crime must be accompanied by mens rea, a guilty mind. The criminal had to know what he or she was doing wrong. This principle has led to the difficult question of responsibility for criminal acts. Persons suffering from mental instability or disorder are not responsible for acts that, committed by others, would be crimes. But the legal criterion of mental responsibility, or competence, has long been a matter of controversy. In general, the British and U.S. courts have followed the so-called McNaghten (M'Naghten) Rule: Insanity is a valid defense if the accused was, at the time of the action, suffering from mental disease such that he or she did not know the nature of the act that he or she was committing, or did not know that it was wrong. Other grounds for diminished criminal responsibility include extreme youth and intoxication. There are also complicated questions of procedure involving arrests, confessions, searches, the right to have a lawyer, the right to confront witnesses, the right not to incriminate oneself, and other means of protecting the individual. Criminal law is not only a means of punishing wrongdoers; it is also a method of dealing with conflict, and as such reflects the weaknesses as well as the ideals of a society.
See also: Crime.