The early colonists brought with them the common law of England and its statutes, both civil and criminal. Thus, most states had common law crimes. A number of states enacted comprehensive statutory criminal codes in the nineteenth century. In most states common law crimes were displaced by specific statutory declaration; in others, the common law was preserved, but today only legislators can create new crimes.
Common law crimes have some strengths. As Viscount Simonds observed, they ensure that the criminal law is always available to punish harmful conduct even if the legislature failed to anticipate its occurrence by enacting an applicable criminal statute. They also discourage the imaginative exploitation of loopholes in the criminal laws. Common law crimes provide flexibility, which permits adjustment to new and unanticipated situations. In the arena of drug crimes, for example, “designer drugs” are created so quickly that Congress (or state legislatures) cannot promptly incorporate their names (or chemical compositions) in statutes prohibiting drug possession (or manufacture). Consequently, federal statutes now allow the Attorney General, working with the Department of Health, to add a newly created drug to the list, for a limited period of time; the listing is valid only for a period sufficient to allow Congress to decide whether to amend the statutorily designated list of drugs.
Common law crimes, however, also have serious weaknesses. First, unless there is a clear precedential case available, an individual could not know beforehand if her contemplated conduct is lawful or criminal. Only when a court decides after the fact, using analogies or cases from other jurisdictions, would a defendant learn whether she had committed a crime. Even someone trying to obey the law must act at her own peril as the defendant in Shaw unhappily learned. Faced with such uncertainty, many individuals may play it safe and avoid engaging in conduct that would not be declared criminal.
Second, under a common law system the limits on governmental authority are not clear. The criminal law is a restriction on individual liberty, but it is also a restriction on governmental authority. Unless the law draws a clear boundary between permissible and impermissible behavior, the government can more easily use the awesome power of the criminal law to convict and incarcerate individuals it considers its enemies for behavior that may have actually been innocent.
The absence of a clear set of rules embodied in criminal statutes thus creates uncertainty in predicting the future. It also weakens the moral justifications for conviction and punishment and diminishes the restraints on government.