Fair Warning

    The United States Supreme Court has invoked the principle of fair warning to place various constitutional limitations upon the making and interpretation of penal law.  These limitations include the legislative principles of prospectivity and specificity, as well as the interpretive principle of lenity.

    The prospectivity requirement derives from the prohibition of "ex post facto" legislation in the U.S. Constitution.  The principles of specificity and lenity have been derived from the constitutional guarantee of due process.

    The principle of fair warning also has been invoked in support of another, related, principle:  the principle of legislativity.  The legislativity principle restricts the power to make penal law to the legislature, as opposed to the judiciary or the executive.  Crimes defined by the judiciary are called common law crimes.  By contrast, crimes defined by the legislature are called statutory crimes.  Although the Supreme Court has interpreted the U.S. Constitution to prohibit the creation of federal common law crimes, the U.S. Constitution has not been found to prohibit the creation of state common law crimes.

    The principles of prospectivity, specificity, lenity, and legislativity are also jointly--and occasionally separately--referred to as the principle of nulla poena sine lege or the principle of legality.