During the nineteenth century American juries came to recognize an unwritten law that an outraged husband, father, or brother could justifiably kill the alleged libertine who had been sexually intimate with the defendant's wife, daughter, or sister. While scholars have occasionally noted this custom, they have not discussed it in depth. This essay will do that, emphasizing especially the origins, sociology and realities ofthe unwritten law and the reasons for its gradual demise. For the most part the unwritten law applied to two types of sexual acts, adultery or sexual relations between a wife and a third party male and seduction1 or sexual relations between an female and a male. Throughout much of history governments have severely punished adulterers and seducers, often executing them usually after a judicial determination of guilt, but England, whose traditions most significantly influenced the origins of American law, at least officially maintained a more tolerant attitude towards such malefactors. In the twelfth century the royal government transferred jurisdiction over adultery to the ecclesiastical courts which did not punish the crime that severely. In 1557 Parliament enacted a statute punishing seduction, defined as the unlawful taking away of a maid or woman^child unmarried from her parents or guardians, but specified only the relatively mild penalty of a fine or from two to five years imprisonment. Such permissiveness encouraged some outraged husbands to exercise private vengeance against adulterers, often killing them. Common law juries sometimes acquitted these avengers and in doing so ignored the English law of provocation that offered partial relief to vengeful husbands by reducing the crime from murder to manslaughter if they had killed immediately after discov? ering the libertine in bed with the wife.2 The private killing of libertines appears not to have occurred in significant numbers in colonial America. At least if such killings did occur they did not become a part of the memory of American law that began to be written down in the late eighteenth and early nineteenth centuries, which recorded memory would be cited by defense lawyers in the unwritten law trials that followed. The presumed paucity of such events may have been due to the existence of colonial statutes that made adultery a crime, and, more importantly, the establishment of an intrusive society that effectively dealt with adultery and seduction and to some extent tolerated it. Although most colonies freely prosecuted alleged perpetrators of sexual immorality, the punishments administered to those who were convicted were usually lenient. The absence of privacy, the vigilance of communities, and the actions of church groups enabled colonial Americans to deal effectively with adultery and seduction without frequent recourse to severe criminal sanctions.