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In law, desuetude (from the Latin desuetudo, outdated, no longer custom) is a doctrine that causes statutes, similar legislation or legal principles to lapse and become unenforceable by a long habit of non-enforcement or lapse of time. It is what happens to laws that are not repealed when they become obsolete. It is the legal doctrine that long and continued non-use of a law renders it invalid, at least in the sense that courts will no longer tolerate punishing its transgressors.
The policy of inserting sunset clauses into a constitution or charter of rights (as in Canada since 1982) or into regulations and other delegated/subordinate legislation made under an Act (as in Australia since the early 1990s) can be regarded as a statutory codification of the common-law doctrine.
British law 
The doctrine of desuetude is not favoured in the common law tradition. In 1818, the English court of King's Bench held in the case of Ashford v Thornton that trial by combat remained available at a defendant's option in a case where it was available under the common law. The concept of desuetude has more currency in the civil law tradition, which is more regulated by legislative codes, and less bound by precedent.
The doctrine has been applied in regard to acts of the pre-1707 Scottish Parliament. Hundreds of Acts dating back to the Middle Ages have also been amended or abolished in current and former British territories by numerous Statute Law Revision Acts from falling into desuetude.
United States law 
Desuetude does not apply to violations of the United States Constitution. In Walz v. Tax Commission of the City of New York, 397 U.S. 664, 678 (1970), the United States Supreme Court asserted that: "It is obviously correct that no one acquires a vested or protected right in violation of the Constitution by long use, even when that span of time covers our entire national existence and indeed predates it."
It may, however, have validity as a doctrine in defense of penal prosecution. In 1825, the Pennsylvania Supreme Court declined to enforce the traditional punishment of ducking for women convicted as common scolds, stating that "total disuse of any civil institution for ages past, may afford just and rational objections against disrespected and superannuated ordinances." Wright v. Crane, 13 Serg. & Rawle 220, 228 (Pa. 1825).
The seminal modern case under U.S. state law is a West Virginia opinion regarding desuetude, Committee on Legal Ethics v. Printz, 187 W.Va. 182, 416 S.E.2d 720 (1992). In that case, the West Virginia Supreme Court of Appeals held that penal statutes may become void under the doctrine of desuetude if:
- The statute proscribes only acts that are malum prohibitum and not malum in se;
- There has been open, notorious and pervasive violation of the statute for a long period; and
- There has been a conspicuous policy of nonenforcement of the statute.
This holding was reaffirmed in 2003 in State ex rel. Canterbury v. Blake, 584 S.E.2d 512 (W. Va. 2003).
While it may not be a violation of due process to enforce a desuetudinal law, the fact that a law has long gone unenforced may present a bar to standing in a suit to prevent its future enforcement. In Poe v. Ullman, the Supreme Court refused to hear a challenge to Connecticut's ban on birth control, writing:
The undeviating policy of nullification by Connecticut of its anti-contraceptive laws throughout all the long years that they have been on the statute books bespeaks more than prosecutorial paralysis . . . . 'Deeply embedded traditional ways of carrying out state policy * * * '—or not carrying it out—'are often tougher and truer law than the dead words of the written text.'
Shortly thereafter, Connecticut's birth control law was enforced, and struck down, in Griswold v. Connecticut.