Definition
A decoy letter is a letter or package prepared and mailed by postal inspectors or law enforcement agents for the purpose of detecting and apprehending persons who commit crimes against the postal or revenue laws — most commonly, theft from the mail. The letter is designed to appear valuable or enticing to a would-be thief; when a postal employee or other person unlawfully takes or tampers with it, the act itself constitutes the offense and supplies the evidence needed for prosecution.
The technique is a form of controlled entrapment-adjacent investigation: the government creates the bait, monitors its handling, and catches the offender in the act. Historically, decoy letters were a standard tool of postal inspectors combating mail depredation, and courts sustained their use as a legitimate investigative method rather than improper inducement.
Common Language
Modern common usage (Wiktionary): "Decoy" in ordinary use refers to a lure or imitation used to attract animals or people — a duck decoy, for example, or a person used to distract attention. A "decoy letter" in everyday speech might loosely describe any misleading or diversionary correspondence.
Historical common usage (Webster's 1913): Webster's defines "decoy" as "a place into which wild fowl, etc., may be enticed and caught" or more broadly "anything intended to lead into a snare." A "decoy letter" does not appear as a defined term in Webster's 1913, though the component meaning of luring into a trap is fully consistent.
The legal meaning is specific and technical: a decoy letter is not merely misleading correspondence but a government-prepared instrument of criminal investigation, deployed within the postal system under legal authority. The common usage captures the general idea of a lure, but the legal term carries with it questions of institutional authority, chain of custody, and the limits on government-created inducement that have no counterpart in ordinary usage.
Common Confusion
Decoy letters are sometimes confused with entrapment in the broader constitutional sense. The distinction matters: courts have consistently held that placing a decoy letter in the mail does not constitute entrapment where the defendant independently decides to steal it. The government creates the opportunity; it does not implant the criminal intent. Researchers should not assume that cases discussing decoy letters will also address the entrapment defense in depth — many do not, because courts resolved the issue on the narrower ground that postal inspection is a lawful investigative function.
Why It Matters in Research
This term is functionally a historical one. Decoy letters were a creature of nineteenth- and early twentieth-century postal law enforcement, and the term appears most densely in federal case law and treatises from that era. Researchers will encounter it primarily in:
— Older federal district and circuit opinions involving postal theft, mail fraud, and revenue fraud, often reported in the Federal Reporter and early Dillon's Reports (the citation "5 Dill. 39" shared by both Black's and Bouvier's points to Dillon's Circuit Court Reports, a pre-West reporter series).
— Historical postal regulations and inspector manuals that governed the preparation and use of decoy mailings.
— Early entrapment jurisprudence, where decoy letter cases sometimes appear as predecessors to the modern entrapment doctrine.
Researchers should note that modern postal inspection practices have evolved considerably. The term "decoy letter" as such rarely appears in contemporary federal statutes or recent case law; analogous techniques are now governed by broader frameworks covering undercover operations and sting investigations. Searching historical databases (pre-1930 federal reporters, early Attorney General opinions) will yield the richest results. Boolean searches pairing "decoy" with "mail" or "postal inspector" will surface relevant material that does not use the compound term.
The truncated Bouvier's entry — cutting off mid-sentence on the question of a postal employee taking a package of value — suggests the full passage addressed the sufficiency of the act as proof of criminal intent. Researchers relying on Bouvier's should locate the complete underlying case text rather than treating the dictionary summary as a complete statement of the rule.
Historical Dictionary Support
Black's and Bouvier's are in full agreement on the core definition and share the same primary citation (5 Dill. 39), suggesting both drew from a common source or that the Dillon opinion was the defining authority for the term. Bouvier's adds the additional citation (40 Fed. Rep. 752) and extends the definition to address the legal justification for the technique explicitly — affirming that use by mail inspectors is "proper and justifiable." This is a meaningful addition: Black's states what a decoy letter is; Bouvier's also answers whether it is permissible.
Neither dictionary addresses what would become more pressing in later decades: the constitutional dimensions of government-created investigative scenarios under the Fourth and Fifth Amendments, or the scope of the entrapment defense as developed in twentieth-century Supreme Court jurisprudence. The historical dictionaries treat decoy letters as an uncontroversial enforcement tool, which was largely accurate for their era but understates the doctrinal complexity that developed later.
Jurisdictional Note
Decoy letter doctrine is overwhelmingly federal, rooted in the postal power and federal mail fraud statutes. State law analogs exist where state postal or courier systems were involved, but these are rare. Researchers focused on state materials are unlikely to find the term used with any precision outside of federal-law commentary.
Encyclopedia Cross-Reference
No Law Mind Encyclopedia entry addresses decoy letters directly. The matching entries provided (the Privileges or Immunities Clause, the IRS 90-Day Letter, and Preliminary Negotiations in Contracts) have no meaningful connection to this term. Cross-reference omitted.