The word entrapment, from the verb "to entrap," meaning to catch in a trap, was first used in this sense in 1899 in the United States Federal Court case of People v Braisted.
The 1828 edition of Noah Webster's American Dictionary of the English Language defines entrap as:
The Supreme Court of Canada developed the doctrine of entrapment in three major decisions: R. v. Amato,  2 S.C.R. 418, R. v. Mack,  2 S.C.R. 903, and R. v. Barnes,  1 S.C.R. 449. There are two different forms of entrapment in Canadian law.
The first type of entrapment, "random virtue testing", occurs when the police offer an individual the opportunity to commit a crime without reasonable suspicion that either that individual or where that individual is located is associated with the criminal activity under investigation. If police have such a reasonable suspicion, they are still limited to providing only an opportunity to commit the offence.
The second form of entrapment occurs when the police go beyond merely providing an opportunity to commit an offence but actually induce the commission of the offense. Some factors a court may consider when deciding whether police have induced the offence include the type of crime being investigated, whether an average person would have been induced, the persistence and number of attempts made by the police, the type of inducement used (fraud, deceit, reward, ...), and the existence of expressed or implied threats.
The question of entrapment is considered only after there has been a finding of guilt. If, after finding the accused guilty, the court determines that the accused was entrapped, the court enters a judicial stay of proceedings. That is similar to an acquittal.
In German law, it is normally forbidden (§ 26 StGB) to induce or persuade someone to commit a crime or to attempt to do so (§ 30 StGB, in German). However, the German Federal Court of Justice has held that entrapment by undercover police agents is not a reason to stay the case per se (e.g. GA 1975, 333, 334). If undercover agents have been used without proper justification, punishment for the committed offense may be reduced (1st Senate's decision in 1 StR 148/84 - 23 May 1984).
In the case of persons who are not initially under suspicion and unlikely to commit a certain crime, a decision from 1999 (18th of November, BGH 1 StR 221/99, in German) stated that entrapment of such persons violates the right to a fair trial, and the punishment for the committed offence may thus be reduced.
The main authority on entrapment in the United Kingdom is the decision of the House of Lords in R. v. Loosely; Attorney-General's Reference (n.3 of 2000). A grant of a stay is awarded if the conduct of the state was so seriously improper that the administration of justice was brought into disrepute. In deciding whether to grant a stay, the Court will consider, as a useful guide, whether the police did more than present the defendant with an unexceptional opportunity to commit a crime.
In Loosely, Lords Hoffman and Hutton indicated certain factors that should be considered in deciding whether proceedings against a defendant should be stayed:Whether the police acted in good faith;
Whether the police had good reason to suspect the accused of criminal activities;
Whether the police suspected that crime was particularly prevalent in the area in which the investigation took place (Williams v. DPP);
Whether pro-active investigatory techniques were necessary because of the secrecy and difficulty of detection of the criminal activity in question;
The defendant's circumstances and vulnerability; and
The nature of the offence.
It has been held that it is generally acceptable for the police to conduct test purchases (DPP v. Marshall) or pose as passengers to catch unlicensed taxi drivers (Nottingham City Council v. Amin).
Entrapment arises when a person is encouraged by someone in some official capacity to commit a crime. If entrapment occurred, then some prosecution evidence may be excluded as being unfair, or the proceedings may be discontinued altogether.
In Scotland the main authority is the case of Brown v. HMA 2002 which stated that entrapment will occur when law enforcement officials cause an offense to be committed which would not have occurred had it not been for their involvement. The remedies available correspond with those in England and are considered to be either a plea in bar of trial or a challenge to the admissibility of evidence obtained through entrapment.
Entrapment defences in the United States, have evolved, mainly through case law. Two competing tests exist for determining whether entrapment has taken place, known as the "subjective" and "objective" tests. The "subjective" test looks at the defendant's state of mind; entrapment can be claimed if the defendant had no "predisposition" to commit the crime. The "objective" test looks instead at the government's conduct; entrapment occurs when the actions of government officers would usually have caused a normally law-abiding person to commit a crime.
Courts took a dim view of the defence at first. "[It] has never availed to shield crime or give indemnity to the culprit, and it is safe to say that under any code of civilized, not to say Christian, ethics, it never will" a New York Supreme Court said in 1864. Forty years later, another judge in that state would affirm that rejection, arguing "[courts] should not hesitate to punish the crime actually committed by the defendant" when rejecting entrapment claimed in a grand larceny case.
Other states, however, had already begun reversing convictions on entrapment grounds. Federal courts recognized entrapment as a defence starting with Woo Wai v. United States, 223 F.1d 412 (9th Cir. 1915). The U.S. Supreme Court declined to consider the question of entrapment in Casey v. United States, since the facts in the case were too vague to definitively rule on the question; but, four years later, it did. In Sorrells v. United States, SCOTUS unanimously reversed the conviction of a North Carolina factory worker who gave in to an undercover Prohibition officer's repeated entreaties to get him some liquor. It identified the controlling question as "whether the defendant is a person otherwise innocent whom the government is seeking to punish for an alleged offense which is the product of the creative activity of its own officials".
In Sherman v. United States, the Court considered a similar case in which one recovering drug addict working with agents of the Federal Bureau of Narcotics (a predecessor agency to today's Drug Enforcement Administration (DEA) ) solicited another to sell him drugs on the premise that his own efforts were failing. Again unanimous, its opinion focused more clearly on the defendant's predisposition to commit the offense and, on that basis, overturned Sherman's conviction as well since, although he had two prior drug convictions, the most recent dated back five years. Furthermore, he was attempting to rehabilitate himself, he had made no profit on the sales, and no drugs were found in his apartment when it was searched, suggesting the absence of a predisposition to break drug laws. "To determine whether entrapment has been established," it said, "a line must be drawn between the trap for the unwary innocent and the trap for the unwary criminal".
Prosecutors won the next two times entrapment came before the Court, in United States v. Russell and Hampton v. United States, albeit by narrow margins. In the former, the Court upheld the conviction of a Washington man for manufacturing methamphetamine even though an undercover agent had supplied some of the ingredients, and also pondered an "outrageous government conduct" defence, though it did not enable it. Hampton let stand, by a similar margin, the conviction of a Missouri man who had, upon seeing track marks on a DEA informant's arms, expressed interest in selling him heroin. After several sales to the informant and undercover agents, he was arrested. The defendant alleged he had been led to believe by the informant that he was not selling heroin but a counterfeit. The Court found he was adequately predisposed to sell heroin in any event.
This became known as the "subjective" test of entrapment, since it focused on the defendant's state of mind. However, in all cases, concurring opinions had advocated an "objective" test, focusing instead on whether the conduct of the police or other investigators would catch only those "ready and willing to commit crime". Under the objective approach the defendant's personality (i.e., his predisposition to commit the crime) would be immaterial, and the potential for the police conduct to induce a law-abiding person considered in the abstract would be the test. This, supporters argued, avoided the dubious issue of an unexpressed legislative intent on which the Sorrells court had relied and instead grounded the entrapment defence, like the exclusionary rule, in the court's supervisory role over law enforcement. And like the exclusionary rule, they would have had judges, not juries, decide whether a defendant had been entrapped as a matter of law.
Since the subjective test focusing on predisposition had, unlike the exclusionary rule, not been applied to the states, they were free to follow it as they saw fit. The state courts or legislatures of 37 states have chosen the subjective test, while the others use the objective test. Some have allowed both the judge and the jury to rule on whether the defendant was entrapped.
In the Supreme Court's last major ruling on entrapment, Jacobson v. United States, which overturned the conviction of a Nebraska man for receiving child pornography via the mail, the subjective vs. objective debate was completely absent. Both the majority and dissenting opinions focused solely on whether the prosecution had established that the defendant had a predisposition for purchasing such material (which had only recently been outlawed at the time of his arrest). Since no other material was found in his home save what he had purchased from the undercover postal inspectors, Justice Byron White believed the operation had implanted the idea in his mind through mailings decrying politicians for assaulting civil liberties by passing laws such as the one the inspectors hoped he would break. Sandra Day O'Connor disagreed in her dissent, arguing that the record did indeed establish that Jacobson was interested in continuing the purchases. Analysts believed that was the Court's indication it considered the subjective vs. objective debate settled.
University of Arizona law professor Gabriel J. Chin points out that the entire federal entrapment defence rests on statutory construction—an interpretation of the will of Congress in passing the criminal statutes. As this is not a Constitutional prohibition, Congress may change or override this interpretation by passing a law.
A subset of the entrapments defence was first recognized by the Supreme Court in Raley v. Ohio. There, four defendants were testifying before a committee of the Ohio State Legislature. The chairman of the committee told them that they could assert their right against self-incrimination. They asserted this right, and refused to answer questions. However, Ohio law provided them immunity from prosecution, so the right against self-incrimination was inapplicable, and they were subsequently prosecuted for their failure to answer questions. The Supreme Court overturned three of the four convictions based on the doctrine of entrapment by estoppel. (The fourth refused to state his address, at which point the committee expressed the view that the right against self-incrimination did not apply to that question.)
As described in United States v. Howell, the defence "applies when, acting with actual or apparent authority, a government official affirmatively assures the defendant that certain conduct is legal and the defendant reasonably believes that official".
The entrapment by estoppel defence exists in both federal and city jurisdictions; however, case law remains inconsistent as to whether the misleading advice of e.g. a state official provides protection against federal criminal charges. Examples exist of an appellate court failing to allow an entrapment by estoppel defence where a municipal official provided misleading instructions regarding a state law.