An important principle of twentieth-century American jurisprudence is that evidence acquired through improper conduct by the state cannot be used to convict a criminal defendant. The U.S., however, is the only industrial democracy, common law or otherwise, in which courts must throw out tainted evidence in criminal trials. The U.S. Supreme Court decisions establishing and expanding on this principle have collectively come to be known as the "exclusionary rule." Although the rule had its origins in arguments about the morality of obtaining a conviction while relying on improperly obtained evidence, its primary modern justification is that it deters illegal conduct by the state.
However, an unexamined premise of this belief is that if illegally acquired evidence may be thrown out, decreasing the probability of conviction, then the police, prosecuting attorneys and other law-enforcement officials have an increased incentive to obey the rules. This paper analyzes the strength of this incentive. In doing so, improper conduct is assumed to be socially costly. Illegal searches, fabricated confessions, and other violations subject to the exclusionary rule are assumed to be worthy of deterrence in their own right. The question to be answered is how the rule performs in deterring such misconduct. The answer, based on principal-agent analysis, is that absent a carefully crafted compensation rule for those personnel, it performs poorly. The rule is questionable not just because of any costs associated with lost convictions but because it does not sufficiently deter law-enforcement violations.
Although the rule can be traced back over 80 years, deterrence was not always an important part of its jurisprudence. The principle of excluding evidence that was obtained in violation of constitutional rights can be traced to Weeks v. U.S. [232 U.S. 383 (1914)]. The Supreme Court overturned the conviction of the defendant, ruling that government efforts to convict a defendant could not be "aided" by evidence obtained through a warrantless search of the defendant's home by a federal marshal. The principle was extended to illegal searches entered into evidence in state courts in Mapp v. Ohio [367 U.S. 643 (1961)], to verbal evidence obtained in the course of a warrantless search by Wong Sun v. U.S. [317 U.S. 417 (1963)], and to confessions obtained without informing the defendant of his constitutional right not to incriminate himself in Miranda v. Arizona [384 U.S. 436 (1966)]. The deterrence principle was completely absent in Weeks, with the Court deriving the rule as necessary to make the Fourth Amendment meaningful, but was included as a rationale for the rule in Mapp and subsequent decisions.(1)
In 1974 deterrence became the centerpiece of exclusionary-rule jurisprudence in U.S.v. Calandra [414 U.S. 338 (1974)].(2) The putative trade-off between deterrence benefits and costs of erroneous trial results has motivated the recent carving out of "good faith" exceptions. U.S. v. Leon [468 U.S. 897 (1984)] established the principle that excluding evidence obtained with a facially valid search warrant that was later ruled invalid served no purpose because no additional deterrence could be obtained by such a rule. The same logic is found in Nix v. Williams [467 U.S. 431 (1984)], in which the Court held that evidence that would have been inevitably discovered absent constitutional violations should be admitted. In Arizona v. Evans [514 U.S. 1 (1995)], the Court's rationale for expelling evidence was that the court clerk whose error was challenged by the defendant was unlikely to be deterred by such an action because of his tenuous relation to frontline law enforcement.(3)
Although deterrence is now inextricably woven into exclusionary-rule jurisprudence, the missing link in this reasoning has been the relation between the expulsion of illegally obtained evidence and law-enforcement incentives. …