Iowa Law Review
In Hudson v. Michigan, the Supreme Court held that evidence need not be excluded despite the fact that the police had violated the Fourth Amendment by failing to knock and announce their presence before conducting a search. The Court said that the constitutional violation was not a but-for cause of the seizure; the police would have obtained the evidence even if they had knocked. Hudson's analysis threatens to withdraw the exclusionary remedy whenever the police have conducted a search in an unconstitutional manner-most notably, when they have failed to obtain a warrant before searching. The Court's decision is likely to withdraw the remedy in the cases in which it is most likely to work and to leave the police with little incentive to conduct searches properly. Even scholars critical of Hudson have seen the Supreme Court's statement of the need for but-for causation as "unassailable". In the administration of the exclusionary rule, however, the Supreme Court generally has not required a but-for causal relationship between a police wrong and discovery of the challenged evidence. Instead it has treated officers who entered without knocking or without obtaining warrants as trespassers and has excluded all evidence that their wrongful presence enabled them to obtain. This approach begged the question of which constitutional violations made the police trespassers and which did not. This Article maintains that the appropriate question in exclusionary rule cases is neither the one traditionally posed by trespass law nor the one posed by Hudson's requirement of but-for causation. It is instead one of "contributory" causation. Courts should ask, not whether a constitutional violation enabled the police to obtain evidence they would not have obtained without it, but whether a constitutional violation facilitated the discovery of evidence either by improving the likelihood of its discovery or by reducing the work required to obtain it.
Albert Alschuler, "Exclusionary Rule and Causation: Hudson v. Michigan and Its Ancestors, The ," 93 Iowa Law Review 1741 (2007).