The exclusionary rule, which states that evidence obtained illegally may not be used by the government, is essential to any robust interpretation of the Fourth Amendment. Without it, the government would be free to violate the Fourth Amendment to obtain evidence, apologize profusely for doing so, and make use of the evidence anyway—defeating the purpose of the restrictions by removing any incentive the government might have to honor them.
Weeks v. United States (1914)Prior to 1914, the U.S. Supreme Court had not clearly articulated the exclusionary rule. This changed with the Weeks case, which established limits on the federal government's use of evidence. As Justice William Rufus Day writes in the majority opinion:
If letters and private documents can thus be seized and held and used in evidence against a citizen accused of an offense, the protection of the Fourth Amendment, declaring his right to be secure against such searches and seizures, is of no value, and, so far as those thus placed are concerned, might as well be stricken from the Constitution. The efforts of the courts and their officials to bring the guilty to punishment, praiseworthy as they are, are not to be aided by the sacrifice of those great principles established be years of endeavor and suffering which have resulted in their embodiment in the fundamental law of the land.This ruling did not, however, affect secondary evidence—federal authorities were still free to use illegitimately-acquired evidence as clues to find more legitimate evidence.
The United States marshal could only have invaded the house of the accused when armed with a warrant issued as required by the Constitution, upon sworn information, and describing with reasonable particularity the thing for which the search was to be made. Instead, he acted without sanction of law, doubtless prompted by the desire to bring further proof to the aid of the government, and, under color of his office, undertook to make a seizure of private papers in direct violation of the constitutional prohibition against such action. Under such circumstances, without sworn information and particular description, not even an order of court would have justified such procedure; much less was it within the authority of the United States marshal to thus invade the house and privacy of the accused.
Silverthorne Lumber Company v. United States (1920)The federal use of secondary evidence was finally restricted six years later in the Silverthorne case. Federal authorities had cleverly copied illegally-obtained documentation pertinent to a tax evasion case in hopes of avoiding the Weeks prohibition—since copying a document that is already in police custody is not technically a violation of the Fourth Amendment. Justice Oliver Wendell Holmes, writing for the Court majority, was having none of it:
The proposition could not be presented more nakedly. It is that, although, of course, its seizure was an outrage which the Government now regrets, it may study the papers before it returns them, copy them, and then may use the knowledge that it has gained to call upon the owners in a more regular form to produce them; that the protection of the Constitution covers the physical possession, but not any advantages that the Government can gain over the object of its pursuit by doing the forbidden act … In our opinion, such is not the law. It reduces the Fourth Amendment to a form of words.Holmes' bold statement, that limiting the exclusionary rule to primary evidence would reduce the Fourth Amendment to "a form of words," has been considerably influential in the history of constitutional law—as has the idea that the statement describes, generally described as the "fruit of the poisonous tree" doctrine.
Wolf v. Colorado (1949)Although the exclusionary role and the "fruit of the poisonous tree" doctrine restricted federal searches, they had not yet been applied to state-level searches. Since most civil liberties violations occur on the state level, this meant that the Supreme Court's rulings on the matter, philosophically and rhetorically impressive though they may be, were of limited practical use. Justice Felix Frankfurter attempted to justify this limitation in Wolf by extolling the virtues of state-level due process legislation:
The public opinion of a community can far more effectively be exerted against oppressive conduct on the part of police directly responsible to the community itself than can local opinion, sporadically aroused, be brought to bear upon remote authority pervasively exerted throughout the country. We hold, therefore, that, in a prosecution in a State court for a State crime, the Fourteenth Amendment does not forbid the admission of evidence obtained by an unreasonable search and seizure.But his argument is uncompelling for contemporary readers, and presumably was not all that impressive by the standards of his time, either; 15 years later, it would be overturned.
Mapp v. Ohio (1961)In Mapp v. Ohio, the Supreme Court finally applied the exclusionary rule and "fruit of the poisonous tree" doctrine articulated in Weeks and Silverthorne to the states by virtue of the incorporation doctrine. As Justice Tom C. Clark writes:
Since the Fourth Amendment's right of privacy has been declared enforceable against the States through the Due Process Clause of the Fourteenth, it is enforceable against them by the same sanction of exclusion as is used against the Federal Government. Were it otherwise, then, just as without the Weeks rule the assurance against unreasonable federal searches and seizures would be "a form of words," valueless and undeserving of mention in a perpetual charter of inestimable human liberties, so too, without that rule, the freedom from state invasions of privacy would be so ephemeral and so neatly severed from its conceptual nexus with the freedom from all brutish means of coercing evidence as not to merit this Court's high regard as a freedom "implicit in the concept of ordered liberty."Today, the exclusionary rule and "fruit of the poisonous tree" doctrine are regarded as basic principles of constitutional law, applicable in all U.S. states and territories.