Wong Sun v. United States

This landmark decision established the "fruit of the poisonous tree" doctrine and affirmed the constitutional prohibition against unreasonable search and seizure. The Wong case added to the exclusionary rule by holding that unless the government can clearly demonstrate that secondary evidence was discovered independently of the "tainted," illegal, primary evidence, then the secondary evidence must be excluded as "fruit of the poisonous tree."

The Wong decision was one of a series of key findings made by the Warren Court in the 1960s, during what was perceived as a "revolution" in the development of criminal procedure. Another landmark case was the Miranda v. Arizona decision in 1966 requiring police to advise suspects of their rights prior to questioning. The Court, by limiting police powers to investigate and prosecute crimes, sought to balance the need to gather evidence against invasion of privacy.

More conservative Supreme Courts since the 1960s sought to limit the exclusionary rule by narrowing the range of evidence considered "fruit" of a Fourth Amendment violation. In United States v. Leon (1984) the Court held that if officers acted with a reasonable belief their conduct did not violate the Fourth Amendment then the search was legal. This amounted to a generalized "good faith" or "reasonableness" exception to the exclusionary rule. In Nix v. Williams (1984), the Court recognized the "inevitable-discovery" exception by holding that illegally gathered evidence could still be used if the government could demonstrate that such evidence would have been "inevitably" discovered through other lawful sources anyhow. The Court further expanded the exception to the "independent source" exception in Murray v. United States in 1988.

With the Wong decision the exclusionary rule became perhaps the most contentious issue associated with the Fourth Amendment. Many believed punishment for Fourth Amendment violations should more appropriately consist of civil or criminal lawsuits or disciplinary actions against police officers responsible for unlawful searches. In 1995, as part of the "Contract with America" war on crime, the newly Republican-controlled 104th Congress introduced the Exclusionary Rule Reform Act. The bill expanded on the Court's "good faith" exception to the exclusionary rule, made electronic search and seizure easier, and suspended the need for search warrants in many situations. The proposed standards only required that law enforcement demonstrate "good intent" instead of obtaining a warrant before searching a business or residence.

Increased emphasis on defendants' constitutional rights by the Court in the 1960s led to a rapid increase in number of public defender offices by the early 1970s. Defense lawyers were no longer considered a luxury, but instead were constitutionally required to protect defendants' rights. By the 1980s public defender offices were a major part of the criminal justice system.