Appellate cases are almost always heard by a panel of judges - or Justices, as they are called if they are members of the Supreme Court of the United States. When these judges decide a case, they do so by majority vote. Judges who disagree with the outcome may write a dissent, explaining why they disagree with the majority.
Although dissents are not the law, they do have persuasive power. The reasoning in the dissent may influence future judges/justices when they review and decide a later case. Over time, the reasoning in certain dissents has been so persuasive that it has become the reasoning of the majority opinion in a future case.
Overall, the importance of a judicial dissent is that it allows judges to state the reasons for their disagreement with a majority argument, and it can give guidance on future cases. The following are examples of instances where a dissenting opinion in one case has influenced the majority opinion in a future case or cases.
The Influence of Dissenting Opinions: Warrantless Wiretaps, Evidence, and Segregation
Warrantless Wiretaps
Olmstead v. United States, 277 U.S. 438 (1928)
The 5-4 majority opinion held that federal agents did not violate the unreasonable search and seizure clause of the Fourth Amendment with their warrantless wiretapping of a suspect's phone. The majority held that the Fourth Amendment only protects physical places and not an intangible phone conversation.
Four dissents were written. The authors were Justices Oliver Wendell Holmes, Louis Brandeis, Pierce Butler, and Harlan Fiske Stone. In his dissent, Justice Louis Brandeis argued that the Fourth Amendment protects not only physical areas, but areas where people have an expectation of privacy. He argued that the government should not break the law in order to prosecute criminals, saying:
"In a government of laws, existence of the government will be imperiled if it fails to observe the law scrupulously. Our government is the potent, the omnipresent teacher. For good or for ill, it teaches the whole people by its example. Crime is contagious. If the government becomes a lawbreaker, it breeds contempt for law; it invites every man to become a law unto himself; it invites anarchy. To declare that in the administration of the criminal law the end justifies the means-to declare that the government may commit crimes in order to secure the conviction of a private criminal-would bring terrible retribution. Against that pernicious doctrine this court should resolutely set its face." 277 U.S. 438, 485.
Katz v. United States 389 U.S. 643 (1961)
In a 7-1 decision, the Court reversed Olmstead and held that a warrantless wiretap of a phone booth by federal agents violated the unreasonable search and seizure clause of the Fourth Amendment. The Court said that the Fourth Amendment protects areas where an individual has a "reasonable expectation of privacy."
Exclusionary Rule in State Courts
Wolf v. Colorado, 338 U.S. 25 (1949)
The 6-3 majority opinion held that state courts were not required to exclude the admission of evidence that was taken in violation of the U.S. Constitution. Although the "exclusionary rule" was applied to federal courts by an earlier Supreme Court ruling, the Court held that the same rule did not automatically apply to the state courts.
In three dissents, Justices Hugo Black, Wiley Rutledge, and Frank Murphy argued that, without the exclusionary rule, the protections of the Constitution would be meaningless because there would be no effective deterrent for the unconstitutional conduct of the police. 338 U.S. 25, 40.
Mapp v. Ohio, 367 U.S. 643 (1961)
In a 6-3 decision, the Supreme Court reversed Wolf and held that the exclusionary rule applied to the states as well as to the federal government. The majority agreed that the exclusionary rule was the only effective means of ensuring that law enforcement complies with the Constitution. The majority specifically quoted Justice Brandeis' Olmstead dissent, saying that the government was a teacher and, thus, it should not break the law in order to enforce the law.
Segregation
Plessy v. Ferguson, 163 U.S. 537 (1896)
By an 8-1 vote, the Supreme Court upheld segregation laws (in this particular context, segregation on a train), saying that "separate but equal" did not violate the Equal Protection Clause of the Constitution.
In his dissent, Justice John Marshal Harlan argued that "Our constitution is color-blind and neither knows nor tolerates classes among citizens." 163 U.S. 537, 559.
Brown v. Board of Education, 347 U.S. 483 (1954)
Some 58 years later, a unanimous court reversed Plessy, holding that segregation in public schools violated the Equal Protection Clause of the Fourteenth Amendment. Chief Justice Earl Warren, writing for the court, said "separate educational facilities are inherently unequal."
Later Cases and Justice Harlan's Dissent
After Brown, Justice Harlan's Plessy dissent became the source of controversy. In University of California v. Bakke 265, 355 (1978), the Court struck down racial quotas in admissions programs at public universities as violating the Equal Protection Clause of the Fourteenth Amendment, but the Court explicitly stated that Harlan's dissent was not the correct interpretation of the Equal Protection Clause. In other words, the Court ruled that a state might consider race under certain circumstances. On the other hand, Justice Clarence Thomas, concurring in part and dissenting in part in Grutter v. Bollinger, 539 U.S. 306, 378 (2003), cited Justice Harlan's dissent as evidence that any use of race in admissions programs at public universities violates the Equal Protection Clause of the Fourteenth Amendment.