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Las organizaciones legales civiles en Arizona buscan su opinión para aumentar su capacidad de satisfacer las necesidades legales civiles de los residentes de bajos ingresos de Arizona. Por favor complete esta encuesta para ayudar a mejorar los servicios legales civiles en Arizona.

Lifelong Legal Learning

Reasonable Expectation of Privacy Under the Fourth Amendment


One of the most important principles of American constitutional law is the idea that individuals in the United States have a reasonable expectation of privacy – that is, a right to be free from invasion in their own private space (for example, the home that they own or the apartment that they rent), whether that invasion is physical or electronic, and whether the invader is a stranger or a government actor (such as the police). This principle became a part of the United States Constitution – which is the supreme law of the United States – through what is known as the Fourth Amendment.

This article provides a brief introduction to constitutional amendments in general and to the Fourth Amendment in particular – with a focus on how the Fourth Amendment protects and promotes the expectation of privacy.

What is a constitutional amendment?

A constitutional amendment is a modification of an existing constitution. The United States Constitution came into effect in 1789. It has since been amended a number of times. The first ten amendments – one of which (the Fourth Amendment) is the topic of this article – were enacted two years later, in 1791. These first ten amendments are known as the “Bill of Rights” because they were intended to secure individual rights and freedoms and to place limits on the powers of government. The most recent amendment – the Twenty-Seventh Amendment, which governs congressional pay – was enacted in 1992. Other well-known amendments include the Thirteenth Amendment (1865) which abolished slavery and the Nineteenth Amendment (1920) which gave women across the country the right to vote.

How is the United States Constitution amended?

Article Five of the United States Constitution describes the process through which the Constitution itself may be modified. Any modification requires two steps: (1) the proposal of an amendment and (2) the ratification (or formal approval) of the proposed amendment.

An amendment may be proposed either by Congress (with a two-thirds vote in both the House of Representatives and the Senate) or by a convention of the states (if the legislatures of at least two-thirds of the states called for the convention). A proposed amendment must be ratified either by the legislatures of three-quarters of the states or by state ratifying conventions in three-quarters of the states. These are difficult thresholds to reach.

What does the Fourth Amendment say?

The Fourth Amendment to the United States Constitution says that “the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

What does the Fourth Amendment do?

The Fourth Amendment prohibits law enforcement and other government agencies from searching and seizing a person’s private space and/or belongings where that person has a “reasonable expectation of privacy” unless the agency first obtains a warrant from a judge that is based on “probable cause” (a good-faith and reasonable belief expressed under oath that contraband or evidence of a crime will be found if the search and/or seizure is permitted).

What is the “reasonable expectation of privacy”?

The “reasonable expectation of privacy” is the legal test that is used to determine whether the protections of the Fourth Amendment apply to a particular search or seizure.

How is the Fourth Amendment interpreted to protect and promote the reasonable expectation of privacy?

Through its decision in Katz v. U.S. (1967) the United States Supreme Court developed a two-part test for determining whether a person has a “reasonable expectation of privacy” which the government may not violate without a search warrant. The first requirement is that the person must exhibit an actual (subjective) expectation of privacy. The second requirement is that the person’s expectation of privacy must be (objectively) reasonable. If both of these requirements are met, then the government may not – as a general rule (there are exceptions) – search the person’s private space or seize the person’s belongings without a warrant.

To whom does the Fourth Amendment apply?

The provisions of the Fourth Amendment apply to governments and government actors (such as law enforcement agencies and agents as well as (for example) private security guards if they have been deputized with the power of arrest (such campus security officers at public universities) and public school administrators.

Where do people usually have a reasonable expectation of privacy?

The spaces and belongings which are protected by the Fourth Amendment include people’s bodies and clothes; their residences and hotel rooms; their papers; and their personal effects (including, for example, their purses and backpacks).

In 2018 the United States Supreme Court ruled that data containing a person’s cellphone location history is protected by the Fourth Amendment and therefore may only be lawfully searched and/or seized with a warrant.

When is a warrant not required?

Even when a person has a reasonable expectation of privacy, warrantless searches and seizures may be lawful under certain conditions. In many situations – including the following – a law enforcement officer may be able to search a person’s private space and/or seize a person’s belongings lawfully without first obtaining a warrant:

1. there are urgent circumstances

  • the evidence will disappear before a warrant is obtained
  • the law enforcement officer is in hot pursuit of a fleeing felon
  • the law enforcement officer believes that a person inside a home is in need of emergency aid to prevent or treat an injury

2. the search is directly related to a lawful arrest

3. the person gives their consent

4. the item seized is in plain view                       

5. the law enforcement officer has probable cause to believe that contraband or evidence of a crime is inside a motor vehicle

6. the special needs of law enforcement requires it

  • government employee drug-testing
  • searches of parolees and their homes and strip-searches of prisoners
  • searches of public schoolchildren and their belongings (for example, to enforce a no-smoking-on-school-grounds policy)
  • searches of airline passengers prior to boarding
  • searches of people and goods at the border                   

Sources and further reading

American Civil Liberties Union – “Supreme Court Rules Police Need a Warrant to Track Cellphones”:

Legal Information Institute – “Electronic Surveillance”:

Legal Information Institute – “Expectation of Privacy”:

United States Congress – “Fourth Amendment”: