When the Fourth Amendment Applies

Like the rest of the Bill of Rights, the Fourth Amendment originally only applied in federal court. However, in Wolf v. Colorado, 338 U.S. 25 (1949), the U.S. Supreme Court ruled that the rights guaranteed by the text of the Fourth Amendment (sans the exclusionary rule to be discussed below) apply equally in state courts via the Fourteenth Amendment, which guarantees to the citizen of every state the right to due process and equal protection of the laws. The process by which the Supreme Court has made certain fundamental liberties protected by the Bill of Rights applicable to the states is known as the doctrine of incorporation.
Not every search and seizure that is scrutinized in state and federal court raises a Fourth Amendment issue. The Fourth Amendment only protects against searches and seizures conducted by the government or pursuant to governmental direction. Surveillance and investigatory actions taken by strictly private persons, such as private investigators, suspicious spouses, or nosey neighbors, are not governed by the Fourth Amendment. However, Fourth Amendment concerns do arise when those same actions are taken by a law enforcement official or a private person working in conjunction with law enforcement.
The Fourth Amendment does not apply even against governmental action unless defendants first establish that they had a reasonable expectation of privacy in the place to be searched or the thing to be seized. The Supreme Court has explained that what "a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection ... " But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected (see Katz v. United States, 389 U.S. 347 [1967]).
Applying this principle, the Supreme Court has ruled that individuals generally maintain a reasonable expectation of privacy in their bodies, clothing, and personal belongings. Homeowners possess a privacy interest that extends inside their homes and in the curtilage immediately surrounding the outside of their homes, but not in the "open fields" and "wooded areas" extending beyond the curtilage (see Hester v. United States, 265 U.S. 57 [1924]). A business owner's expectation of privacy in commercial property is less than the privacy interest afforded to a private homeowner and is particularly attenuated in commercial property used in "closely regulated" industries (i.e., airports, railroads, restaurants, and liquor establishments), where business premises may be subject to regular administrative searches by state or federal agencies for the purpose of determining compliance with health, safety, or security regulations. Automobile owners have a reasonable expectation of privacy in the cars they own and drive, though the expectation of privacy is less than a homeowner's privacy interest in his or her home.
When the Fourth Amendment Applies
No expectation of privacy is maintained for property and personal effects held open to the public. Things visible in "plain view" for a person of ordinary and unenhanced vision are entitled to no expectation of privacy and thus no Fourth Amendment protection. Items lying in someone's backseat, growing in someone's outdoor garden, or discarded in someone's curb-side garbage all fall within this category. However, items seen only through enhanced surveillance, such as through high-powered or telescopic lenses, may be subject to the strictures of the Fourth Amendment. Public records, published phone numbers, and other matters readily accessible to the general public enjoy no expectation of privacy. Similarly, the Supreme Court has said that individuals do not possess an expectation of privacy in their personal characteristics (see United States v. Dionisio, 410 U.S. 1 [1973]). Thus, the police may require individuals to give handwriting and voice exemplars, as well as hair, blood, DNA, and fingerprint samples, without complying with the Fourth Amendment's requirements.
Finally, to raise a Fourth Amendment objection to a particular search or seizure, a person must have "standing" to do so. Standing in this context means that the rights guaranteed by the Fourth Amendment are personal and may not be asserted on behalf of others. Thus, a passenger may not generally object to a police search of the owner's car and a houseguest may not generally object to a search of the homeowner's premises. These rules can become murky, however, as when a houseguest is actually living with the homeowner or owns things stored on the owner's premises.