The exclusionary rule prevents evidence collected or analyzed in violation of the defendant's constitutional rights from being used in a court of law.
When the Exclusionary Rule Applies
Is there Standing?
Determining who can raise the exclusionary rule is whether the person's 4th amendment rights were violated, which generally turns on whether the person has a REP. If no REP, then no exclusionary rule. For example, being a passenger in a car that is searched does not convey standing to the passenger (Rakas). However, if the passenger is also an owner of the car, then the person does have standing. The person must have a property or possessory interest in the item searched or seized.
Visitors to Homes
If the transaction is purely commercial in nature, of a relatively short period of time on the premises, and there is not a previous connection between the visitors and the householder, then the visitors have no standing (Minnesota v. Carter). Kennedy concurs, but modifies the test to say almost all social guests have REP and standing, but not people with only a fleeting or insubstantial connection with the home owners.
Exclusionary Rule and States
The exclusionary rule applies to states under the 14th amendment. It's necessary to preserve judicial integrity, deter violations of government officials, maintain public trust in the system, and so on (Mapp v. Ohio).
Exclusionary Rule Definition/Purpose
The exclusionary rule prohibits the introduction into evidence of the tangible materials seized during an unlawful search, and of testimony concerning knowledge acquired during an unlawful search. It also prohibits derivative evidence, both tangible and testimonial, that is the product of the primary evidence, or is otherwise acquired as an indirect result of the unlawful search (Murray v. US).
Exceptions to Exclusionary Rule
Overnight guests have standing (Minnesota v. Olson).
Knock and Announce
The exclusionary rule doesn't apply to knock and announce violations because the cost of letting criminals free outweighs the potential of police misconduct (Hudson v. Michigan).
If the link between the illegal police action and evidence is attenuated, then the evidence is admissible. Attenuation is dependent on temporal proximity, intervening circumstances, and the flagrancy of the misconduct (UT v. Strieff, Brown v. Illinois).
If police obtain evidence in violation of the 4th amendment, it is still admissible if it is also obtained through an independent source (Murray v. US).
If the evidence would have been obtained legally eventually anyway, then it's admissible (Nix v. Williams). To be inevitable, the preponderance of the evidence has to suggest the evidence would be found. Additionally, police cannot say, "Well, we could've got a warrant if we wanted."