What keeps the police from knocking down people’s doors at random, looking for evidence of crimes? One answer is the Fourth Amendment to the U.S. Constitution. The Fourth Amendment provides some of the most important criminal law protections the American people have, including the right to be free from “unreasonable searches and seizures.”
The Fourth Amendment plays an important role in many criminal defense cases, and can often mean the difference between a conviction and an acquittal.
Over the past 200 years since the amendment was ratified, courts have argued many times about what it means, exactly, and have had to decide what counts as a “search,” what counts as a “seizure,” and, most controversial of all, what counts as “unreasonable.” At times, the courts have seemed to expand the power of the Fourth Amendment, and at other times they have seemed to rein it in.
Today, courts generally agree that the protection against unreasonable search and seizure means that before police can arrest a person or search their property, they must get a warrant supported by probable cause. Essentially, this means they must convince a court that it is more likely than not that they have evidence of a crime committed by the person, or that they will find the evidence in a search.
However, there are many exceptions to the warrant requirement.
One exception occurs in emergency situations. Another occurs when the police see evidence of a crime in plain view. For instance, if a police officer pulls over a driver and sees illegal drugs sitting on the passenger seat of their car, the police do not need a warrant to search the car for evidence of drugs.
Things get trickier when evidence is not in plain view. The Fourth Amendment says the people should be free in their “persons, houses, papers and effects” from unreasonable searches and seizures. Over the years, courts have determined this means that warrants are more important in places where the person has a “reasonable expectation of privacy.”
A person who carries evidence of a crime freely in public has no reasonable expectations of privacy, and so the police are not violating the Fourth Amendment if they stop the person and search them. However, it’s reasonable for people to expect privacy in their homes. If the police are going to enter someone’s home to search for evidence of a crime, they had better have a good reason. Better yet, they should have a warrant supported by probable cause.
If the police violated a defendant’s Fourth Amendment rights in a search, the prosecution cannot use evidence the police found in that search. Without this evidence, the prosecution has a much harder time proving its case, and the defendant may beat the charges.