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I spent 3 weeks in law school and one summer internship to master the 4th Amendment. I'll do my best to briefly summarize it in this post.
The 4th Amendment states in part 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..." This Amendment says that the government cannot unreasonably seize or search a person, their home, papers, and effects (which means communication including mail and telephone) without a warrant and probable cause.
When and what is a seizure?
Even though many of you might not know it, you have probably been seized before. A person is seized when he/she does not feel free to walk away. The police can seized someone either by a show of force or physical restraint. By this definition, every traffic stop is a seizure! Certainly nobody feels free to walk away (or leave) when lights and sirens are coming from behind.
What is required for a seizure?
The Constitution says that no unreasonable search without a warrant and probable cause. This has been interpreted by the Supreme Court to mean that reasonable searches are okay. So what is a reasonable search?
In the famous case of Terry v. Ohio, the Supreme Court ruled that if the police has a reasonable suspicion that a crime is afoot or a reasonable suspicion that the suspect is armed and dangerous, the police can conduct a reasonable search. In that case, a police officer spotted some guys walking around and looking into store windows. The officer approached the suspects, flipped them around, and frisked them for weapons. When he discovered a gun, he arrested them. When it came time for trial, the defendants questioned whether the cop had a right to stop and search them without a warrant or probable cause. The Supreme Court ultimately allowed this search, deciding that the government's interest outweighed the defendant's individual privacy interest.
The Court was careful in applying this search. This search is limited to the pat down of the suspects outer clothing and can only last so long. This has been known as the Terry Search or the "Pat Down." This is the only known search allowed where the police does not have probable cause that a crime has been committed and no warrant is present.
Later, the Court applied the Terry Search to cars in Michigan v. Long and to homes in Maryland v. Buie. Over the years, the Supreme Court has expanded its interpretation of the 4th Amendment to say that an unreasonable search without a warrant can be allowed in limited circumstances. I won't elaborate on every exception (such as exigent circumstances, to preserve evidence, incident to an arrest, etc.), but I will talk about one exception called "consent."
Consent
The police can always search a person upon consent. Consent must be knowing, intelligent, and voluntary ("KIV"). Even though refusing consent cannot give rise to any probable cause, this is the number one reason why people consent to searches. For example, a police officer pulls you over for speeding, and you have a kilo of cocaine in the trunk (that you took from your drug-addict brother in an intervention). The officer says, "do you mind if I look in your trunk?" What do you say?
Most people feel that if you say no to the popo (pissed off police officer) that he'll think that you're hiding something, so they voluntarily give the police permission to search the trunk! Next time you watch COPS, pay attention to the people who voluntarily give consent to a search. The Court is clear that refusing to give consent, alone, is not enough for the police to get a warrant to search. So next time you have cocaine in your trunk, don't let the popo search it!
There are other exceptions such as the "plain-view" doctrine which says that if the police can see something in plain view (like through your car window), that it is okay for the police to use it as evidence.
What happens if the search is improper?
If the police violates your 4th Amendment right to obtain evidence, then that evidence cannot be used against you in the court of law. Criminal defense attorneys heavily rely on this defense to exclude harmful evidence. When I was in Michigan, East Lansing Police used to walk up and down sidewalks and ask people to take breathalyzers. Based on the results, people could get charged with a "Minor in Possession" of alcohol. Anyone who refuses to take the breathalyzer was charged a $100 fine. The Court eventually held that it was an unconstitutional search because (1) simply walking on a sidewalk cannot be enough reason to seize someone and (2) consent was not voluntary. As a result, the breathalyzer results couldn't be used to charge anyone with an MIP. No evidence, no crime.
*Please remember that refusing a breathalyzer if you are driving a car could result in your license being suspended. This is because you consented to this search when you got your license.*
I'm not sure what the lesson here is, except that you should never consent to a search of your trunk if you are hiding cocaine in there.
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