"Fruit of the poisonous tree" is a rather Gothic-sounding phrase for an important evidentiary principle designed to protect the rights of defendants in criminal cases. It expands the boundaries of the exclusionary rule, which prohibits the government from using unconstitutionally obtained evidence to prosecute a criminal defendant.
Why some evidence needs to be excluded
The court takes constitutional protections seriously. Among those are the protection against unreasonable search and seizure under the Fourth Amendment, the right to protect oneself against self-incrimination under the Fifth Amendment and the right to assistance of counsel under the Sixth Amendment.
The exclusionary rule was developed as a remedy for defendants in situations where violations of their constitutional rights resulted in law enforcement gathering evidence they would not have been able to acquire otherwise.
Throwing out illegally obtained evidence provides a strong incentive for police officers and prosecutors to be careful about making sure all evidence is properly obtained.
Providing additional deterrence
The fruit of the poisonous tree doctrine takes this idea a step further and prohibits the use of any evidence that can be traced back to illegally obtained evidence. For example, a police officer performs an illegal search of a defendant's apartment and finds a list of criminal contacts. He or she gets in touch with a person on this list, and this person voluntarily provides the officer with additional evidence against the defendant. Neither the contact list nor the additional evidence would be admissible. This is the case even though, had the officer found this person through legitimate sources, the additional evidence would likely be fine.
The reason for establishing the fruit of the poisonous tree doctrine is to provide an extra measure of deterrence to police officers and prosecutors. With only the exclusionary rule in play, some officers may well decide that it is worth foregoing the admissibility of the first piece of evidence if they believe that it could lead them to something they could use.
Looking back to the example of the contact list, police officers may undertake an illegal search because they suspect the existence of the list. The evidence they can potentially get from contacts may be far more valuable to their case than just the list. However, if they know they will not be able to use the evidence either, they will not undertake the illegal search.
Because the main reason for this rule is deterrence of unlawful conduct, courts have set up exceptions to the rule when circumstances indicate that allowing the evidence will not encourage illegal searches. There are several exceptions, the main one being the good-faith exception that applies when the officers assume their search is lawful based on a genuine and reasonable mistake.
The rules of evidence are highly complex and technical. They can also determine the ultimate outcome of your case. If you are accused of a crime, you need to consult a criminal defense attorney who can assess the facts of your case and develop an optimal strategy.