The term "fruit of the poisonous tree" is used to describe a rule of evidence exclusion in the United States in which evidence obtained through illegal means is excluded from trial. The term references the idea that if the means — the "tree" — are illegal, then the evidence — the "fruit" — is tainted by association. This doctrine often comes up in the context of evidence associated with illegal searches and interrogations; even if the evidence itself is very solid proof of illegal activity, it is not considered admissible because it was obtained illegally.
Concerns about the fruit of the poisonous tree have led law enforcement agencies to train their representatives extensively in the protocols of evidence collection. No law enforcement agency wants to lose a case or have a case not go to trial because the evidence is not admissible. Law enforcement officers learn about how to collect evidence legally, and how to legally conduct searches, interrogations, and other activities to avoid uncovering and tainting evidence during illegal activities. On the flip side, citizens who educate themselves about the law can protect themselves from illegal law enforcement actions which may compromise their rights.
The United States Supreme Court hinted at this doctrine in the 1920 case Silverthorne Lumber Co. versus United States, in which the defendant argued that accounting information used in the case was seized illegally and was therefore not valid. This case set a precedent, laying the groundwork for additional cases and the eventual emergence of the term "fruit of the poisonous tree."
There are some cases in which evidence normally considered inadmissible under the fruit of the poisonous tree doctrine may be accepted by the court. The first case applies to evidence which would have been outed inevitably through legal means. The second case applies to evidence which is only weakly linked to an illegal action; if it can be demonstrated that the link is tenuous, the evidence may be allowed. Evidence is also allowed when it can be linked to independent and legal means of investigation.
The rules of evidence can get quite complicated, and lawyers often use this to their advantage when they work to get evidence suppressed or admitted. The fruit of the poisonous tree is one among several tools which a lawyer can use to try and argue that evidence should not be brought before the court. Since search and seizure are especially controversial in the United States, a lawyer who can successfully demonstrate that a search was illegal can often suppress the resulting evidence. If the bulk of a case rests on that evidence, it may be thrown out.