Prosecutors and defendants in criminal proceedings may present evidence in support of their cases. The state has the burden of proving guilt beyond a reasonable doubt, while the defendant may present evidence to challenge the state’s case. Each side should have the opportunity to review the other side’s evidence before trial and object to the introduction of certain evidence before or during trial. In criminal cases, defendants may move the court to exclude evidence that the state obtained in violation of their constitutional rights. The Federal Rules of Evidence govern the admission of evidence in the federal court system. Each state has its own evidence rules, which are often similar to the federal rules.
The term “evidence” broadly refers to materials relating to the subject matter of a legal proceeding, such as:
The most important factor in determining whether a piece of evidence is admissible is its relevance to the proceeding. “Relevant evidence” includes any evidence that would make the existence of a material fact “more probable or less probable than it would be without the evidence.” As a general rule, relevant evidence is admissible, while evidence deemed irrelevant is not.
Even if evidence is deemed relevant by a judge, it could be excluded if the possibility that it would confuse a jury, mislead jurors, or unfairly prejudice jurors against a defendant is greater than its “probative value.”
Evidence must also be sufficiently reliable to be admitted at trial. Evidence from expert witnesses, which might be used to establish the validity of or to challenge drug test results, ballistics, or computer forensics, to name but a few, must meet standards defined by the U.S. Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), and Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999).
“Hearsay” is defined as any statement made outside of court that is “offered in evidence to prove the truth of the matter asserted.” An example would be evidence that a person, in a non-court setting, said to another person that the defendant committed a robbery, if the state tried to introduce it as evidence that the defendant committed robbery.
Hearsay is generally inadmissible, since the judge or jury is unable to form an opinion regarding whether the person making the out-of-court statement is reliable. Multiple exceptions to the hearsay rule exist, and a defendant’s own out-of-court statements are excluded from the definition of hearsay entirely.
The Fifth Amendment states that no one may “be compelled in any criminal case to be a witness against himself.” During a criminal trial, neither the state nor the court may compel a defendant to testify, nor may they compel a defendant to provide evidence that would incriminate him or her.
Defendants may move to suppress evidence obtained by police or prosecutors in violation of their constitutional rights, including the Fourth Amendment right against warrantless searches and seizures, the Fifth Amendment right against self-incrimination, and the Sixth Amendment right to an attorney in a criminal case. Evidence obtained in violation of a defendant’s rights is known as “fruit of the poisonous tree.” See Silverlight Lumber Co. v. United States, 251 U.S. 385 (1920). The rule requiring suppression of such evidence, known as the exclusionary rule, applies in all federal and state cases, according to the Supreme Court’s ruling in Mapp v. Ohio, 367 U.S. 643 (1961).
Since its ruling in Mapp, the Supreme Court has set limits on the applicability of the exclusionary rule. A defendant may only seek suppression of evidence obtained in violation of the defendant’s own rights. Evidence against the defendant obtained in a warrantless search of someone else’s home may not be subject to suppression by the defendant. “Fruit of the poisonous tree” also may be admitted if police could have obtained it through lawful means. The Supreme Court has held that constitutional violations and the suppression of evidence obtained as a result are two separate questions, and that the “mere fact that a constitutional violation” occurred does not require suppression. Hudson v. Michigan, 547 U.S. 586, 592 (2006).
