Evidence Law Guide: Rules of Evidence in Criminal Trials

Evidence is the lifeblood of a criminal trial. Without evidence, there is no case. The rules governing what a jury may hear, what documents may be read, and what physical objects may be examined are among the most intricate and consequential in all of law. They determine the story that the jury hears, and that story determines the verdict.

The Federal Rules of Evidence, adopted in 1975, govern proceedings in federal courts, and nearly every state has adopted similar rules. These rules reflect centuries of common law experience and balance the goal of admitting all relevant evidence against the need to exclude unreliable, prejudicial, or constitutionally problematic proof.

Relevance

The starting point of evidence law is relevance. Evidence is relevant if it has any tendency to make a fact more or less probable than it would be without the evidence, and if that fact is of consequence in determining the action. This standard is intentionally low — the rules favor admissibility, and even evidence with minimal probative value is generally admissible.

Balancing Prejudice

Relevant evidence may still be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusing the issues, misleading the jury, undue delay, or wasting time. This balancing test is one of the most important tools available to trial judges and is frequently invoked in motions argued under Rule 403 of the Federal Rules.

Evidence is unfairly prejudicial when it invites the jury to decide based on emotion rather than reason. Graphic photographs of a crime scene, evidence of a defendant’s bad character, and testimony about prior bad acts are all subject to exclusion if their prejudicial effect outweighs their probative value. The criminal defense strategies guide explains how defense attorneys use Rule 403 objections to keep damaging evidence from the jury.

Hearsay

The hearsay rule is perhaps the most famous and most misunderstood rule of evidence. Hearsay is an out-of-court statement offered to prove the truth of the matter asserted. The rule against hearsay reflects the understanding that statements made outside of court and without cross-examination are inherently unreliable.

Hearsay is excluded unless it falls within one of the exceptions. There are over two dozen exceptions in the Federal Rules, reflecting situations where out-of-court statements have sufficient guarantees of trustworthiness. Common exceptions include:

• Excited utterances: statements made under the stress of a startling event
• Present sense impressions: statements describing or explaining an event while the declarant was perceiving it
• Business records: records kept in the regular course of business
• Statements for medical diagnosis: descriptions of symptoms or medical history
• Dying declarations: statements by a person who believes death is imminent concerning the cause or circumstances of the death

Confrontation Clause

Beyond the hearsay rules, the Sixth Amendment’s Confrontation Clause gives criminal defendants the right to confront witnesses against them. Under Crawford v. Washington (2004), testimonial out-of-court statements are inadmissible unless the declarant is unavailable and the defendant had a prior opportunity for cross-examination. Under Crawford v. Washington (2004), testimonial out-of-court statements are inadmissible unless the declarant is unavailable and the defendant had a prior opportunity for cross-examination.

This decision revolutionized hearsay law in criminal cases. Testimonial statements include affidavits, depositions, grand jury testimony, and statements made to police during interrogations that the declarant would reasonably expect to be used in prosecution. The Confrontation Clause does not apply to nontestimonial hearsay, which is governed by the ordinary hearsay exceptions.

Character Evidence

The rules sharply limit the use of character evidence in criminal trials. The prosecution may not introduce evidence of the defendant’s bad character to prove that they acted in conformity with that character on the occasion in question. This is the propensity rule — the jury is not allowed to infer that because the defendant is a bad person, they probably committed the crime.

However, the defendant may introduce evidence of their good character, and if they do, the prosecution may rebut with evidence of bad character. The prosecution may also introduce evidence of prior bad acts for non-propensity purposes, such as proving motive, opportunity, intent, preparation, plan, knowledge, identity, or absence of mistake. These so-called 404(b) issues are fertile ground for pretrial motions.

The Exclusionary Rule

The exclusionary rule is a constitutional remedy, not an evidentiary rule, but it has profound effects on what evidence reaches the jury. When law enforcement obtains evidence in violation of the Fourth Amendment (unreasonable search or seizure), the Fifth Amendment (failure to give Miranda warnings), or the Sixth Amendment (denial of counsel), the court may suppress that evidence.

The Miranda rights guide explains when statements obtained during custodial interrogation must be suppressed. The bail bond system discussion notes that evidence obtained during an illegal detention may also be excluded.

Exceptions to the Exclusionary Rule

The Supreme Court has created several exceptions that limit the exclusionary rule’s reach. The good faith exception allows evidence obtained in reasonable reliance on a defective search warrant. The inevitable discovery rule allows evidence that would have been discovered anyway through lawful means. The independent source doctrine allows evidence obtained through means independent of the constitutional violation. Attenuation breaks the causal chain between the violation and the evidence.

Expert Testimony

Expert witnesses play a critical role in criminal trials, providing opinions on subjects beyond the knowledge of ordinary jurors. Forensic experts testify about DNA, fingerprints, ballistics, and digital evidence. Mental health experts testify about competency, insanity, and diminished capacity.

The admissibility of expert testimony is governed by Daubert v. Merrell Dow Pharmaceuticals (1993) in federal courts and by the Frye standard in some states. Daubert requires judges to act as gatekeepers, ensuring that expert testimony is based on reliable methodology. The judge considers whether the theory or technique can be tested, whether it has been subjected to peer review, its known or potential error rate, and its general acceptance in the relevant scientific community.

Authentication and Best Evidence

Documents, photographs, recordings, and other tangible evidence must be authenticated — shown to be genuine — before they can be admitted. Authentication can be accomplished through witness testimony, distinctive characteristics, chain of custody evidence, or other means.

The best evidence rule requires the original document to prove its contents unless the original is unavailable. This rule applies to writings, recordings, and photographs. In the digital age, courts have adapted the rule to allow electronic duplicates when the original is not reasonably available.

Judicial Notice and Presumptions

Courts may take judicial notice of facts that are not subject to reasonable dispute — scientific principles, historical events, and geographical facts. The jury is instructed to accept judicially noticed facts as true. Presumptions shift the burden of production on certain issues, requiring the opposing party to come forward with evidence to rebut the presumed fact.

Forensic Evidence Challenges

The reliability of forensic evidence has come under increasing scrutiny. The 2009 National Academy of Sciences report “Strengthening Forensic Science in the United States” identified significant problems with many forensic disciplines, including bite mark analysis, hair comparison, and firearm tool mark analysis. The FBI has acknowledged that flawed forensic testimony contributed to wrongful convictions in numerous cases.

The appeal process criminal guide discusses how defendants can challenge convictions based on discredited forensic science through post-conviction proceedings and habeas corpus petitions.

Burden of Proof in Evidentiary Motions

The burden of proof for evidentiary motions varies depending on the issue. At a suppression hearing, the defendant typically bears the burden of proving that a search or seizure was unconstitutional. Once the defendant makes a prima facie showing, the burden shifts to the prosecution to justify the search. At trial, the proponent of evidence bears the burden of establishing its admissibility.

Frequently Asked Questions

What is the difference between direct and circumstantial evidence? Direct evidence proves a fact directly, such as eyewitness testimony that the defendant pulled the trigger. Circumstantial evidence requires an inference, such as the defendant’s fingerprints on the murder weapon. Both are legally sufficient to support a conviction.

Can a defendant be convicted solely on circumstantial evidence? Yes. Circumstantial evidence is not inherently less reliable than direct evidence. Many convictions rest entirely on circumstantial evidence, and juries are instructed that they should give circumstantial evidence the same weight as direct evidence.

What happens if the judge admits evidence that should have been excluded? The defendant may raise the issue on appeal. If the appellate court finds that the error was not harmless, it may reverse the conviction and order a new trial.

Can a witness’s prior criminal record be used to impeach them? Yes, subject to limitations. Felony convictions and crimes involving dishonesty may be used to attack a witness’s credibility. The court may exclude evidence of a conviction if its prejudicial effect substantially outweighs its probative value.

Are rape shield laws constitutional? Yes. Rape shield laws, which limit the admission of evidence about a victim’s sexual history, have been upheld against constitutional challenges. They balance the defendant’s right to present a defense against the victim’s privacy and dignity.

Criminal Defense Strategies — Appeal Process Criminal — Beyond a Reasonable Doubt