Evidence Essay

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Evidence is the raw material of the criminal justice system, and it comes in many sizes, shapes, and types. At the core, it is the proof on which the fact finder in a criminal trial—judge or jury—bases its verdict or decision of guilty or not guilty. In the modern U.S. criminal justice system, evidence and its admissibility in court are guided by rules of evidence adopted by the U.S. Supreme Court and by state supreme courts. Ideally, those rules are designed and intended to ensure that only relevant and reliable evidence comes before the fact finder so verdicts are not based on irrelevant or unfairly prejudicial evidence.

Key Categories of Evidence

Although there are a number of different general concepts and categories of evidence, four sets of definitions are essential to understanding evidence concepts. First, one should differentiate between direct evidence and circumstantial evidence. Direct evidence is that which is directly seen, heard, felt, smelled, or otherwise sensed. For example, if a witness saw snow falling on the night of the crime, if that witness testified that the snow fell that night, that witness would be offering direct evidence that snow fell that night. On the other hand, if a witness saw no snow on the ground the day before the crime, but saw snow on the ground the next morning, if that witness offered testimony about the before and after observations, that witness would be offering circumstantial evidence that snow fell the night of the crime. Although the law purports to not favor direct or circumstantial evidence over the other, juries often subconsciously prefer direct evidence.

Second, one should differentiate between testimonial evidence and tangible evidence. Testimonial evidence includes oral and written testimony given, under oath, by a witness at a trial or deposition. Typically, that testimony is presented subject to cross-examination by the opposing attorney. Tangible evidence is predominantly exhibits offered and received into evidence in court. Tangible evidence is not subject to cross-examination to test its truthfulness or import; however, tangible evidence that would mislead or not be helpful to the fact finder at trial will be excluded from trial by the presiding judge, whose role is to ensure that only admissible evidence comes into court.

Third, one should distinguish between real evidence and demonstrative evidence. Real evidence is tangible evidence that was directly involved the crime and related events leading up to trial. For example, the actual gun used in a shooting is real tangible evidence. Demonstrative evidence is tangible evidence that was not directly involved in the crime or related events, but would be useful for the fact finder to consider during the trial. For example, a diagram of a traffic accident scene would be demonstrative tangible evidence helping the jury consider the relative positions of the vehicles during the accident.

Finally, one should distinguish between lay testimony and expert testimony. Lay testimony may be offered by anyone who personally observed the events the witness is describing from the witness stand. Expert testimony, on the other hand, does not involve direct observations. Rather, experts are permitted to testify about their educated, experienced analyses about how to interpret others’ observations at the scene. Although the expert did not witness the crime, the expert may be allowed to testify when the expert’s opinion is within the expert’s training, experience, and expertise, so long as that expert testimony is deemed helpful to the fact finder in interpreting the other testimony and exhibits in the case.

Only Relevant and Reliable Evidence Is Admissible

The judge presiding at the criminal trial stands as the gatekeeper, deciding which evidence the fact finder is allowed to hear and see. And the judge’s decisions on evidence are guided by rules of evidence. Those rules, adopted by each jurisdiction’s supreme court, specify what types of evidence are admissible, and what types are not. Most states have patterned their evidence rules on the federal rules of evidence, which apply in federal courts.

Rules of evidence are designed to allow only relevant evidence to be admitted in court. Relevant evidence is evidence of any of the types discussed above that make any fact important to the case more or less likely. For example, evidence that the defendant was angry at the victim before the crime is relevant to whether the defendant committed the crime. However, evidence that the offender used drugs the previous year, if not related at all to the crime, would not be considered relevant or admissible—that does not make any fact in the case more or less likely.

Rules of evidence are also designed to allow only reliable evidence to be admitted in court. If a potential witness was unable reliably to perceive the crime, or unable to remember it well, or unable to relate it clearly in court, then testimony from that potential witness would be unreliable, and would likely not be admitted in court. Evidence from a potential expert witness, who plans to testify about his findings based on a new branch of scientific testing that no other experts view as recognized in the field, would be deemed unreliable, and would not be admitted in court.

Irrelevant evidence and unreliable evidence, if admitted in court, would confuse or distract the fact finder from the relevant and reliable evidence in the case and thus could compromise the defendant’s right to a fair trial.

Key Categories of Admissible and Inadmissible Evidence

Evidence rules can almost always be understood by considering three factors; does the rule (1) make logical sense, (2) “feel” right, and (3) avoid unnecessary expenditures of time and money. Generally, a rule of evidence must meet all three factors, which may be presented in shorthand as the head, heart, and wallet factors. Although just five categories are considered here, the head, heart, and wallet factors may be applied to all rules of evidence.

First, evidence of a defendant’s character traits is inadmissible to prove the defendant acted in conformity with such character traits when committing the crime at issue. For example, if the defendant has past misdemeanor convictions for a series of bar fights, those past convictions would not be admissible in a domestic assault case to show that the defendant has an assaultive character, and his assaultive character led to the domestic assault incident at issue. Under the head factor, it makes sense that prior bar fights are not relevant to whether the defendant committed this domestic assault. Under the heart test, it does not seem fair to have the new jury dwell on past events that may have no relevance to the new domestic assault. Finally, applying the wallet test, it is a waste of time and money to admit irrelevant and unfairly prejudicial evidence at trial.

Second, evidence of a defendant’s past convictions for crimes of dishonesty and false statement, such as perjury, are admissible against the defendant if the defendant testifies. Under the head test, that is logical because truthfulness is a critical trait for judging all witnesses’ testimony, including a testifying defendant. Under the heart test, it feels right to hold the defendant accountable for past convictions when those convictions are relevant to truthfulness. Finally, under the wallet test, admitting that testimony permits the system to work more efficiently because it allows the fact finder to consider this key untruthfulness evidence.

Third, evidence of the defendant’s prior bad acts is admissible, whether each resulted in a conviction or not, to show a common plan or scheme. For example, in a child sexual assault case where the defendant had engaged in grooming behavior with the current victim, the prosecutor would be allowed to offer proof that the defendant had previously used the same grooming behavior with each of his prior child sex assault victims. Under the head test, that makes logical sense. It is relevant that the defendant consistently follows the same modus operandi in grooming his victims. Under the heart test, it feels right to hold the defendant accountable for such a pattern of grooming behavior, where it is a true pattern. Finally, under the wallet test, admitting grooming pattern evidence will allow a more efficient process by giving the fact finder all the relevant evidence in every trial.

Fourth, hearsay evidence is inadmissible unless it fits into one of the recognized exceptions. Hearsay evidence is an out-of-court statement offered to prove the truth of the matter asserted. Typically, hearsay evidence is a statement made by a person who does not testify, and instead, one who overheard the speaker’s statement is put on the witness stand to testify about what was overheard. That is typically inadmissible. Under the head test, it makes sense to exclude hearsay evidence because, since the speaker does not testify, the speaker cannot be cross-examined about the statement and cannot explain what the speaker meant by making that statement. Thus, that hearsay evidence is unreliable. Under the heart test, it feels right to exclude that evidence because no one can say exactly what the speaker meant, so it feels wrong to allow that statement into evidence. Under the wallet test, although it is perhaps inefficient in a single case to exclude that hearsay evidence, at the overall level it is more efficient to exclude unreliable hearsay. Allowing it would result in postconviction appeals that would increase the overall system cost.

Finally, one exception to the hearsay inadmissibility rule arises when the out-of-court statement was made by the speaker to a medical professional when the speaker was seeking diagnosis and treatment. Under the head test, that is logical. When a person seeks medical diagnosis or treatment, the person does not try to mislead the medical professional when describing the symptoms; thus, it makes sense that although hearsay is typically presumed to be irrelevant, that presumption of unreliability is overborne by the high likelihood the statement to the doctor was reliable. Under the heart test, it feels right to allow that hearsay evidence, but only if the statement is related to diagnosis or treatment.

For example, if the speaker told the doctor (1) “I was robbing that bank,” when (2) “I was shot three times in the leg,” the statement in (2) would be admissible since it was related to diagnosis and treatment, but the statement in (1) would not be admissible since it was not relevant to diagnosis or treatment. Finally, under the wallet test, where an out-of-court statement is particularly reliable, such as a statement to a doctor while seeking diagnosis and treatment, it would be inefficient to preclude the fact finder from hearing and considering that clearly relevant and reliable evidence.

Rules of evidence, designed to make sense, feel right, and not be unduly expensive to apply, guide the presiding judge’s decisions as to which evidence is admissible and which is not. Only relevant and reliable evidence should be considered in criminal courts; otherwise, the fact finder would be distracted or confused or worse, and the defendant’s right to a fair trial would be compromised.


  1. Broun, Kenneth, et al., eds. McCormick’s Evidence, 6th ed. Eagan, MN: West, 2006.
  2. Imwinkelried, Edward J., et al. Courtroom Criminal Evidence, 5th ed. New Providence, NJ: LexisNexis,
  3. Park, Roger. The New Wigmore: A Treatise on Evidence, New York: Aspen, 2013.
  4. Weinstein’s Evidence Manual, 6th ed. New York: Matthew Bender, 2012.
  5. Wharton’s Criminal Evidence. Eagan, MN: West, 2008. Wright, Charles Alan and Arthur R. Miller. Federal Practice and Procedure. 4th ed. Eagan, MN: West, 2007–2013.

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