Evidence in Criminal Trials

During criminal trials, evidence rules restrict both the content of evidence presented and the manner that evidence can be presented during a trial. Evidence rules not only ensure the smooth running of a criminal trial, but also, protect a defendant's right to a fair trial. Typically, rules of evidence are set forth on a state-by-state basis, however, since the Federal Rules of Evidence were established, nearly forty states abide by these regulations. Additionally, judges are not required to strike or restrict violations of evidence rules on their accord, but rather, it is the duty of the defense or prosecution to challenge actions potentially violating rules of evidence.

Rules Regulating Testimony

The premier reason testimony is presented during a trial is to influence the opinion of a judge or jury that is acting as the decider of the facts. Therefore, certain rules and methods for offering testimony in a trial are enforced to ensure a fair trial for defendants. Some of the more notable rules regulating testimony during a criminal trial, include:

  • Testimony submitted must be deemed logically connected to the issues at hand to be deemed admissible in the court. On the contrary, however, not all logically connected evidence may be deemed admissible at all times.
  • The "personal knowledge rule" of the Federal Rules of Evidence requires witnesses to testify only on information they received or witnessed firsthand.
  • The "mercy rule" allows defendants to actually testify and have others testify to their own personal good character
  • Typically, prosecutors cannot enter "bad character evidence" unless the defendant has entered "good character evidence"
  • Witnesses during a trial can have their own personal character attacked or scrutinized regarding matters involving honesty or criminal activity to assess their credibility as a witness
  • Defendants can protect their right to avoid informing jury members of their past criminal convictions, if they do not enter good character evidence, which if done, can be disputed by the prosecution using a defendant's past convictions
  • Rape shield laws essentially protect a victim or accuser's right to avoid being subjected to questions into their sexual history deemed irrelevant to the case
  • Hearsay rules prevent non-firsthand statements from being admitted to the court, as well as preventing statements being made without allowing either side to cross-examine the person making these statements

Expert statements or testimony can be entered into a trial, which may include personal opinions and references to previous testimony of other witnesses. Additionally, expert witnesses can be paid for their time in court.

Prosecutors typically already have experts in various fields on payroll prior to a trial, but an indigent defendant with court-appointed counsel can obtain expert testimony at no cost if the presiding judge feels that without the expert, a fair trial will not prove possible.

The "chain of custody" rules regulate the admissibility and credibility of evidence during a trial to ensure evidence was not tampered with or somehow altered prior to trial

Rules Regarding Scientific Evidence

Scientific evidence, or forensic evidence, is information derived through the "scientific method". Commonly, scientific evidence, such as DNA, fingerprints, ballistics, and other items, is regularly entered during a criminal trial by both sides. Contrary to some beliefs, polygraph evidence is not typically deemed reliable enough to be admitted to a criminal trial, nor are most statements made under hypnosis or other forms of altered consciousness. The ability to admit scientific evidence, however, is at the discretion of the presiding judge, who must consider the validity of the evidence, the credibility of the science behind it, and how influential each piece of evidence may prove during a given case. Typically, turning to the "chain of custody" rules may immediately put into question the validity of an admitted piece of evidence. Additionally, this piece of evidence may have undergone scientific testing that returned a given result, which can be ruled inadmissible if the "chain of evidence" was not properly followed. Additionally, disputes over admitting evidence are typically heard during a "minitrial", which allows the jury to leave, while a decision to admit or suppress a given piece of evidence is established. This "minitrial" event prevents jurors from being influenced by evidence, which may be inadmissible.

Rules Regarding Confidential Information

During a criminal trial, evidence rules may contain "privileges", which prevent disclosing private information exchanged during privileged relationships. State laws greatly vary regarding what constitutes privileged information, however, all states grant privileges for communications between:

  • Spouses
  • Medical doctors and their patients
  • Attorneys and their clients
  • Ministers and their congregants

Additionally, some jurisdictions respect the privilege of journalists and their informants, as well as psychotherapists and their clients. In essence, privileged communications do not need to be disclosed by the holder, nor can the other party release this information without the consent of the holder. Privilege communication, however, is not always protected. For instance, if a third party overhears this information it may prove admissible, a client telling their attorney an intention to commit a future crime cannot be kept confidential, or if the holder of the privilege elects to also include a third party in the communication.


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