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.
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:
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
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.
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:
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.