Evidence

The most common form of evidence is the testimony of witnesses. A witness can be a person who actually viewed the crime or other event at issue, or a witness can be a person with other relevant information—someone who heard a dog bark near the time of a murder, or who saw an allegedly injured plaintiff lifting weights the day after his accident, or who shared an office with the defendant and can describe her character and personality. Any competent person may testify as a witness, provided that the testimony meets other requirements, such as relevancy.

The Federal Rules of Evidence contain broad competency requirements. To testify, a witness must swear or affirm that he or she will testify truthfully; possess personal knowledge of the subject matter of the testimony; have the physical and mental capacity to perceive accurately, record, and recollect fact impressions; and possess the capacity to understand questions and to communicate understandably, with an interpreter if necessary. When an issue of state law is being determined, the state rules of evidence govern the competency of a witness. States that have not adopted the Federal Rules of Evidence may have other grounds for INCOMPETENCY, such as mental incapacity, immaturity, religious beliefs, and criminal convictions. The Federal Rules of Evidence and most jurisdictions state that jurors and presiding judges are not competent to testify in the case before them.

To be admissible, testimony must be limited to matters of which the witness has personal knowledge, meaning matters that the witness learned about using any of his or her senses. Second, the witness must declare under oath or affirmation that the testimony will be truthful. The purpose of this requirement is to "awaken the witness' conscience and impress the witness' mind with the duty to [be truthful]" (Fed. R. Evid. 603). The oath or affirmation requirement also serves as a ground for perjury if the witness does not testify truthfully. Although the oath frequently invokes the name of God, the witness need not possess any religious beliefs; a secular affirmation is sufficient.

Witnesses may be called to testify by any party to the lawsuit. The party who calls a witness to testify generally questions the witness first, in what is known as direct examination. The judge may exercise reasonable control over the questioning of witnesses in order "(1) to make the interrogation and presentation effective for the ascertainment of the truth; (2) to avoid needless consumption of time, and (3) to protect the witnesses from harassment, or undue embarrassment" (Fed. R. Evid. 611(a)). Thus, the judge may prevent a witness from rambling in a narrative fashion and may require an attorney to ask specific questions in order to ascertain the truth quickly and effectively.

The federal rules and most jurisdictions discourage the use of leading questions on direct examination. These are questions that are designed to elicit a particular answer by suggesting it. For example, the question "Didn't the defendant then aim the gun at the police officer?" is a leading question, and normally it would not be permitted on direct examination. By contrast, "What did the defendant do next?" is a nonleading question that would be permitted on direct examination. In most cases, questions that can be answered with either "Yes" or "No" are considered to be leading questions. Courts generally will permit leading questions during direct examination if the witness is adverse or hostile toward the questioning party.

Leading questions are permitted, and are common practice, during cross-examination. Once a party conducts a direct examination, the opposing party is entitled to cross-examine the same witness. The scope of questions asked during cross-examination is limited to the subject matter that was covered during direct examination, and any issues concerning the witness's credibility. Attorneys use cross-examination for many purposes, including eliciting from a witness favorable facts; having the witness modify, explain, or qualify unfavorable versions of disputed facts elicited during direct examination; and impeaching, or discrediting, the witness.

If a witness is a lay witness (i.e., not testifying as an expert), the witness generally may testify as to facts and not as to opinions or inferences, unless the opinions or inferences are "(a) rationally based on the perception of the witness and (b) helpful to a clear understanding of the witness' testimony or the determination of a fact in issue" (Fed. R. Evid.). For example, a witness may not testify that she smelled marijuana unless she can sufficiently establish that she knows what marijuana smells like. Lay witnesses commonly testify about such things as the speed that a car was going, or someone's approximate age, but these types of inferences are less likely to be permitted the more closely they address critical issues in the case.