The competency of a witness depends upon the circumstances that exist when the testimony is given.  Age, mental power, and capacity to understand the nature and obligation of an oath are the tests that determine the competency of a witness.  An expert witness should possess special knowledge of the subject on which the jury’s knowledge would be inadequate without expert assistance.

The competency of a witness is distinguished from credibility of a witness and from a witness’s possession of knowledge sufficient to enable him or her to testify concerning a specified matter.

In federal courts, the Federal Rules of Evidence apply.  However, in civil actions and proceedings, the competency of a witness is determined according to the state law.  The Federal Rules of Evidence is consulted when determining a defendant’s mental competency to stand trial.

A person can be a witness if s/he has sufficient intelligence to understand the nature of an oath and to give a reasonably accurate account of what the person has seen and heard regarding the matter in question.  A competent witness should be capable of receiving, remembering, and narrating impressions.  A witness should also be sensible to the obligation of an oath before the person can be permitted to testify.

The trial court has the irreversible discretionary power to determine the competency of a witness.  However, if the ruling constitutes an abuse of discretion or was plainly erroneous the decision can be reversed.

The general presumption is that a person is competent to be a witness.  A mature person of normal appearance and demeanor offered as a witness is presumed to be a competent witness.  Unless a mature person comes under exceptions provided in statutes, a person is presumed to be competent to testify.

A person should only testify to those facts that the person knows personally.  Facts should not be given that are beyond the personal knowledge of the person.  When two witnesses have equal means to have knowledge about a fact, they are equally competent to be witnesses in a case[i].  Even if the person had acquired knowledge about a fact in an unlawful manner, the witness will be considered competent[ii].  If a witness is biased in favor of one litigant, also the person will be considered a competent witness.

Negative testimony is admissible in courts.  A witness is considered competent to testify about the absence of a thing or the nonappearance of a matter when it can be proved that the person was in a position to see and hear the thing inquired about.  A person will be competent to testify even if the person has only heard a part of a conversation.  However, s/he will be competent to testify only about that part heard or understood.

Generally, the evidence provided by witnesses when information is derived from statements or writings of others is excluded.  However, there are certain exceptions to the rule.  A banker can testify about the details of a bank account, or transactions although the information is derived from the books of the bank[iii].

A person’s testimony should not be excluded because the witness has a hearing problem[iv].  If the witness’s answers to the questions were responsive, then that person’s evidence need not be excluded.  Deaf and mute persons are also competent witnesses.  If deaf and mute persons are able to communicate the matter and are of sufficient mental capacity to observe the matters as to which they will testify and to appreciate the obligation of an oath their evidence will have value.  However, trial courts can have interpreters for deaf and mute witnesses.  Their evidence will be considered as direct evidence and not hearsay evidence.

A witness cannot be considered incompetent because of intoxication.  However, if the person was virtually unconscious at the time of the event, the person can be considered incompetent.  Persons intoxicated at the time they are offered as witnesses are excluded from testifying.  Use of drugs also does not render a witness incompetent.  However, addiction to drugs can affect the credibility and weight of the person’s testimony.

When a person is insane or mentally ill, it does not automatically render him/her an incompetent witness[v].  The court tests whether the person affected with insanity has sufficient knowledge to apprehend the obligation of an oath and whether h/she is capable to give a correct account of the matters which the person has seen or heard, then the person can be considered a competent witness.  The person should also be capable to perceive and narrate.

The testimony of a convicted felon cannot be considered inadmissible.  However, the weight of the evidence can be questioned.  In federal court, even a convicted perjurer is competent to testify.

In most of the states, the general disqualification by interest is excluded.  Interest in the outcome of the litigation does not render a person incompetent to testify.  The credibility of the witnesses is determined by the courts or the jury.  A party to a case can be a competent witness on their own behalf or can be compelled to testify for others.  The defendant in a criminal case cannot be compelled to be a witness against himself/herself.  However, the defendant is entitled to be sworn as a witness and can testify on his/her own behalf.  If a witness has an interest in the outcome of the litigation s/he will be incompetent to testify.

A child can be a competent witness if the child has an understanding of the obligation to speak the truth on the witness stand, the mental capacity at the time of the occurrence concerning which s/he is to testify to receive an accurate impression of it, a memory sufficient to retain an independent recollection of the occurrence, the capacity to express in words his/her memory of the occurrence, and the capacity to understand simple questions about the matter.  A child of any age can testify if the child meets the statutory requirements of a competent witness.

An attorney can be a competent witness on behalf of his/her client.  In exceptional cases, and in the absence of disqualifying interest, an attorney for a case can testify without withdrawing from the litigation.  An attorney can also be a competent witness against his/her client.  With regard to privileged communications, an attorney cannot testify against his/her clients.

The common law rule is that neither party to a marriage can be a witness in favor of or against the other, in a suit to which the other is a party, or has a direct or immediate interest[vi].  In a criminal proceeding in which one spouse is the defendant, the other is not a competent witness either for or against the defendant.  However, if the case is brought by one spouse against the other spouse, spouses are competent to testify against each other.

A judge is not incompetent to testify as a witness in a cause not on trial before him/her.  However, a judge presiding in a trial cannot be a witness in the same trial.  It is because a judge should avoid not only impropriety but the appearance of impropriety[vii].  Arbitrators can be made competent witnesses in the same cause.  Judges cannot act as expert witnesses because it would be prejudicial to the other party against whom the judge gives expert judicial opinions.

[i] United States v. Taveras, 585 F. Supp. 2d 327 (E.D.N.Y. 2008).

[ii] NLRB v. Western Clinical Laboratory, Inc., 571 F.2d 457 (9th Cir. 1978).

[iii] Kincaid v. Wade, 196 Kan. 174 (Kan. 1966).

[iv] Geiger v. Pfizer, Inc., 2009 U.S. Dist. LEXIS 34982 (S.D. Ohio Apr. 15, 2009).

[v] Commonwealth v. Ware, 459 Pa. 334 (Pa. 1974).

[vi] Bowlin v. Commonwealth, 65 Ky. 5 (Ky. 1867).

[vii] Spector v. State Com. on Judicial Conduct, 47 N.Y.2d 462 (N.Y. 1979).