ER 601 -- General Rules of Competency. According to this rule, every person is presumed to be competent to testify. Generally, a witness is competent to testify if (1) he or she has personal knowledge of facts relevant to the case (ER 602), (2) understands the requirement that he or she tell the truth, and (3) be willing to take an oath or affirmation to tell the truth.
However, in Washington state, as in most states and federal courts, there can be limitations on the competency of certain witnesses to testify. If witnesses are not "competent", they can be excluded by the court from testifying.
People who are potentially not competent as witnesses are: (1) children; (2) the mentally impaired, whether by virtue of a mental disease, or one who is currently under the influence of a mind-altering substance. The burden of proving incompetence rests with the party opposing the witness.
Child Witnesses - Factors Relating to Competency
In order to be competent children must demonstrate that they have sufficient intelligence, the ability to remember and describe past events, and that they know the difference between the truth and a lie. The United States Supreme Court permitted child testimony in court for the first time over 115 years ago. If an attorney challenges the competence of a child, a hearing will be held (outside the presence of the jury) to determine whether the child is competent. The Judge will rule on the competence of the child.
Special considerations may be permitted by the court when a child is testifying. For example, while attorneys are not generally permitted to ask leading questions of witnesses, courts often relax this rule when children are testifying. Also, many states now permit children to testify by closed-circuit television so that they will not have to be in the same room as a person charged with abusing them.
Mentally Impaired - Factors Relating to Competency
As with any other witness, the competency of a witness with a perceived mental impairment may be challenged by counsel. Opposing counsel must establish that the witness has no personal knowledge of relevant facts, does not possess the ability to remember and describe past events, or that the witness does not know the difference between the truth and a lie. Since Evidence Rule 601 declares "every person" is competent as a witness, opposing counsel bears a heavy burden in challenging the testimony.
The fact that the witness with mental retardation has been found in a court proceeding to be incompetent to make informed decisions of a personal, medical or financial nature or that he is under a guardianship or conservatorship does not necessarily preclude that witness from being found competent to testify by the trial court.
A person who is under the influence of drugs or alcohol at the time of their testimony, however, may be found incompetent to testify.