Haw. Rev. Stat. § 603.1
A person is disqualified to be a witness if the person is (1) incapable of expressing oneself so as to be understood, either directly or through interpretation by one who can understand the person, or (2) incapable of understanding the duty of a witness to tell the truth.
HRS § 603.1
RULE 603.1 COMMENTARY
The intent of this rule, which is similar to Cal. Evid. Code §701, is to complement Rule 601 supra, and to require disqualification of witnesses whose incapacity either to articulate in an understandable fashion or to understand the truthtelling obligation renders their testimony valueless.
Under this rule the competency of a witness is a matter for determination by the court. Competency has traditionally embodied a level of threshold capacity "to understand the oath and to perceive, recollect, and communicate that which he is offered to relate." Law Revision Comm'n Comment to Cal. Evid. Code §701. Capacity to perceive and to recollect are implicit in Rule 602's personal knowledge requirement. This rule covers the oath and the ability to communicate, matters which may be of concern in cases of youthful or mentally infirm witnesses.
This rule generally restates existing Hawaii law. A superseded statute, Hawaii Rev. Stat. § 621-16 (1976) (repealed 1980) (originally enacted as L 1876, c 32, §50; am L 1972, c 104, §1(k)), provided that the court could "receive the evidence of any minor; provided, that the evidence of the minor is given upon his affirmation to tell the truth...; provided also, that no such evidence shall in any case be received unless it is proved to the satisfaction of the court...that the minor perfectly understands the nature and object of the affirmation...." To the extent that the previous law required a "perfect" understanding, the current rule effects a liberalization of the competency standard for children. In Republic v. Ah Wong, 10 H. 524, 525 (1896), the court said: "There is no precise age within which children are excluded from testifying. Their competency is to be determined, not by their age, but by the degree of their knowledge and understanding."
In Territory v. Titcomb, 34 H. 499, 502 (1938), the court announced that "the proper test must always be, does the lunatic understand what he is saying, and does he understand the obligation of an oath?.... [I]f he can stand the test proposed, the jury must determine all the rest." Rule 603.1 is consistent with the Ah Wong and Titcomb decisions.
Issue of complainant's competency to testify was reasonably called into question, and trial court committed plain error in failing to engage in independent inquiry and make an express finding as to whether complainant was competent to testify before allowing complainant's substantive testimony to be exposed to jury; supreme court not convinced beyond reasonable doubt that error harmless.74 Haw. 479,849 P.2d 58. Testimonial capacity of mentally defective person. 5 H. App. 659, 706 P.2d 1333.