What is the legal test for determining whether a witness is competent to testify?

The test for competency is not whether the witness is testifying truthfully, but whether the witness has the capacity to understand his duty to testify truthfully. (In re Crystal J. (1990) 218 Cal.App.3d 596, 602.) For example, in People v. Lyons (1992) 10 Cal.App.4th 837, the court held one of the victims in a sexual assault case was not competent to testify. Lyons held the trial court should have excluded her testimony because she was so delusional, and her testimony so contradictory and fantastic, that she lacked the ability to understand the duty to testify truthfully. The witness's testimony included claims that the defendant sexually assaulted her in an imaginary part of her body, he murdered two of her husbands, and he blew up a plane on which her husband was flying. There was also evidence the witness suffered from multiple personality disorder, and the trial court had been unable to determine which of the different personalities was testifying. (Id. at pp. 842-844.)