Can an expert testify about the credibility of a witness who has changed his story?

gang witness who has changed his story is lying. (Id. at p. 947.) However, an expert witness may not testify about a specific witness at trial and ultimately offer an opinion about his or her credibility. (See People v. Vang (2011) 52 Cal.4th 1038, 1047 ["Obviously, there is a difference between testifying about specific persons and about hypothetical persons."].)

We see little difference between an expert witness offering an opinion that states that a specific witness was afraid to testify and thus lying during his testimony and offering an opinion as to the guilt of a defendant. "A witness may not express an opinion on a defendant's guilt. [Citations.] The reason for this rule is not because guilt is the ultimate issue of fact for the jury, as opinion testimony often goes to the ultimate issue. [Citations.] 'Rather, opinions on guilt or innocence are inadmissible because they are of no assistance to the trier of fact. To put it another way, the trier of fact is as competent as the witness to weigh the evidence and draw a conclusion on the issue of guilt.' " (People v. Coffman and Marlow (2004) 34 Cal.4th 1, 77.)