Dr. Matteson's explanation of how she knows when the victim is telling the truth makes sense if we view her testimony in this manner. In effect, Dr. Matteson testified that the victim is telling the truth about having sex with defendant, and this is how I know she is telling the truth. No matter whether we view her testimony this way, or as an opinion on the prosecuting witness's credibility in general, it was error to admit it at trial. Defendant did not object to this testimony at trial. He asks that we view the admission of this evidence as plain error.
Plain error is error that results in a miscarriage of justice or denies a defendant a fair trial.
It is fundamental to a fair trial that the credibility of the witnesses be determined by the jury. Our Courts have held numerous times that an expert's opinion to the effect that a witness is credible, believable, or truthful is inadmissible. This Court in Holloway determined that the admission of such an opinion is plain error when the State's case depends largely on the prosecuting witness's credibility. In this case there was no evidence of sexual intercourse other than the prosecuting witness's testimony. Therefore, her credibility was of critical importance.