An expert witnesses may not offer opinions on relevant events based on their personal assessment of the credibility of another witness's testimony. The credibility of witnesses is exclusively for the determination by the jury, United States v. Richter, 826 F.2d 206, 208 (2d Cir. 1987), and witnesses may not opine as to the credibility of the testimony of other witnesses at the trial.
Even an expert witnesses possessed of medical knowledge and skills that relate directly to credibility may not state an opinion as to whether another witness is credible, United States v. Azure, 801 F.2d 336, 340-41 (8th Cir. 1986), although such witnesses may be permitted to testify to relevant physical or mental conditions. See generally Annotation, Necessity and Admissibility of Expert Testimony as to Credibility of Witnesses, 20 A.L.R.3d 684 (1968).
Expert testimony assists the trier of fact by providing 'a resource for ascertaining truth in relevant areas outside the ken of ordinary laity.' Townsend, 103 Nev. at 117, 734 P.2d at 708. Specialized knowledge which is the proper subject of expert testimony is knowledge not possessed by the average trier of fact who lacks the expert's skill, experience, training, ...