California, United States of America
The following excerpt is from People v. Robles, F068542 (Cal. App. 2016):
Moreover, an expert "is not allowed to give an opinion on whether a witness is telling the truth because the determination of credibility is not a subject sufficiently beyond common experience that the expert's opinion would assist the trier of fact. [Citations.]" (People v. Long (2005) 126 Cal.App.4th 865, 871; see also People v. Sergill (1982) 138 Cal.App.3d 34, 39-40 [lay opinion about the veracity of a witness is inadmissible].)
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