California, United States of America
The following excerpt is from People v. Dews, F060623 (Cal. App. 2013):
Generally, an expert may testify about the reasons for his opinions. (People v. Montiel (1993) 5 Cal.4th 877, 918.) "However, prejudice may arise if, '"under the guise of reasons,"' the expert's detailed explanation '"[brings] before the jury incompetent hearsay evidence."' [Citation.]" (Id. at pp. 918-919.) Thus, a trial court may "exclude from an expert's testimony any hearsay matter whose irrelevance, unreliability, or potential for prejudice outweighs its proper probative value." (Id. at p. 919, italics added.)
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