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1. Experts Can Rely on Inadmissible Hearsay to Form Their Opinions.
Experts may rely on inadmissible hearsay in forming their opinions. Under Evidence Code Section 802, an expert witness may state on direct examination both the reasons for his or her opinion and the matters on which it is based. (See also People v. Catlin, (2001) 26 Cal.4th 81, 137.) The opinion may be based on matters “perceived by ... the witness ... before the hearing, whether or not admissible" if of a type that experts reasonably rely upon in forming such opinions. (Evidence Code § 801(b); Catlin, Id. at 137; People v. Dean, (2009) 174 CA4th 186, 193.)
An expert may rely on a non-testifying expert's report because it can come in as part of the information used by a doctor in “diagnosis and treatment." (Kelley v. Bailey, (1961) 189 Cal.App.2d 728, 738.) For example, it would be proper for a doctor to read portions of a specialist’s report if he used this report to formulate his own opinion as to plaintiff’s condition. Such a report would stand on a parity with patient's history of an accident and injuries given to his physician. (Id.) Such reports are not offered for their truth and thus are not hearsay.
(Id., (Specialist's report admissible “not as independent proof of the facts but as a part of the information upon which the physician based his diagnosis and treatment.")
Nonetheless, the court may exclude from an expert's testimony any hearsay matter whose irrelevance, unreliability or potential for prejudice outweighs its proper probative value. (Evidence Code Section 352; Catlin, 26 Cal.4th at 137.) Ultimately, the fact that the judge allows the expert to give his or her opinion does not end the matter. The jury determines whether the expert's opinion is based upon reliable information. (Evidence Code Section 405(b)(2), People v. Stoll, (1989) 49 Cal.3d 1136, 1155, (a jury may reject expert's opinion testimony if it questions matter upon which expert's conclusions were based.) Opposing counsel has the right to an instruction limiting the evidence to its proper purpose in that such evidence cannot be considered for its truth, but only for the limited purpose of showing the basis of the expert's opinion. (People v. Coleman, (1985) 38 Cal.3d 69, 92.) Where an expert is basing his or her opinion in whole or in part on inadmissible evidence, the judge, upon request, must instruct the jury that such inadmissible matters are to be considered only in determining the reliability of the opinion, and not for their truth. (Kelley, Id. at 738, Emphasis Added.) 
While an expert may state on direct examination the matters on which he or she relied, the expert may not testify as to the details of those matters if they are otherwise inadmissible. “He may not under the guise of reasons bring before the jury incompetent hearsay evidence." (Coleman, Id. at 69.) For example, experts may testify on direct that they consulted other experts in formulating their own opinion, but they cannot reveal the content of those other experts' opinions. (Whitfield v. Roth, (1974) 10 Cal.3d 874, 894, Emphasis added, (an expert witness cannot use inadmissible hearsay to “fortify" the expert's own opinion); Caitlin, Id. at 137–138.) This is because there is no opportunity to cross-examine the other experts as to the basis for their opinions. (People v. Campos, (1995) 32 CA4th 304, 308.)
Similarly, an expert may testify that they based their opinions on research and review of medical literature, but it is error to allow a doctor to testify as to details in individual case histories in medical journals to support his or her opinion. (Furtado v. Montebello Unified School Dist.,(1962) 206 Cal.App.2d 72, 79.)
Although an expert may testify on direct that his or her opinion as to the nature of an injury is based on the injured person's hearsay statements, such hearsay cannot be used to prove the cause of the injury. (In re Cheryl H., (1984) 153 Cal.App.3d 1098, 1120, (overruled on other grounds in People v. Brown, (1994) 8 C4th 746, 763.) For example, in Johnson v. Aetna Life Ins. Co., a doctor was able to rely on a patient’s statement that he slipped and fell and use it as the basis for his opinion as to the nature of the injury. ((1963) 221 Cal.App. 2d 247, 252.) However, this information was not admissible at trial to prove the patient actually slipped and fell. (Id.)
An expert may rely on a non-testifying expert’s opinion in forming their basis of their own opinion. They cannot testify as to the details of the information relied on or have the inadmissible hearsay admitted for its truth.
 There is no duty for the court to provide this instruction on its own motion. The party who wants the limiting instruction must request it. (Grimshaw v. Ford Motor Co., (1981) 119 Cal.App. 3d 757, 789.)