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California expert witness law requires attorneys to consider more than an expert’s qualifications, including the foundation, reasoning, scientific methodology, hypothetical assumptions, and source material supporting an opinion. This article examines seven essential California Supreme Court cases that shape expert admissibility and explains how their principles can guide expert selection, preparation, examination, and challenges.
Expert witnesses can play an important role in California litigation. Depending on the case, an expert may be asked to address issues such as medical causation, economic losses, accident reconstruction, engineering questions, scientific evidence, or other matters requiring specialized knowledge.
But credentials alone do not make an expert's opinion admissible. California courts may examine whether the witness is qualified, whether expert testimony will assist the trier of fact, whether the opinion rests on an appropriate foundation, and whether the reasoning connecting that foundation to the expert's conclusion is sound.
Several California Supreme Court decisions have shaped how these principles work in practice. Some apply broadly across civil and criminal litigation. Others address more specialized questions involving new scientific techniques, hypothetical questions, or an expert's use of hearsay.
For California attorneys selecting, preparing, examining, or challenging expert witnesses, the following cases provide an important starting point.
Before turning to the cases, it helps to understand the statutory framework.
California Evidence Code section 720 provides that a person may qualify as an expert through sufficient special knowledge, skill, experience, training, or education relating to the subject of the proposed testimony.
Evidence Code section 801 generally limits expert opinion testimony to opinions that concern a subject sufficiently beyond common experience that expert testimony would assist the trier of fact and that rest on matter personally known to or perceived by the expert, or made known to the expert before the hearing, of a type that experts reasonably may rely upon in forming an opinion on the subject, unless reliance is otherwise prohibited by law.
Evidence Code section 802 permits examination into the reasons for an expert's opinion and the matter on which it is based.
Together, these provisions establish important qualification and foundational requirements for expert testimony. As Sargon explains, sections 801 and 802 also support the trial court's gatekeeping role in evaluating expert opinions.
Other Evidence Code provisions may impose additional requirements depending on the issue presented. For example, Evidence Code section 801.1, operative January 1, 2024, addresses certain competing expert testimony concerning medical causation in general civil cases.
Expert witnesses can play an important role in California litigation. Depending on the case, an expert may be asked to address issues such as medical causation, economic losses, accident reconstruction, engineering questions, scientific evidence, or other matters requiring specialized knowledge.
But credentials alone do not make an expert's opinion admissible. California courts may examine whether the witness is qualified, whether expert testimony will assist the trier of fact, whether the opinion rests on an appropriate foundation, and whether the reasoning connecting that foundation to the expert's conclusion is sound.
Several California Supreme Court decisions have shaped how these principles work in practice. Some apply broadly across civil and criminal litigation. Others address more specialized questions involving new scientific techniques, hypothetical questions, or an expert's use of hearsay.
For California attorneys selecting, preparing, examining, or challenging expert witnesses, the following cases provide an important starting point.
Before turning to the cases, it helps to understand the statutory framework.
California Evidence Code section 720 provides that a person may qualify as an expert through sufficient special knowledge, skill, experience, training, or education relating to the subject of the proposed testimony.
Evidence Code section 801 generally limits expert opinion testimony to opinions that concern a subject sufficiently beyond common experience that expert testimony would assist the trier of fact and that rest on matter personally known to or perceived by the expert, or made known to the expert before the hearing, of a type that experts reasonably may rely upon in forming an opinion on the subject, unless reliance is otherwise prohibited by law.
Evidence Code section 802 permits examination into the reasons for an expert's opinion and the matter on which it is based.
Together, these provisions establish important qualification and foundational requirements for expert testimony. As Sargon explains, sections 801 and 802 also support the trial court's gatekeeping role in evaluating expert opinions.
Other Evidence Code provisions may impose additional requirements depending on the issue presented. For example, Evidence Code section 801.1, operative January 1, 2024, addresses certain competing expert testimony concerning medical causation in general civil cases.
Case: Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747.
Sargon is one of California's most important modern decisions concerning expert testimony.
The dispute involved a dental implant company seeking substantial lost-profit damages after alleging that the University of Southern California breached a contract to clinically test its implant. The company's damages expert projected lost profits ranging into hundreds of millions of dollars.
The California Supreme Court upheld the exclusion of the testimony and discussed the trial court's gatekeeping responsibility under Evidence Code sections 801 and 802.
Sargon makes clear that a court does not simply ask whether an expert has impressive credentials. The court may examine the material on which the expert relies and the reasoning connecting that material to the expert's conclusions.
At the same time, gatekeeping does not authorize a trial court to choose between competing reasonable expert opinions merely because it finds one more persuasive. The inquiry focuses on whether the opinion has an adequate foundation and reasoned analytical basis.
For counsel, that means examining what information supports each material opinion, why experts in the field reasonably rely on it, what methodology or reasoning was used, and whether the conclusions logically follow from the underlying material.
Case: People v. Kelly (1976) 17 Cal.3d 24.
California lawyers dealing with evidence based on a potentially new scientific technique should know People v. Kelly.
The case involved spectrographic voice identification, commonly called "voiceprint" evidence. The decision became the foundation of what is commonly known as California's Kelly test for evidence produced by a new scientific technique.
Where Kelly applies, the proponent must satisfy the applicable Kelly requirements, including the required showing of general acceptance in the relevant scientific community, appropriate expert qualifications, and proper use of the technique in the particular case.
Importantly, Kelly is not a universal test for every expert opinion. Not every disagreement involving scientific evidence presents a Kelly issue. Counsel should distinguish a qualifying new scientific methodology from disputes concerning the application, interpretation, or weight of an established technique.
Case: People v. Leahy (1994) 8 Cal.4th 587.
After the U.S. Supreme Court decided Daubert v. Merrell Dow Pharmaceuticals, Inc. in 1993, California courts faced an important question: Would California abandon its existing approach to new scientific techniques?
In People v. Leahy, which involved horizontal gaze nystagmus testing in a driving-under-the-influence prosecution, the California Supreme Court retained the Kelly formulation for new scientific methodology falling within its scope.
California state courts therefore do not simply apply the federal Daubert framework as the governing state-law test for expert testimony. California has its own Evidence Code and body of expert-admissibility law.
Federal courts sitting in California, by contrast, apply the Federal Rules of Evidence to expert admissibility in federal proceedings. For lawyers practicing in both state and federal courts, identifying the governing framework at the beginning of an expert assignment can prevent confusion later.
Case: People v. Vang (2011) 52 Cal.4th 1038.
Expert witnesses are often asked hypothetical questions, and People v. Vang explains an important rule governing their use.
The California Supreme Court held that hypothetical questions posed to an expert may closely track the evidence presented at trial. Counsel does not have to disguise the relationship between the hypothetical and the facts of the case.
The hypothetical, however, must be rooted in facts shown by the evidence. It cannot properly depend on unsupported assumptions, speculation, or conjecture.
Vang also illustrates the distinction between an expert answering a properly supported hypothetical and an expert directly opining on what particular defendants actually did or intended. The court did not establish a categorical rule governing every circumstance in which testimony concerning a specific defendant may be permissible.
When constructing a hypothetical, counsel should identify the evidence supporting each important assumption and anticipate challenges to assumptions that may lack adequate evidentiary support.
Case: People v. Sanchez (2016) 63 Cal.4th 665.
People v. Sanchez significantly affected the treatment of expert basis testimony in California.
The case addressed a gang expert who related case-specific information obtained from police documents and other sources. The California Supreme Court drew an important distinction between an expert's general background knowledge and case-specific hearsay.
An expert may rely on hearsay in forming an opinion when permitted by the Evidence Code and may generally explain that such material contributed to the opinion. But an expert may not relate case-specific facts asserted in hearsay statements as true unless those facts are independently proven by competent evidence or are covered by an applicable hearsay exception.
In a criminal prosecution, testimonial hearsay presented through a prosecution expert may also raise separate Confrontation Clause issues under governing federal constitutional law.
For counsel, the key is to distinguish general background information, facts personally known to the expert, case-specific facts established through admissible evidence, and case-specific hearsay the expert has merely read or been told.
Case: People v. Veamatahau (2020) 9 Cal.5th 16.
People v. Veamatahau helped clarify the distinction developed in Sanchez.
The case involved expert testimony identifying pills as alprazolam. The California Supreme Court concluded that the expert did not relate inadmissible case-specific hearsay. The case-specific information about the markings on the seized pills came from the expert's own observations, while a pharmaceutical database supplied general information used to interpret those observations.
General background information that does not describe the particular people or events involved in the case is analytically different from an out-of-court statement offered to establish what happened in the specific case.
Classifying information as general background rather than case-specific does not, however, end every admissibility inquiry. The expert's reliance and testimony remain subject to applicable Evidence Code requirements.
Case: People v. Valencia (2021) 11 Cal.5th 818.
People v. Valencia further developed the Sanchez framework in the context of gang predicate offenses.
In Valencia, the California Supreme Court held that the predicate offenses required for the gang allegations had to be proven by independently admissible evidence. Under Sanchez, those facts could not be established solely through testimony from an expert who lacked personal knowledge of the facts necessary to satisfy the prosecution's burden.
More generally, an expert's ability to rely on otherwise inadmissible material does not permit the expert to prove case-specific facts for their truth by repeating inadmissible hearsay.
For litigators, one useful preparation technique is to create an evidence map identifying which facts the expert personally knows, which will be established through other witnesses or admissible exhibits, which information constitutes permissible background knowledge, and which statements may create hearsay or other admissibility problems.
The same exercise can help when evaluating an opposing expert by allowing counsel to compare each material opinion with the evidence and assumptions supporting it.
Taken together, these decisions illustrate several recurring principles:
These principles can guide expert selection, discovery, deposition preparation, admissibility motions, direct examination, and cross-examination.
Reviewing potential admissibility issues before trial gives counsel an opportunity to address weaknesses in an expert's qualifications, foundation, assumptions, methodology, or proposed basis testimony.
Counsel should consider:
This framework can also be used when evaluating an opposing expert. Counsel can work backward from each important conclusion and examine the evidence, assumptions, and reasoning necessary to support it.
California expert witness law cannot be reduced to a single admissibility test.
Evidence Code sections 720, 801, and 802 provide core statutory rules governing expert qualifications, permissible opinion testimony, and examination of an expert's reasons and basis. Other provisions, including section 801.1 in applicable general civil medical-causation cases, may also matter.
Sargon emphasizes scrutiny of expert foundation and reasoning. Kelly and Leahy address qualifying new scientific techniques. Vang explains important principles governing evidence-based hypothetical questions. Sanchez, Veamatahau, and Valencia establish important limits involving case-specific hearsay and expert background information.
For attorneys, selecting a qualified expert is only the beginning. Counsel should also understand the factual foundation, methodology, assumptions, source material, and evidentiary path supporting each significant opinion.
Careful attention to those issues can help attorneys identify vulnerabilities earlier, prepare stronger examinations, and reduce the risk of avoidable admissibility problems.
What Is the Most Important California Case on Expert Witness Admissibility?
There is no single case governing every expert-testimony issue. Sargon is particularly important for expert foundation and reasoning under Evidence Code sections 801 and 802. Kelly addresses qualifying new scientific techniques, while Sanchez is central to the treatment of case-specific hearsay in expert testimony.
Does California Follow Daubert?
California state courts do not generally apply the federal Daubert framework as the governing state-law test. California expert testimony is governed by the California Evidence Code and California precedent. Leahy retained the Kelly approach for new scientific methodology within its scope after Daubert changed federal law.
Federal courts sitting in California apply the Federal Rules of Evidence in federal proceedings.
Can a California Expert Rely on Hearsay?
In appropriate circumstances, Evidence Code section 801 permits an expert to base an opinion on matter that may itself be inadmissible if it is of a type reasonably relied upon by experts in the field and reliance is not otherwise prohibited by law.
But relying on information to form an opinion is different from communicating inadmissible case-specific hearsay to the factfinder as true. Sanchez places important limits on the latter.
Can an Expert Give an Opinion Based on a Hypothetical Question?
Yes. Vang confirms that an expert may answer hypothetical questions based on facts shown by the evidence. The hypothetical may closely track the actual evidence, but it cannot properly depend on unsupported assumptions or speculation.
Case: Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747.
Sargon is one of California's most important modern decisions concerning expert testimony.
The dispute involved a dental implant company seeking substantial lost-profit damages after alleging that the University of Southern California breached a contract to clinically test its implant. The company's damages expert projected lost profits ranging into hundreds of millions of dollars.
The California Supreme Court upheld the exclusion of the testimony and discussed the trial court's gatekeeping responsibility under Evidence Code sections 801 and 802.
Sargon makes clear that a court does not simply ask whether an expert has impressive credentials. The court may examine the material on which the expert relies and the reasoning connecting that material to the expert's conclusions.
At the same time, gatekeeping does not authorize a trial court to choose between competing reasonable expert opinions merely because it finds one more persuasive. The inquiry focuses on whether the opinion has an adequate foundation and reasoned analytical basis.
For counsel, that means examining what information supports each material opinion, why experts in the field reasonably rely on it, what methodology or reasoning was used, and whether the conclusions logically follow from the underlying material.
Case: People v. Kelly (1976) 17 Cal.3d 24.
California lawyers dealing with evidence based on a potentially new scientific technique should know People v. Kelly.
The case involved spectrographic voice identification, commonly called "voiceprint" evidence. The decision became the foundation of what is commonly known as California's Kelly test for evidence produced by a new scientific technique.
Where Kelly applies, the proponent must satisfy the applicable Kelly requirements, including the required showing of general acceptance in the relevant scientific community, appropriate expert qualifications, and proper use of the technique in the particular case.
Importantly, Kelly is not a universal test for every expert opinion. Not every disagreement involving scientific evidence presents a Kelly issue. Counsel should distinguish a qualifying new scientific methodology from disputes concerning the application, interpretation, or weight of an established technique.
Case: People v. Leahy (1994) 8 Cal.4th 587.
After the U.S. Supreme Court decided Daubert v. Merrell Dow Pharmaceuticals, Inc. in 1993, California courts faced an important question: Would California abandon its existing approach to new scientific techniques?
In People v. Leahy, which involved horizontal gaze nystagmus testing in a driving-under-the-influence prosecution, the California Supreme Court retained the Kelly formulation for new scientific methodology falling within its scope.
California state courts therefore do not simply apply the federal Daubert framework as the governing state-law test for expert testimony. California has its own Evidence Code and body of expert-admissibility law.
Federal courts sitting in California, by contrast, apply the Federal Rules of Evidence to expert admissibility in federal proceedings. For lawyers practicing in both state and federal courts, identifying the governing framework at the beginning of an expert assignment can prevent confusion later.
Case: People v. Vang (2011) 52 Cal.4th 1038.
Expert witnesses are often asked hypothetical questions, and People v. Vang explains an important rule governing their use.
The California Supreme Court held that hypothetical questions posed to an expert may closely track the evidence presented at trial. Counsel does not have to disguise the relationship between the hypothetical and the facts of the case.
The hypothetical, however, must be rooted in facts shown by the evidence. It cannot properly depend on unsupported assumptions, speculation, or conjecture.
Vang also illustrates the distinction between an expert answering a properly supported hypothetical and an expert directly opining on what particular defendants actually did or intended. The court did not establish a categorical rule governing every circumstance in which testimony concerning a specific defendant may be permissible.
When constructing a hypothetical, counsel should identify the evidence supporting each important assumption and anticipate challenges to assumptions that may lack adequate evidentiary support.
Case: People v. Sanchez (2016) 63 Cal.4th 665.
People v. Sanchez significantly affected the treatment of expert basis testimony in California.
The case addressed a gang expert who related case-specific information obtained from police documents and other sources. The California Supreme Court drew an important distinction between an expert's general background knowledge and case-specific hearsay.
An expert may rely on hearsay in forming an opinion when permitted by the Evidence Code and may generally explain that such material contributed to the opinion. But an expert may not relate case-specific facts asserted in hearsay statements as true unless those facts are independently proven by competent evidence or are covered by an applicable hearsay exception.
In a criminal prosecution, testimonial hearsay presented through a prosecution expert may also raise separate Confrontation Clause issues under governing federal constitutional law.
For counsel, the key is to distinguish general background information, facts personally known to the expert, case-specific facts established through admissible evidence, and case-specific hearsay the expert has merely read or been told.
Case: People v. Veamatahau (2020) 9 Cal.5th 16.
People v. Veamatahau helped clarify the distinction developed in Sanchez.
The case involved expert testimony identifying pills as alprazolam. The California Supreme Court concluded that the expert did not relate inadmissible case-specific hearsay. The case-specific information about the markings on the seized pills came from the expert's own observations, while a pharmaceutical database supplied general information used to interpret those observations.
General background information that does not describe the particular people or events involved in the case is analytically different from an out-of-court statement offered to establish what happened in the specific case.
Classifying information as general background rather than case-specific does not, however, end every admissibility inquiry. The expert's reliance and testimony remain subject to applicable Evidence Code requirements.
Case: People v. Valencia (2021) 11 Cal.5th 818.
People v. Valencia further developed the Sanchez framework in the context of gang predicate offenses.
In Valencia, the California Supreme Court held that the predicate offenses required for the gang allegations had to be proven by independently admissible evidence. Under Sanchez, those facts could not be established solely through testimony from an expert who lacked personal knowledge of the facts necessary to satisfy the prosecution's burden.
More generally, an expert's ability to rely on otherwise inadmissible material does not permit the expert to prove case-specific facts for their truth by repeating inadmissible hearsay.
For litigators, one useful preparation technique is to create an evidence map identifying which facts the expert personally knows, which will be established through other witnesses or admissible exhibits, which information constitutes permissible background knowledge, and which statements may create hearsay or other admissibility problems.
The same exercise can help when evaluating an opposing expert by allowing counsel to compare each material opinion with the evidence and assumptions supporting it.
Taken together, these decisions illustrate several recurring principles:
These principles can guide expert selection, discovery, deposition preparation, admissibility motions, direct examination, and cross-examination.
Reviewing potential admissibility issues before trial gives counsel an opportunity to address weaknesses in an expert's qualifications, foundation, assumptions, methodology, or proposed basis testimony.
Counsel should consider:
This framework can also be used when evaluating an opposing expert. Counsel can work backward from each important conclusion and examine the evidence, assumptions, and reasoning necessary to support it.
California expert witness law cannot be reduced to a single admissibility test.
Evidence Code sections 720, 801, and 802 provide core statutory rules governing expert qualifications, permissible opinion testimony, and examination of an expert's reasons and basis. Other provisions, including section 801.1 in applicable general civil medical-causation cases, may also matter.
Sargon emphasizes scrutiny of expert foundation and reasoning. Kelly and Leahy address qualifying new scientific techniques. Vang explains important principles governing evidence-based hypothetical questions. Sanchez, Veamatahau, and Valencia establish important limits involving case-specific hearsay and expert background information.
For attorneys, selecting a qualified expert is only the beginning. Counsel should also understand the factual foundation, methodology, assumptions, source material, and evidentiary path supporting each significant opinion.
Careful attention to those issues can help attorneys identify vulnerabilities earlier, prepare stronger examinations, and reduce the risk of avoidable admissibility problems.
What Is the Most Important California Case on Expert Witness Admissibility?
There is no single case governing every expert-testimony issue. Sargon is particularly important for expert foundation and reasoning under Evidence Code sections 801 and 802. Kelly addresses qualifying new scientific techniques, while Sanchez is central to the treatment of case-specific hearsay in expert testimony.
Does California Follow Daubert?
California state courts do not generally apply the federal Daubert framework as the governing state-law test. California expert testimony is governed by the California Evidence Code and California precedent. Leahy retained the Kelly approach for new scientific methodology within its scope after Daubert changed federal law.
Federal courts sitting in California apply the Federal Rules of Evidence in federal proceedings.
Can a California Expert Rely on Hearsay?
In appropriate circumstances, Evidence Code section 801 permits an expert to base an opinion on matter that may itself be inadmissible if it is of a type reasonably relied upon by experts in the field and reliance is not otherwise prohibited by law.
But relying on information to form an opinion is different from communicating inadmissible case-specific hearsay to the factfinder as true. Sanchez places important limits on the latter.
Can an Expert Give an Opinion Based on a Hypothetical Question?
Yes. Vang confirms that an expert may answer hypothetical questions based on facts shown by the evidence. The hypothetical may closely track the actual evidence, but it cannot properly depend on unsupported assumptions or speculation.
Expert Witness Guides
Expert Witness Guides