By Judge J. Michael Byrne (Ret.) & Judge Gregory H. Ward

California Objections

Excerpted from California Objections

You may object to expert testimony on several grounds, including the following:

  • The question does not require or lend itself to expert testimony; see §§17:10-17:20.
  • The party offering the expert testimony failed to comply with the disclosure requirements of Code Civ. Proc. §2034.010 et seq.   or Pen. Code §1054; see §17:70.
  • The individual is not qualified to testify as an expert; see §17:110.
  • The individual’s qualifications do not extend to the subject matter at issue; see §17:110.
  • The question is improper expert cross-examination. For cross-examination generally, see Ch. 7.
  • The evidence is prejudicial or cumulative; see Evid. Code §§352, 723. For Evid. Code §352 generally, see Ch. 8.

Whether the trial court will sustain the objection depends on the circumstances of the case. Given the broad discretion of the trial judge and trial courts’ liberal approach toward admitting expert testimony in civil and criminal cases, objections are not often sustained. See People v. Ramos (1997) 15 Cal. 4th 1133, 1174-1175, 64 Cal. Rptr. 2d 892 (trial court properly exercised its discretion to not admit expert testimony, but Court stated in dictum that trial court could have allowed opinion, leaving its weight to the jury).

Unless a party makes a proper and timely objection, the grounds for objection are not preserved for appeal. People v. Valdez(1997) 58 Cal. App. 4th 494, 505, 68 Cal. Rptr. 2d 135. While a trial court may exclude an improper or unqualified opinion on its own motion, it is not required to exclude the testimony absent an objection.In re Powell (1988) 45 Cal.3d 894, 905-906, 248 Cal.Rptr. 431. If a party does not test the basis or foundation for an opinion and the expert’s credibility during the expert’s testimony, the party may not raise challenges to the admissibility of the testimony later. In re Marriage of Hargrave (1985) 163 Cal.App. 3d 346, 352, 209 Cal.Rptr. 764.

§17:100   Examining Expert Witness at Trial

During direct examination, an expert witness may state the basis of his or her opinion. The trial court has discretion to require the reasons for any conclusion before the opinion is given and to allow the witness to recite the sources upon which he or she relied. Evid. Code §802. When the witness describes the basis for his or her opinions, the court must control the use of background material to prevent the admission of improper hearsay. People v. Gardeley (1996) 14 Cal. 4th 605, 618, 59 Cal. Rptr. 2d 356.

Since different experts may have a different focus and may base their opinions on different facts, it is an abuse of discretion to prohibit a party from calling a witness to state an opinion that has already been elicited from another expert if the limitation destroys a party’s presentation of the evidence. Monroy v. City of Los Angeles (2008) 164 Cal. App. 4th 248, 266-267, 78 Cal. Rptr. 3d 738.

If the jury requires the benefit of an expert’s specialized knowledge, the trial court must allow the expert’s opinion even though it bears on an ultimate question. Evid. Code §805; People v. Olquin (1994) 31 Cal. App. 4th 1355, 1371, 37 Cal. Rptr. 2d 596. An expert may not use opinion testimony to improperly usurp the fact-finding function of the jury. Summers v. A. L. Gilbert Co. (1999) 69 Cal. App. 4th 1155, 1183, 82 Cal. Rptr. 2d 162 (witness was not allowed to give opinions on law). If the ultimate issue is a matter of ordinary intelligence and day-to-day experience, the judge may preclude the giving of an opinion. People v. Gardeley (1996) 14Cal4th 605, 617, 59 Cal. Rptr. 2d 356.

An expert may be cross-examined to the same extent as any other witness and also as to all of the following [Evid. Code §721(a)]:

  • His or her qualifications.
  • The subject to which his or her testimony relates.
  • The matter upon which his or her opinion is based and the reasons for the opinion.

The scope of cross-examination is broad and includes questions directed to whether the expert took into consideration matters arguably inconsistent with the opinion. People v. Ledesma (2006) 39 Cal. 4th 641, 695, 47 Cal. Rptr. 3d 326. Although the court may place limitations on cross-examination, it must recognize that repetitive questions and laborious inquiry into foundational facts are of greater importance in the examination of adverse expert witnesses. Monroy v. City of Los Angeles (2008) 164 Cal. App. 4th 248, 267, 78 Cal. Rptr. 3d 738. For cross-examination of witnesses generally, see Ch. 7.

After a party questions an expert about papers the expert has written, the court does not admit the entire documents into evidence, but, under Evid. Code §1235, only the parts that are inconsistent with the testimony. Benson v. Honda Motors Co. (1994) 26 Cal. App. 4th 1337, 1349, 32 Cal.Rptr. 322.

If the source of the material the expert used to form an expert opinion is protected by the attorney-client relationship or work product [see Ch. 10], the expert’s status as an agent is lost once the expert testifies, and the expert may be questioned fully on the basis of his or her opinions. People v. Milner (1988) 45 Cal. 3d 227, 241, 246 Cal. Rptr. 713.

The expert must be prepared to be fully questioned on his or her qualifications, the basis of opinions, and any issue related to his or her testimony on cross-examination. Evid. Code §721;People v. Osband (1996) 13 Cal. 4th 622, 711-712, 55 Cal. Rptr. 2d 26. On cross-examination, counsel may attack the expert’s opinions by questions on material relevant to the opinion, but it is improper to denigrate the witness or make insinuations. People v. Visciotti (1992) 2 Cal. 4th 1, 80-81, 5 Cal. Rptr. 2d 495. The credibility, biases, and professional motives of the expert are proper subjects for cross-examination. People v. Davenport (1995) 11 Cal.4th 1171, 1208, 47 Cal.Rptr. 2d 800. Cross-examination may challenge the expert’s knowledge in the field, the reasons for the evaluations and opinions, and the documents and exhibits reviewed in preparation for testimony. Miller v. Silver (1986) 181 Cal.App. 3d 652, 661, 226 Cal.Rptr. 479.

Cross-examination cannot be used to get prejudicial information or material not relevant to the opinions on direct before the jury or as a way to circumvent the hearsay rule. People v. Smithey (1999) 20 Cal. 4th 936, 960, 86 Cal. Rptr. 2d 243. Further, a party can challenge the use in cross-examination of highly emotional and inflammatory material under Evid. Code §352. People v. Coleman (1985) 38 Cal. 3d 69, 92, 211 Cal. Rptr. 102. When the expert uses hearsay to form an expert opinion and the expert gives detailed testimony about it in cross-examination, the court must instruct the jury on its limited use. People v. Montiel (1993) 5 Cal. 4th 877, 918-919, 21 Cal. Rptr. 2d 705.

On cross-examination, counsel may not confront an expert witness with a passage from a scientific, technical or professional publication unless any one of the following occurs [Evid. Code §721(b)]:

  • The witness referred to, considered or relied on the publication.
  • The court has admitted the publication into evidence.
  • The publication has been established as a reliable authority by the witness, by other expert testimony or by judicial notice.

Relevant portions of the text may then be read into the record, but the court will not admit the publication itself. The expert need not admit that he or she has read the book or article on which he or she is being questioned if any of the three conditions of Evid. Code §721(b) of the Evidence Code are fulfilled.


Use reliable authorities to cross-examine experts. If you have a publication that contains language that contradicts statements made by the expert you have, in effect, an additional expert witness. Although no appellate court has yet interpreted the “reliable authorities” provision of Evid. Code §721(b), attempt to lay the foundation for use of the publication by asking the expert if he or she is aware of it and, if so, if it is considered to be a reliable authority in the field. If the witness is unfamiliar with the publication, ask if he or she agrees with the statement you are about to read. If this question is met with an objection, request permission to approach the bench and make an offer of proof that your expert will establish the publication as a reliable authority. If the objection is sustained, ask that the witness be subject to recall in the event you are able to establish the publication as a reliable authority through other witnesses.

A witness who testifies that he or she considered or relied on all of his or her training in rendering an opinion and who indicates an awareness of the publication may be found to have considered or relied upon the publication and cross-examined as to its contents. People v. Clark (1993) 5 Cal. 4th 950, 1013, 22 Cal. Rptr. 2d.

After the expert expresses an opinion, the fact that an expert has given a contrary opinion in another case is a proper subject for impeachment on cross-examination. Kennemur v. State of California (1982) 133 Cal. App. 3d 907, 922, 184 Cal. Rptr. 393.


Present direct evidence of the expert in a manner understood by lay people. A common mistake in the examination of expert witnesses is the use of terms or the discussion of professional procedures not explained to the jury. Most experts are teachers and enjoy describing what they do and why they do it. Use exhibits and diagrams to demonstrate the conclusions and do not be in a hurry to get the witness into cross-examination before the trier of fact truly understands his or her testimony. It will pay off during your argument.

Review an expert’s past testimony and published work. In preparing to cross-examine an expert witness, always examine the witness’ published works and review transcripts of previous testimony the expert has given in other cases for inconsistencies. Note books and treatises the expert has recognized as authoritative, and determine whether they square with the testimony the expert is giving in the present case. Review deposition testimony for purposes of cross-examination and to hold the expert to the scope of the expert’s opinions at the deposition. Kennemur v. California (1982) 133 Cal.App. 3d 907, 924, 184 Cal.Rptr. 393.

You can also cross-examine an expert witness regarding the following:

  • How often he or she has testified in the past. Evid. Code §721.
  • The party for whom past testimony was given (whether plaintiff or defendant). Evid. Code §721.
  • The amount of remuneration the expert has or will receive for assisting in preparation of the case, for reviewing necessary materials to arrive at an opinion, and for testifying in court and at depositions. People v. Pride (1991) 1Cal. 4th 324, 457, 3Cal. Rptr. 106.

II. Objections

§17:110   Lack of Qualifications

Objection, Your Honor. This witness is not qualified to testify as an expert on this topic.


The trial judge has broad discretion to determine whether an expert witness is qualified to testify based on knowledge, skill, education, and experience. People v. Davenport (1995) 11 Cal. 4th 1171, 1207, 47 Cal. Rptr. 2d 800. It is error for a trial court to admit the testimony of a witness if the witness clearly lacks the qualifications to render the opinions for which the party is asking.People v. Chavez (1985) 39 Cal. 3d 823, 827-829, 218 Cal. Rptr. 49.

Judges generally qualify experts whose experience or training bears a reasonable relation to the area of purported expertise. Trial courts are more likely to exercise discretion in favor of admitting an expert’s testimony than excluding it, allowing the trier of fact to assign weight to the testimony based on the witness’ credentials. People v. Bolin (1998) 18 Cal. 4th 297, 322, 75 Cal. Rptr. 2d 412; Mann v. Cracchiolo (1985) 38 Cal. 3d 18, 37-38, 210 Cal. Rptr. 762 (weight to be accorded expert testimony is within province of jury, not trial court). The court may also limit the scope of the expert’s testimony based on its assessment of expertise. Korsak v. Atlas (1992) 2 Cal. App. 4th 1516,1523, 3 Cal. Rptr. 2d 833.

The trial court is not required to formally certify or qualify a witness as an expert. Evid. Code §§402, 801. Thus, it is unnecessary to “proffer” or “tender” the witness as an expert. Most judges, if asked to certify or qualify, will state merely that the witness is acceptable or may testify. This avoids giving the witness an imprimatur of undue credibility.


Do not stipulate to your expert’s qualifications. If your expert is obviously qualified, or has a particularly impressive background, your opponent may offer to stipulate to the expert’s qualifications. Politely reject the offer, since questioning the witness concerning his or her qualifications in front of the jury enhances the witness’ credibility, though, of course, the court can limit “grandstanding.” Offering your expert’s resume or curriculum vitae can shorten the testimony and enhance the appearance of modesty, but remember that the jury will compare opposing expert witnesses’ experience when they evaluate conflicting opinions. The questioning should emphasize the training or recognition that puts your expert above others practicing in the same area and be understandable to the average juror. See CALCRIM 332.

Maximize the jury’s awareness of the expert’s qualifications. Most attorneys lose the full impact of their expert’s qualifications by mechanically going through them at the start of the testimony. Consider presenting the minimum qualifications and then asking general opinions. If the opposition objects, the jury will be alerted, and you can detail your expert’s experience. If there is no objection, have the witness explain the opinions with further references to special experience or research that directly relates to the expert’s conclusions. Use the entire direct examination to show his or her special knowledge or experience that make the opinions particularly trustworthy.


    • If you can mount a sustainable challenge to your adversary’s expert’s qualifications, consider seeking an in limineruling before your adversary calls the expert as a witness. Courts are often more likely to disqualify an expert before he or she is actually in front of the jury. Further, you do not want the judge to rule that the witness is qualified after you have objected in the jury’s presence. For motions in limine generally, see Ch. 1.


  • A good cross-examination concerning the expert’s experience within the particular field, or a comparison of your expert’s credentials with that of your opponent’s, may be more effective than trying to prevent the witness from testifying.
  • Consider stipulating to the opposing expert’s qualifications or asking a few voir dire questions challenging the witness’ expertise, then sit down and accept the testimony.
  • If an opposing expert is a professional witness or has testified frequently and on diverse subjects, effective cross-examination can make the witness seem like a “hired gun.”
  • Establish that the witness is being paid for testimony or “time in court.”
  • If the expert advertises his or her services in legal journals, elicit that fact through questioning.
  • Check the expert’s prior testimony or writings to prepare to cross-examine and impeach if there is a discrepancy between the testimony and opinions offered by the expert in the past. Bring transcripts of any relevant testimony from the current or prior trials to challenge any inconsistencies in the witness’ answers. Explore the percentage of times the witness has testified for the plaintiff/prosecution and for the defense. Know the testimony of the expert at the deposition thoroughly and have the transcript organized to allow quick access to points for impeachment.



  • Since courts generally allow experts to testify, respond to a challenge to your expert’s qualifications that the quality or quantity of the witness’ credentials go to the weight and not the admissibility of evidence.
  • If the judge is reluctant to allow your expert to testify, suggest the court issue a limiting instruction that the jury is free to accept or reject the witness’ opinion.
  • Argue that opposing counsel waived the objection if he or she failed to object to your expert immediately after you called the expert as a witness.
  • Use your judgment in asking for or agreeing to a stipulation concerning qualifications. A stipulation will prevent you from questioning the witness concerning his or her qualifications, which can enhance your witness’ credibility.
  • Too much time spent on detailing your witness’ credentials can become tedious and may come across as “showing off.”
  • If your expert witness has appeared as an expert in many trials, and opposing counsel characterizes the expert as a “hired gun,” make it clear that many people have sought the expert’s services because of the expert’s excellent credentials.


The proponent of an expert witness should establish some of the following foundational bases:

  • Educational background.
  • Licenses and certifications.
  • Experience and personal research in relevant fields.
  • Teaching experience.
  • Membership or offices held in professional organizations.
  • Books and articles written.
  • Awards won.
  • Number of times previously qualified as an expert.


Expert Opinion Admissible

People v. Catlin (2001) 26 Cal.4th 81, 131-138, 109 Cal.Rptr. 2d 31. In a murder case, a clinical toxicologist    with a Ph.D. in physiology and pharmacology was qualified to give an opinion on the cause of death and on the effect of hypertension and age on the kidneys, despite the fact that he was not a medical doctor. The witness had qualifications and experience with poisons and their effect on the human body. The expert could base his opinion on the results of the autopsy by another doctor, the results of the laboratory test and tissue analysis, and the history of disease.

People v. Champion (1995) 9 Cal.4th 879, 924-925, 39 Cal.Rptr. 2d 547. An expert on gangs was allowed to interpret words used by gang members because it was beyond the common experience and would assist the trier of fact.

Mann v. Cracchiolo (1985) 38 Cal.3d 18, 35, 210 Cal.Rptr. 762. A neurosurgeon was qualified to testify to the standard of care for a nonspecialist for the reading of X-rays, the submitting of X-ray reports, and diagnosis. The physician-specialist was familiar with the standard even though he worked under a higher standard.

People v. Williams (1996) 46 Cal.App. 4th 1767, 1779, 54 Cal.Rptr. 2d 521. An expert called to testify to the reliability of tumor diagnosis equipment need not be a medical doctor to testify about the process and explain the resultant photographs.

Osborn v. Irvin Memorial Blood Bank (1992) 5 Cal.App. 4th 234, 273, 7 Cal.Rptr. 2d 101. A physician need not be a specialist in the pertinent field of medicine to qualify as an expert for purposes of offering an opinion.

Miranda v. National Emergency Services, Inc. (1995) 35 Cal. App. 4th 894, 905, 41 Cal. Rptr. 2d 593. An emergency medical expert must have substantial professional experience acquired on the job within five years of the trial as an emergency room physician in a location in which emergency care is provided [Health & Safety Code §1799.110(c)].

Schreidel v. American Honda Motor Co. (1994) 34 Cal.App. 4th 1242, 1252, 40 Cal.Rptr. 2d 576. A mechanic with extensive experience could show the cause of a defect by the process of the elimination of other causes and the strength of the assumptions was a question of fact.

Expert Opinion Inadmissible

People v. Ramos (1997) 15 Cal. 4th 1133, 1174-1175, 64 Cal.Rptr. 2d 892. A professor of sociology and expert on prisons was asked what might provoke an inmate to possess a weapon in San Quentin in June of 1984. The witness had no personal information since 1983 and talked to persons working in the location. The trial court properly sustained an objection, because the witness was only interpolating others’ information and the questions were not within the expert’s area of expertise. In dictum, the Supreme Court found that it also would have been a proper exercise of discretion to admit the opinion.

People v. Davenport (1995)11 Cal. 4th 1171, 1206-1207, 47 Cal. Rptr. 2d 800. A homicide investigator’s opinion as to whether the victim was impaled on a stake ante mortem or post mortem was not admissible for lack of the qualifications necessary to give an expert opinion. The witness had no medical, serology, or pathology training. He had not investigated similar cases and had limited knowledge of the manner in which the cause and time of death are determined clinically.

Westrec Marina Management, Inc. v. Jardine Ins. Brokers Orange County, Inc. (2000) 85 Cal. App. 4th 1042, 1051, 102 Cal.Rptr. 2d 673. An expert on insurance was not allowed to state an opinion on the profit or loss of insurance sales for boats at a marina during a period when insurance was not available because it was not based on reasonably reliable information. The witness needed a factual basis to show that reliable statistical data was used to analyze the potential market, and the witness’ information did not meet that standard. The expert had not done a study or survey of marinas and based his opinions on insurance programs in businesses other than marinas.

Korsak v. Atlas (1992) 2 Cal. App. 4th 1516, 1525, 3 Cal.Rptr. 2d 833. An expert in hotel maintenance could not give an opinion on the usual maintenance of shower heads based on general discussions with unidentified hotel persons who worked in the area without some scientific study or investigation or some showing of expertise of the persons interviewed or authenticity of their information because the proponent of the witness made no showing of reliability of the basis for the opinion testimony.

§17:120   Province of Court and Jury

Objection, Your Honor. The expert is usurping the [jury’s/court’s] function.


An opinion that embraces the ultimate issue to be decided by the trier of fact may be admitted if it is otherwise admissible. Evid. Code §805. Notwithstanding §805, an expert must not usurp the function of the jury, and an opinion that amounts to nothing more than an expression of belief as to how a case should be decided is inadmissible. Summers v. A.L. Gilbert Co. (1999) 69 Cal. App. 4th 1155, 1183, 82 Cal. Rptr. 2d 162.

In a criminal case, an expert may not give an opinion concerning the guilt or innocence of the defendant or whether a crime has been committed. People v. Torres (1995) 33 Cal. App. 4th 37, 46-47, 39 Cal.Rptr. 2d 103. The jury is equally able to reach this conclusion and the opinion may give the impression that the issue has been decided. People v. Prince (2007) 40 Cal.4th 1179, 1227, 57 Cal.Rptr. 3d 543. An opinion in a civil case that a defendant is liable also goes beyond merely addressing an ultimate issue and usurps the role of the jury. Summers v. A.L. Gilbert Co. (1999) 69 Cal. App. 4th 1155, 1185, 82 Cal. Rptr. 2d 162.

An expert may not give an opinion that a witness is telling the truth. People v. Coffman and Marlow (2004) 34 Cal.4th 1, 82, 17 Cal.Rptr. 3d 710. Testimony that a witness is a pathological liar is also inadmissible, since the jury is capable of determining a witness’ credibility without expert testimony. People v. Avila (2006) 38 Cal.4th 491, 43 Cal.Rptr. 3d 1.

Experts may not give opinions on questions of law to be decided by the court. Sheldon Appel Co. v. Albert & Oliker (1989) 47 Cal. 3d 863, 254 Cal. Rptr. 336 (not proper in malicious prosecution action for attorney-witnesses to opine as to whether a reasonable attorney would conclude there was probable cause to file an action); Cooper Companies v. Transcontinental Ins. Co. (1995) 31 Cal. App. 4th 1094, 37 Cal. Rptr. 2d 508 (expert testimony on the interpretation of an insurance contract is inappropriate). Allowing an expert to give an opinion on an issue of law usurps the role of the trial judge and the evidence is incompetent. Amtower v. Photon Dynamics, Inc. (2008) 158 Cal. App. 4th 1582, 1599, 71 Cal. Rptr. 3d 361 (opinion on whether defendant had a fiduciary relationship); Summers v. A.L. Gilbert Co. (1999) 69 Cal. App. 4th 1155, 1181, 82 Cal. Rptr. 2d 162 (whether defendant had a nondelegable duty); Adams v. City of Fremont (1998) 68 Cal. App. 4th 243, 265-266, 80 Cal. Rptr. 2d 196 (whether defendants had a duty of care).


Avoid general questions. Avoid questions that are so general that they can be viewed as asking for legal opinions or credibility questions.


  • In the pretrial conference, resolve any issues of law that apply to the case and ask the judge to limit the opposition in raising issues that would conflict with those resolutions.
  • If an area of law could be unclear to the jury, request a pre-instruction to the jury. For jury conduct and management, see Ch. 3.
  • Object to opinions on the law or any issues within the province of the jury.


  • Prepare questions to avoid questions asking for legal conclusions by having the expert formulate opinions that are specific to the practices in the area of expertise.
  • Use argument to show how facts and opinions are applied to the law.


People v. Coffman and Marlow (2004) 34 Cal.4th 1, 82-83, 17 Cal.Rptr. 3d 710. Although defendant’s psychologist could offer testimony on the battered woman syndrome, it was error for her to testify that she believed the defendant’s claims of abuse and domination by the codefendant.

Piscitelli v. Friedenberg (2001) 87 Cal.App. 4th 953, 972-974, 105 Cal.Rptr. 2d 88. In a legal malpractice case, expert testimony on the expected result of arbitration or what the arbitrator would do invaded the fact-finding province of the jury to decide the facts of the underlying case.

Asplund v. Selected Investments (2000) 86 Cal.App. 4th 26, 50, 103 Cal.Rptr. 2d 34. The existence and scope of a defendant’s duty to supervise an agent is a legal question, and an expert may not give an opinion in an area that is within the province of the court to decide.

Loth v. Truck-A-Way ( 1998) 60 Cal.App. 4th 757, 767, 70 Cal.Rptr. 2d 571. Opinions on the value of enjoyment of life are within the province of the trier of fact and not for expert testimony.

Adams v. City of Fremont (1998) 68 Cal. App. 4th 243, 266, 80 Cal. Rptr. 2d 196. Expert opinion on the questions of law such as the standard of care or duty in negligence cases is not admissible.

People v. Valdez (1997) 58 Cal. App. 4th 494, 507, 68 Cal. Rptr. 135. An expert was permitted to give an opinion as to whether the defendant’s conduct in committing the charged crime was to assist his gang. The information charging the crime had allegations that the defendant, while committing the crime, acted for the gang. The expert was in a better position to form the opinion and possessed experience to assist the jury. Opinions on the information and its allegations were proper so long as they were not tantamount to an opinion that the defendant is guilty or the allegations are true.

People v. Erickson (1997) 57 Cal.App. 4th 1391, 1401, 67 Cal.Rptr. 2d 740. Evid. Code §§805 and 1107, which allow opinions on ultimate issues and battered woman syndrome respectively, do not allow expert opinion on the defendant’s state of mind at the time of the crime. Pen. Code §29 controls.

§17:130   Inappropriate Hypothetical Question

Objection, Your Honor. Counsel’s hypothetical question is misleading because it [omits important facts in evidence] [assumes facts not in evidence].


Unless the court orders otherwise, questions calling for an expert’s opinion need not be in a hypothetical format. Evid. Code §§801, 721. On the other hand, counsel has wide latitude in using hypothetical questions in cross-examination. Evid. Code §721.

When counsel poses a hypothetical question to an expert witness, the hypothetical may include facts that are not established beyond all controversy. People v. Zepeda (2001) 87 Cal.App. 4th 1183, 1208, 105 Cal.Rptr. 2d 187. Each side may formulate the question according to its theory of the case, so long as the basis of the question is sufficiently supported by the evidence adduced at trial or reasonably inferred from the evidence. People v. Boyette (2002) 29 Cal. 4th 381, 449-451, 127 Cal. Rptr. 2d 544. The hypothetical question may be based on facts not admitted into evidence if it is reasonable information for an expert in the field to rely on. People v. Gardeley (1993) 14 Cal.4th 605, 618, 59 Cal.Rptr. 2d 356.

However, the hypothetical question must be rooted in fact and must be reliable. People v. Gardeley (1993) 14 Cal.4th 605, 617-618, 59 Cal.Rptr. 2d 356. The question may not be misleading or unfair. In re Marriage of S. (1985) 171 Cal. App. 3d 738, 750, 217 Cal. Rptr. 561. The court may exclude expert testimony that is based on guess, surmise or conjecture. People v. Richardson (2008) 43 Cal. 4th 959, 1008, 77 Cal. Rptr. 3d 163.


Plan your hypothetical question carefully. Summarizing the facts in hypothetical form allows you to focus the jury’s attention on important, favorable evidence, and to state facts in a light favorable to your client. Write out the facts to be summarized before attempting to pose a hypothetical to an expert witness. Writing out the facts ensures that the summary accurately and fairly reflects the facts in evidence, and does not omit any necessary elements. Successful objections to your questions can prevent a clear presentation by your expert.

Making the Objection

  • Object to any hypothetical that:
    • Mischaracterizes the evidence.
    • Omits or misstates underlying facts.
    • Assumes facts not in evidence.
    • Sounds like a summation because it presents an argumentative view of the evidence.
    • If the hypothetical is based on evidence the court has admitted subject to a connection to be made later, ask for a continuing objection, then remember to move to strike the expert’s opinion if the connection is not made. For continuing objections, see Ch. 1.
    • If the court admits the testimony, use cross-examination to modify or restate the facts underlying the opinion testimony.
    • Request that the court charge the jury on the definition and use of hypothetical questions.

Responding to the Objection

  • If opposing counsel objects to your use of hypothetical questions, argue that your opponent will have an opportunity to question your underlying assumptions during cross-examination.


To use hypothetical questions, lay a foundation in the following manner:

  • Ask the expert witness to assume specific facts relevant to forming an opinion.
  • Ask the witness whether, based on those facts, he or she has an opinion.
  • If yes, ask the witness to state the opinion.
  • Ask the witness to explain the basis for the opinion.


People v. Richardson (2008) 43 Cal. 4th 959, 1008, 77 Cal. Rptr. 3d 163. Court properly excluded expert testimony on the outflow rate of water from a bathtub, offered to show when the tub had been filled and the victim murdered, when there was no evidence whether the tub had been filled to the overflow pipe, whether the victim’s face blocked the drain and the effect of the victim’s removal from the tub.

People v. Gardeley (1996) 14 Cal.4th 605, 619, 59 Cal.Rptr. 2d 356. A gang expert was asked to assume the facts of an assault as testified to and evidence not admitted that the attackers were Family Crips. The gang expert was permitted to opine that the activity was gang related.

People v. Sims (1993) 5 Cal. 4th 405, 435-436, 20 Cal.Rptr. 2d 537. Hypothetical questions posed to an expert forensic pathologist who performed the autopsy about the circumstance of death based on testimony and the autopsy examination were proper.

In re Marriage of S. (1985) 171 Cal.App. 3d 738, 750, 217 Cal.Rptr. 561. Expert recommendations on child custody by a psychiatrist could be based on the assumption that the wife-mother knew of abuse to the child, because evidence in the trial showed that she had been warned of it.

§17:140   Scientific Testimony

Objection, Your Honor. The expert testimony about to be offered [lacks foundation] [does not satisfy the Kelly rule] [is of dubious scientific validity].


The Kelly Rule. If the expert testimony to be offered is novel scientific evidence, it must satisfy the standard set forth in Frye v. United States (D.C. Cir. 1923) 293 F. 1013, and People v. Kelly (1976) 17 Cal. 3d 24, 130 Cal. Rptr. 144. People v. Leahy(1994) 8 Cal. 4th 587, 594, 34 Cal. Rptr. 663. The Kelly rule, known as the “general acceptance test,” allows for the admission of expert opinion grounded in a scientific theory or technique if the theory or technique is generally accepted as reliable in the relevant scientific community. Frye v. United States (D.C. Cir. 1923) 293 F. 1013; People v. Kelly (1976) 17 Cal. 3d 24, 30, 130 Cal. Rptr. 144.

California does not follow the judicial “gatekeeper” procedure used in the federal courts. See Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993) 509 U.S. 579, 113 S. Ct.2786, 125 L. Ed. 2d 469. Californiacourts generally use the terms “Kelly test” for the standard and “Kelly-Frye hearing” for pretrial judicial hearings to establish the foundation for the evidence. People v. Venegas (1998) 18Cal. 4th 47, 76, 74 Cal. Rptr. 2d 262.

Under the Kelly rule, evidence based on a new scientific method must satisfy three requirements to be admissible [People v. Diaz (1992) 3 Cal. 4th 495, 526, 11 Cal. Rptr. 2d 353]:

  • The technique has gained general acceptance in its field.
  • The witness furnishing the testimony is qualified to give evidence on the acceptance.
  • Correct scientific procedures were used.

The scientific opinion need not be accepted by every scientist, nor need procedures or tests establishing validity be totally perfected. General acceptance means a consensus drawn from a typical cross-section of major voices in the relevant qualified scientific community. It is not satisfied merely by showing that the technique has been used by law enforcement for a long time, nor generally is a police officer qualified to give expert opinion about the test’s general acceptance in the scientific field. People v. Leahy (1994) 8 Cal. 4th 587, 608-612, 34 Cal. Rptr. 2d 663. The testimony of a single expert can be sufficient to establish the general acceptance of the procedure within the relevant scientific community. People v. Allen (1999) 72 Cal. App. 4th 1093, 1098-1099, 85 Cal. Rptr. 2d 655.

Testimony on the general acceptance of the test or technique should be from an impartial expert who satisfies each of the following [People v. Pizarro (1992) 10 Cal. App. 4th 57, 79-80, 12 Cal. Rptr. 2d 436]:

  • The expert is qualified to state the views of impartial scientists in the relevant fields.
  • The expert has formal training in the applicable scientific discipline.
  • The expert is aware of the relevant scientific literature.

Once an appellate court affirms a trial court’s decision to admit evidence based on a new scientific technique in a published opinion, that precedent controls subsequent trials, at least until new evidence is presented reflecting a change in the attitude of the scientific community. People v. Venegas (1998) 18 Cal. 4th 47, 76, 74 Cal. Rptr. 2d 262. California courts give the published appellate decisions of other jurisdictions similar effect.People v. Allen (1999) 72 Cal. App. 4th 1093, 1099, 85 Cal.Rptr. 2d 655.

There is no requirement that the proponent of the evidence establish absolute certainty for the test results to be presented. People v. Slone (1978) 76 Cal. App. 3d 611, 625, 143 Cal. Rptr. 61. The court will not reject a recent application of a scientific method that has been accepted as reliable unless contrary evidence is presented. People v. Cooper (1991) 53 Cal. 3d 771, 812-813, 281 Cal. Rptr. 90.

Once the proponent establishes that correct scientific procedures were used, questions of contamination or faulty laboratory practices are for the trier of fact. People v. Venegas (1998) 18 Cal. 4th 47, 80-81, 74 Cal. Rptr. 2d 262. The rule applies to scientific procedures, not new machines or devices. People v. Nolan (2002) 95 Cal.App. 4th 1210, 116 Cal.Rptr. 2d 331.

A procedure that isolates physical evidence whose existence, appearance, nature and meaning are obvious to a layperson is not subject to the Kelly rule. People v. DePriest (2007) 42 Cal. 4th 1, 63 Cal. Rptr. 3d 896 (shoe prints).

High-tech demonstrative evidence, such as the computer animation of a crime, does not involve scientific techniques or procedures when used merely to illustrate testimony. People v. Hood (1997) 53 Cal. App. 4th 965, 969, 62 Cal. Rptr. 2d 137. Expert medical opinion is not subject to the Kelly rule unless there is a special feature that would effectively “blind side the jury.” People v. Rowland (1992) 4 Cal. 4th 238, 266, 14 Cal. Rptr. 2d 377. Medical opinions include an expert’s testimony based on personal evaluations of a patient and diagnosis.Wilson v. Phillips (1999) 73 Cal.App. 4th 250, 254, 86 Cal.Rptr. 2d 204. The use of accepted scientific observations and applying scientific analysis to them is not a scientific technique subject to theKelly rule. People v. Bui (2001) 86 Cal. App. 4th 1187, 1195, 103 Cal.Rptr. 2d 908. An expert’s personal opinion based on the expert’s own experience is not subject to the Kelly rule unless the opinion is based on a new method of proof. Texaco Producing, Inc. v. Countyof Kern (1998) 66 Cal. App. 4th 1029, 1048-1049, 78 Cal. Rptr. 2d 433.

Procedure. If you intend to introduce scientific evidence of a new method or novel technique or theory, file an in limine motion to alert the court, either before taking testimony or before offering your expert. For motions in limine generally, see Ch. 1.

The court may hold a hearing outside the jury’s presence to establish the validity of scientific evidence, and to establish whether proper protocols have been followed. The hearings are discretionary with the trial judge, but if new scientific procedures are at issue, expect a hearing. People v. Rowland (1992) 4 Cal. 4th 238, 265, 14 Cal. Rptr. 2d 377; Evid. Code §402.

The proponent of the evidence must be prepared to define the relevant scientific community through expert testimony and then to establish that the theory or technique is generally accepted as reliable. People v. Leahy (1994) 8 Cal. 4th 587, 594, 34 Cal. Rptr. 2d 663. The court is free to evaluate the scientific material in the area to determine general acceptance, including a fair overview of the subject or independent study of journals and treatises in the appropriate field. People v. Wallace (1993) 14 Cal. App. 4th 651, 658-659, 17 Cal. Rptr. 2d 721.

In addition to satisfying the general acceptance test, scientific evidence must meet the ordinary foundational requirements of admissibility. For example, the evidence must be relevant, helpful, and neither unduly prejudicial nor confusing. Evid. Code §352; for objecting based on relevance or prejudice, see Ch. 8. An inconclusive test is not necessarily irrelevant, and may be admissible simply to show the jury that the test was done. People v. Horning (2004) 34 Cal. 4th 871, 900-901, 22 Cal. Rptr. 3d 305. The evidence should not be cumulative or involve an undue consumption of time. People v. Espinoza (2002) 95 Cal.App. 4th 1287, 1309-1312, 116 Cal.Rptr. 2d 700.


  • If opposing counsel attempts to introduce the results of a scientific test, object and request an in limine hearing or voir dire questioning outside the presence of the jury.
  • If the evidence or results have not been established as scientifically valid or accepted, ask the court to hold a Kelly/Frye hearing.
  • Question the witness about peer review articles condoning or accepting the procedures or findings, acceptance in other courts, and general acceptance in the relevant scientific community.
  • Object if the results or tests do not satisfy the Kelly rule.
  • If the evidence is the type generally accepted in the scientific community, such as results of a blood or Breathalyzer test, and your focus is on questions on methodology, accuracy of equipment, competence of the technician, and chain of custody, save these points for cross-examination.
  • Consider objecting based on lack of relevance or undue emphasis on a peripheral issue, or on the ground that the tests are confusing or misleading.


  • If you offer test results that are traditionally accepted, such as blood, alcohol level, or DNA tests, be prepared to establish:
    • A clear chain of custody.
    • That the tests were properly administered.
    • Have the person who administered the test or someone with personal knowledge concerning how and when the test was administered available as a witness. If that person is unavailable, someone with knowledge of the usual customs and routines of the laboratory may be acceptable. People v. Clark (1992) 3 Cal. 4th 41, 158, 10 Cal. Rptr. 2d 554.
    • To offer “new” scientific evidence, first show that it is generally accepted in the relevant scientific community.
    • Give the court and opposing counsel a memorandum setting forth the basic principles involved in obtaining the scientific data and a list of articles discussing the science involved.
    • If possible, furnish copies of peer review articles to the court and opposing counsel in advance of calling a witness.
    • Offer an expert witness to explain the science to the court and to your adversary.
    • Notify the court if other courts in your jurisdiction or in any other jurisdiction have admitted or rejected the evidence as soon as possible, preferably before trial.
    • In response to objections based on relevancy or confusion, be prepared to suggest limiting or cautionary instructions that help focus the jury’s attention on the relevant or connecting aspects of your evidence.


Follow these foundation steps where appropriate [People v. Leahy (1994) 8 Cal. 4th 587, 34 Cal. Rptr. 2d 663; People v. Diaz (1992) 3 Cal. 4th 495, 525-526, 11 Cal. Rptr. 2d 353]:

  • Have the witness establish his or her credentials as an expert in the field.
  • Have the witness explain the purpose of the test.
  • Have the witness describe procedures for obtaining test results.
  • Have the witness demonstrate compliance with accepted procedures and protocols.
  • Have the witness testify that test equipment or machinery is properly maintained and accurate.
  • Have the witness testify that test equipment was in good working order at the time of the test.
  • Establish the competence of the individual administering the test.
  • Establish that test results are properly preserved.
  • Have the witness describe methodology where applicable.
  • Establish the general acceptance of the methodology in the scientific community, either by calling other experts or through the testimony of the expert who made the test or scientific evidence involved.
  • If the scientific evidence is not a new procedure or technique requiring a Kelly foundation, establish through a qualified expert the following [People v. Venegas (1998) 18 Cal.4th 47, 79-81, 74 Cal.Rptr. 2d 262]:
    • The accuracy of any machine, equipment or mechanism used for testing.
    • That any machinery or mechanism involved in conducting the test was in good working order when the test was administered.
    • The test was properly administered.
    • The test results were properly preserved.


Admissibility Generally

People v. Horning (2004) 34 Cal.4th 871, 900-901, 22 Cal.Rptr. 3d 305. Expert testimony that bullets from the crime scene were compared with bullets from the defendant’s gun, and it could not be determined for sure if both were fired from the same gun, was relevant to ensure that the jurors did not erroneously conclude that no testing was done.

People v. Venegas (1998) 18 Cal.4th 47, 79-81, 74 Cal.Rptr. 2d 262. In a case involving DNA evidence, the Court held that once a methodology or procedure has been accepted by the courts as reliable and generally accepted in the relevant scientific community, an expert does not have to evaluate the procedures anew as long as the witness is qualified and understands the technique and procedures followed to implement the technique.

People v. Leahy (1994) 8 Cal. 4th 587, 605-612, 34 Cal.Rptr. 2d 663. The test for admissibility of scientific evidence is whether it is generally accepted as reliable in the scientific community and whether the witness is qualified to give opinion in the area. General acceptance means a consensus drawn from a cross section of the relevant qualified scientific community.

People v. Wash (1993) 6 Cal. 4th 215, 242, 24 Cal. Rptr. 2d 421. The Kelly rule requires the proponent of expert testimony based on the application of new scientific technique to establish:

  • The technique or method is sufficiently established to have gained general acceptance in its field.
  • Testimony with respect to the technique and its application is offered by a properly qualified expert.
  • Correct scientific procedures have been used in the particular case.

People v. Rowland (1992) 4 Cal.4th 238, 265, 14 Cal.Rptr. 377. Medical testimony based on an examination and diagnosis does not need to be qualified under the Kelly rule.

People v. Bui (2001) 86 Cal. App. 4th 1187, 1194-1197, 103 Cal.Rptr. 2d 908. A forensic toxicologist testified that the level of intoxication from the methamphetamine in a defendant’s blood system would impair his ability to drive. This was not a new scientific technique or device or a novel process so no Kelly analysis was required. It was reasonable for the expert to rely upon literature and scientific studies in the area that were based on statistical information.

Wilson v. Phillips (1999) 73 Cal. App. 4th 250, 254, 86 Cal.Rptr. 2d 204. Psychiatric expert testimony is medical testimony and not covered by the Kelly rule unless some special feature might confuse the jury.

People v. Allen (1999) 72 Cal.App. 4th 1093, 1098-1099, 85 Cal.Rptr. 2d 655. Testimony of a single expert was sufficient to establish that the test had general acceptance in the professional community.

Bite Mark

People v. Slone (1978) 76 Cal.App. 3d 611, 143 Cal.Rptr. 61. Bite-mark-identification fulfills the three prong Kelly test. “There is a probability factor in even the most carefully structured scientific inquiry; seldom is it possible to exclude all possible chance for error in human endeavor. But there is no requirement in our law that the admissibility of scientific-test evidence must be predicated on a 100 percent degree of accuracy.”


People v. Morganti (1996) 43 Cal.App. 4th 643, 671, 50 Cal.Rptr. 2d 837. Tests of blood samples by gamma marker blood analysis and polymerase chain reaction tests are generally accepted as reliable techniques by the relevant scientific community. Further, the expert was qualified and correct scientific procedures were used.

People v. Bury (1996) 41 Cal. App. 4th 1194, 1199, 49 Cal.Rptr. 107. A preliminary alcohol screening breath test given in the field at the time of arrest to determine if the suspect is under the influence of alcohol is admissible and not a new scientific procedure that would trigger a Kelly analysis.

People v. Clark (1993) 5 Cal. 4th 950, 1017-1018, 22 Cal.Rptr. 2d 689. Testimony on blood-spatter is not new to science or the law, and because it does not have a misleading aura of scientific infallibility, it does not require a Kelly ruling. It is common knowledge that blood hitting a surface will have an effect which creates a pattern.

Bullet Comparison

People v. Ayala (2000) 24 Ca. 4th 243, 281, 99 Cal. Rptr. 2d 532. A radiologist’s testimony that the deformed bullet observed within the victim most likely was the same caliber as a bullet that was taped to the body before the x-ray was taken was not subject to the Kelly rule. The testimony was neither ballistics evidence nor an experiment, but was an expression of an obvious physical condition.

Chemical Tests

People v. Wash (1993) 6 Cal. 4th 215, 241, 24 Cal. Rptr. 2d 421. The electrophoretic analysis of semen samples is generally accepted in the scientific community and therefore a valid test.

People v. Fierro (1991) 1 Cal. 4th 173, 214, 3 Cal.Rptr. 2d 426. Electrophoretic analysis of dried bloodstains found in the defendant’s car was admissible as scientific evidence.

Melaleuca, Inc. v. Clark (1998) 66 Cal.App. 4th 1344, 1357, 78 Cal.Rptr. 2d 627. Use of a syncrometer to establish the presence of benzene in the defendant’s product was not allowed because the procedure was not generally accepted in the field of chemistry.

Computer Enhancement

People v. Williams (1996) 46 Cal.App. 4th 1767, 54 Cal.Rptr. 2d 521. Segmentation of photographs is a technique involving the use of computers and mathematical formulas to breakdown a photograph and then reformulate it. It is a new scientific technique which is generally accepted in its field. The director of research for the company that developed the process was a proper expert to comment on that acceptance of the process.

DNA. Note: These cases are scientifically complex and require careful review before planning the presentation of or defense against DNA evidence. They are detailed accounts of the presentations of new scientific evidence and can be learning tools for an attorney about to try a lawsuit involving new and complex scientific evidence.

People v. Soto (1999) 21 Cal. 4th 512, 515, 88 Cal.Rptr. 2d 34. Use of the unmodified product rule is permissible to assist in assessing the probative significance of a DNA match.

People v. Venegas (1998) 18 Cal.4th 47, 79-81, 74 Cal.Rptr. 2d 262. An expert may use R.F.L.P. methodology as a basis to interpret DNA information. Calculations made under the modified ceiling approach are admissible.

People v. Wright (1998) 62 Cal.App. 4th 31, 38-39, 72 Cal.Rptr. 246. DNA evidence derived from a PCR machine is not a novel type of scientific procedure or technique which requires a Kelly/Frye hearing because it is reliable and scientifically valid.


People v. Webb (1993) 6 Cal. 4th 494, 524, 24 Cal.Rptr. 779. Fingerprints analyzed by a laser process which isolates parts of physical evidence whose appearance, nature, and meaning are obvious to the senses of lay people do not need a Kelly foundation because the reliability of the process is apparent.


People v. Pride (1992) 3 Cal. 4th 195, 239, 10 Cal.Rptr. 2d 636. Human hair comparisons made by expert witnesses are admissible in evidence.

Horizontal Gaze Nystagmus

People v. Leahy (1994) 8 Cal. 4th 587, 611, 34 Cal.Rptr. 2d 663. The horizontal gaze nystagmus test used by police in driving under the influence cases is not a generally accepted scientific technique to measure the amount of alcohol in a driver’s blood system at the time of arrest. Long-standing use by law enforcement does not establish general acceptance by the scientific community.

People v. Joehnk (1995) 35 Cal.App. 4th 1488, 1507-1509, 42 Cal.Rptr. 2d 6. The horizontal gaze nystagmus test is generally accepted by the scientific community as a useful tool when combined with other tests and observations of the suspect to reach an opinion on whether the arrestee was intoxicated.


People v. Shirley (1982)31 Cal. 3d 18, 66-67, 181 Cal.Rptr. 243. Hypnosis of witness to bring out suppressed memory does not have the general acceptance in the scientific community and if hypnosis is used on a witness, makes the witness incompetent to testify. For witness competence, see Ch. 6.

Schall v. Lockheed Missiles & Space Co. (1995) 37 Cal. App. 4th 1485, 1492-1493, 44 Cal. Rptr. 2d 191. The testimony of a witness who was hypnotized is inadmissible as to any memory restored by the session. Matters recalled and related to others before the hypnosis are admissible. The trial court should hold a hearing to determine whether the witness underwent hypnosis, if the purpose of the hypnosis was to restore memory, and whether the proposed testimony was recalled and related to another before the hypnosis.

Medical and Psychiatric

People v. Smithey (1999) 20 Cal.4th 936, 966-967, 86 Cal.Rptr. 2d 243. A qualified expert’s opinions regarding mental state based upon standardized tests are not subject to a Kelly foundation. Rebuttal evidence that the standardized tests are not reliable similarly does not require a Kelly ruling.

Wilson v. Phillips (1999) 73 Cal. App. 4th 250, 254, 86 Cal.Rptr. 2d 204. The opinions of a psychiatrist who specializes in sex abuse cases on repressed memory were based on personal evaluations and diagnosis of the patient and did not require a Kelly analysis.

Polygraph or Lie Detector. The results of a polygraph examination, the opinion of the examiner and any reference to the taking of or offer to take an examination are excluded in criminal proceedings unless all parties stipulate to their admission. Evid. Code §351.1(a).

People v. Wilkinson (2004) 33 Cal.4th 821, 845, 852, 16 Cal.Rptr. 3d 420. Evidence Code §351.1 abrogated the Kelly/Frye rule with respect to the admission  of polygraph evidence in criminal cases and the defendant was not denied the right to introduce evidence critical to her defense. Defendant was merely barred from introducing expert testimony to bolster her  own credibility.

Rape Trauma Syndrome

People v. Bledsoe (1984) 36 Cal.3d 236, 246-251, 203 Cal.Rptr. 450. Evidence of “rape trauma syndrome” has met the Kelly test, to the extent that expert testimony may be admitted to explain a complainant’s post-rape behavior that seems inconsistent with the claim of rape. However, because the syndrome has not yet met the general acceptance test as a method of explaining how the victim would act during the actual rape, the testimony is inadmissible if offered to prove that the rape actually occurred.

Sodium Amytal

Ramona v. Superior Court (1997) 57 Cal.App. 4th 107, 115-121, 66 Cal.Rptr. 2d 766. Plaintiff was given sodium amytal to refresh her memory, and she then remembered sex abuse occurring when she was a child. UnderKelly, this was a scientific procedure, and it was not generally accepted in the relevant expert community. She was not allowed to testify because her testimony was tainted and she could not be cross-examined.

Voice Identification

People v. Kelly (1978) 17 Cal. 3d 24, 40, 130 Cal.Rptr. 144. In the case that established the Kelly-Frye rule, the Court found voice print identification was not generally accepted in the relevant scientific community.

§17:150   Lay Opinion

Objection, Your Honor. There is no foundation for lay opinion evidence.


The witness is speculating.


A witness testifying in the form of an opinion may state on direct examination the reasons for the opinion and the matter on which it is based, unless the witness is precluded by law from using the reasons or matter as a basis for the opinion. The court, in its discretion, may require the witness before testifying in the form of an opinion, to be first examined concerning the matter on which the opinion is based. Evid. Code §802. The court may, and on objection must, exclude testimony in the form of an opinion based in whole or in significant part on matter that is not a proper basis for the opinion. The witness may state the opinion after excluding from consideration the matter determined to be improper, if there remains a proper basis for the opinion. Evid. Code §803.

The general opinions or conclusions of percipient witnesses or nonexperts are admissible when they are both of the following [Evid. Code §800]:

  • Rationally based on the witness’ perceptions.
  • Helpful to a clear understanding of his or her testimony.

The opinion must not go beyond the facts the witness personally observed or otherwise perceived or acquired through the senses. Evid. Code §170; People v. McAlpin (1991) 53 Cal.3d 1289, 1308, 283 Cal.Rptr. 382. The opinions must be within the general knowledge of lay people and taken from the witness’ own perception; otherwise, only an expert’s opinions are admissible. Jambazian v. Borden (1994) 25 Cal. App. 4th 836, 848-849, 30 Cal. Rptr. 2d 768. They must involve concepts within the common experience of most people and must not be based on scientific analysis. People v. Williams (1992) 3 Cal. App. 4th 1326, 1332, 5 Cal.Rptr. 2d 130.

A lay opinion is helpful to an understanding of the witness’ testimony when the concrete observations on which the opinion is based cannot otherwise be conveyed. People v. Melton (1988) 44 Cal. 3d 713, 744, 244 Cal.Rptr. 867. A witness may state an impression or opinion when it rests on subtle or complex interactions that are difficult to put into words. People v. Hinton (2006) 37 Cal. 4th 839, 889, 38 Cal. Rptr. 3d 149. Although a witness generally may not give an opinion about another’s state of mind, he or she may testify about behavior and describe that behavior as consistent or inconsistent with a state of mind. People v. Chatman (2006) 38 Cal. 4th 344, 397, 42 Cal. Rptr. 3d 621.

Lay witnesses may express opinions on their perceptions and observations of a declarant, but the believability of a declarant is not the proper subject of lay opinion. People v. Melton (1988) 44 Cal. 3d 713, 744, 244 Cal. Rptr. 867.

Lay opinion is admissible when no particular scientific knowledge is required or a matter is too complex or subtle to enable a witness to accurately convey it to the jury. People v. Williams (1988) 44 Cal. 3d 883, 914, 245 Cal. Rptr. 336. But the conclusions must be based on facts the witness personally perceived; otherwise, they are merely speculative. People v. McAlpin (1991) 53 Cal. 3d 1289, 1308, 283 Cal. Rptr. 382.

The opinion may not invade the province of the jury to decide truthfulness or guilt and innocence. People v. Melton (1988) 44 Cal. 3d 713, 744, 244 Cal.Rptr. 867; People v. Torres (1995) 33Cal. App. 4th 37, 47, 39 Cal. Rptr. 2d 103. An opinion that is otherwise admissible and does not relate to witness credibility or guilt or innocence is not objectionable because it embraces the ultimate issue to be decided by the trier of fact. Paez v. Alcoholic Beverage Control Appeals Bd. (1990) 222 Cal.App. 3rd 1025, 1026, 272 Cal.Rptr. 272.

A witness not otherwise qualified to testify as an expert may state an opinion regarding whether a writing is in the handwriting of a supposed writer if the court finds that the witness has personal knowledge of the handwriting of the supposed writer. Evid. Code §1416. Opinion testimony as to the value of property by certain lay witnesses is proper, including the owner or spouse of the property owner. Evid. Code §813; In re Marriage of Stoll (1998) 63 Cal. App. 4th 837, 843, 74 Cal. Rptr. 506.


  • If the deposition or preliminary hearing testimony of a lay witness indicates the witness’ tendency to make unfounded conclusions, attempt to get a pretrial ruling limiting the witness’ testimony. For motions in limine, see Ch. 1.
  • Object that the witness is speculating or has no personal knowledge.
  • Object that the opinions will not assist the jury and are cumulative, invade the fact-finding responsibilities of the jury, or are prejudicial.
  • Have the court instruct the jury on the role of lay opinion.
  • If the opinion is unfounded or outrageous or shows a bias by the witness, consider saving your attack for cross-examination so you can maximize its impact.
  • If the testimony merely contains non-damaging conclusions, but the witness has personal knowledge, it may be the best strategy to not object and allow the opponent to clarify the testimony and then raise the deficiencies in cross-examination or final argument.


  • Before you request an opinion from a lay witness, ask the witness about all the observations or perceptions on which the witness bases the opinions.
  • Argue that the witness’ conclusions will assist the jury in understanding the testimony or that the witness is not articulate enough to describe the events without some generalizations.



Osborn v. Mission Ready Mix (1990) 224 Cal.App. 3d 104, 112-114, 273 Cal.Rptr. 457. A witness gave an opinion that the condition of a ramp area was open and obvious. The opinion was proper to describe the area to show the obviousness of the risk. However, the witness’ opinion that the condition was reasonable was not helpful to understand the condition, since the testimony invaded the province of the jury to determine the reasonableness of both parties’ conduct.


People v. Gonzales (1968) 68 Cal.2d 467, 472, 67 Cal.Rptr. 551. A police officer gave an opinion that the defendant was the person whom he saw leave the bar based on his clothes and characteristics but was not positive because he did not see his face. The identification was admissible.

People v. Ingle (1986) 178 Cal.App. 3d 505, 513, 223 Cal.Rptr. 723. A lay witness who viewed a surveillance film taken at the time of a robbery can be permitted to identify the person portrayed on the film, if the witness has personal knowledge of the defendant at or before the robbery and the testimony aids the trier of fact in determining the crucial issue of identity.

Intent, Motive or Knowledge

People v. Hinton (2006) 37 Cal.4th 839, 889, 38 Cal.Rptr. 3d 149. Witness gave his opinion that the defendant appeared to be directing another person in the conduct of a drug transaction. The opinion was proper if it was based on observations of the interactions between these persons that were difficult to put into words.

People v. Harris (1969) 270 Cal.App. 2d 863, 872, 76 Cal.Rptr. 130. A witness’ opinion that it appeared as if the defendant was trying to break up a fight and was not participating in an assault was admissible because the witness could not give a precise description of every aspect of the fight.


People v. Williams (1992) 3 Cal.App. 4th 1326, 1332, 5 Cal.Rptr. 130. A lay person may give an opinion on someone’s state of intoxication when the testimony is based on the witness’ observations of the signs of intoxication. In this case, an experienced police officer made the conclusions regarding intoxication after observing the defendant.


Dean v. Feld (1946) 77 Cal.App. 2d 327, 330-331, 175 P.2d 278. Speed, distance, size, weight and other physical descriptions cannot be exactly testified to. Estimates or comparative expressions are permissible.


Jambazian v. Borden (1994) 25 Cal.App. 4th 836, 849, 30 Cal.Rptr. 2d 768. A nonexpert may not testify to a diagnosis of diabetes and its risks.

Vandi v. Permanente Medical Group Inc. (1992) 7 Cal.App. 4th 1064, 1071-1072, 9 Cal.Rptr. 2d 463. Expert opinion is required for an opinion on the standard of care in the practice and the diagnosis and treatment of neurological disorder.

Windeler v. Scheers Jewelers (1970) 8 Cal.App. 3d 844, 853, 88 Cal.Rptr. 39. A witness may testify about wounds the witness received and describe them.

Mental State

People v. Chatman (2006) 38 Cal.4th 344, 397, 42 Cal.Rptr. 3d 621. Witness who testified that he saw defendant kick someone four or five times could properly opine as to whether the defendant seemed to be enjoying it.

People v. DeSantis (1992) 2 Cal.4th 1198, 1228, 9 Cal.Rptr. 2d 628. The court erred in excluding an out-of-court statement of a witness that he had trouble remembering “because of his brain cells.” The statement was lay opinion of the witness’ mental condition.

People v. Medina (1990) 51 Cal. 3d 870, 887, 274 Cal. Rptr. 849. A deputy sheriff testified in a competency hearing that the defendant appeared to understand a conversation they had shortly after the defendant’s arrest and the defendant was very responsive and very descriptive during the conversation about his housing problems. A nonexpert may state an opinion on a person’s mental state and degree of awareness based on his or her observations and conversation with the person.

Physical State

Marriage of Teegarden (1986) 181 Cal.App. 3d 401, 409, 226 Cal.Rptr. 417. A wife was competent to testify to an opinion of her husband’s pain and physical condition and symptoms from lupus based on her observations.

Sexual Deviancy

People v. McAlpin (1991) 53 Cal.3d 1289, 1308-1309, 283 Cal.Rptr, 382. Character witnesses who had observed the defendant with their daughters and saw no unusual behavior could give opinions that he was not a sexual deviant, but witnesses who had not seen the defendant with children could not because they had never seen his behavior with children.

Sexual Relationships

People v. Sandoval (2008) 164 Cal.App. 4th 994, 1002, 79 Cal.Rptr. 3d 634. It was not error to exclude testimony of an expert witness on the subject of “make-up” sex offered to support defendant’s argument that the victim consented. The concept that some people find sex more arousing after a fight was a matter of common knowledge and within the experience of the jurors.

Shoe Prints

People v. Lucero (1998) 64 Cal.App. 4th 1107, 1110, 75 Cal.Rptr. 2d 806. It was acceptable for a police officer to testify about his comparison of shoes the defendant was wearing when arrested and the imprints of shoe soles found at the crime scene.

Speed, Distance, Light, etc.

Albrecht v. Broughton (1970) 6 Cal.App. 3d 173, 181, 85 Cal.Rptr. 659. Witnesses to an accident were properly asked if had they been driving at that time, would they have had their lights on. The amount of illumination at the time and place was material and witnesses could opine based on what they knew. This is also proper in areas of speed, distance, and size.

Statistical Analysis

Everett v. Superior Court (2002) 104 Cal.App. 4th 388, 393, 128 Cal.Rptr. 2d 418. Statistical analysis requires an expert witness, but not all number issues do. A simple comparison of percentages does not exceed the common experience of the trier of fact.

Value of Property

Schroeder v. Auto Driveaway Co. (1974) 11 Cal.3d 908, 921, 114 Cal.Rptr. 622. An owner’s opinion on the value of personal property is competent evidence of its value and usually sufficient to support a judgment based on that value.

Crail v. Blakely (1973) 8 Cal. 3d 744, 754-755, 106 Cal.Rptr. 187. An estate administrator is similar to an owner and possesses the sufficient personal knowledge to give an opinion on the value of the estate property.

In re Marriage of Stoll (1998) 63 Cal.App. 4th 837, 843, 74 Cal.Rptr. 2d 506. The owner of property may give an opinion on the value of his or her property.

Sacramento & San Joaquin Drainage Dist. v. Goehring (1970) 13 Cal. App. 3d 58, 65, 91 Cal. Rptr. 375. An owner’s opinion on the value of real property is admissible only if based on a proper legal basis for the opinion. Here, the owner improperly valued the loss in value of his property by the loss in use to him and not the fair market loss.

Value of Services

Donahue v. Ziv Television Programs, Inc. (1966) 245 Cal. App. 2d 593, 609, 54 Cal. Rptr. 130. A witness could testify to the value of his services.


People v. Melton (1988) 44 Cal. 3d 713, 744, 244 Cal. Rptr. 867; People v. Smith (1990) 214 Cal. App. 3d 904, 915, 263 Cal. Rptr. 155. A witness may not give an opinion as to the believability of a declarant.

J. Michael Byrne is a private mediator and arbitrator for ADR Services. From 1989 to 2003, Judge Byrne served as a superior court judge in Los AngelesCounty. His many judicial assignments have included civil fast-track court, civil and criminal trial court, and civil law and motion. While presiding in Pasadenaand Downtown Los Angeles Civil fast-track courts, Judge Byrne tried an average of fifteen to twenty civil jury trials a year. From 1982 to 1988, he was a municipal court judge.

Before his judicial appointment, Judge Byrne was a trial attorney and later a supervising attorney in the Los Angelesand Sacramento District Attorney offices, where he tried numerous jury and court trials on a regular basis. For a number of years, he was assigned to the Consumer & Environmental Crimes Section and Special Investigation Divisions of the Los Angeles District Attorney’s Office. For two years, he was the legal advisor to the Los Angeles Grand Jury.

Gregory H. Ward has served as a Judge of the Santa Clara County Superior Court since 1990. He has presided over more than one-thousand jury and non-jury trials in civil, criminal, juvenile delinquency, juvenile dependency, family and probate cases.

Prior to his appointment to the Superior Court, Judge Ward was in private practice in Palo Alto, California. He also served as a trial attorney for the United States Department of Justice, Organized Crime & Racketeering Section, in Washington,D.C., Chicagoand San Juan, Puerto Rico and as an Assistant United States Attorney in San Jose,California.

They are the authors of California Objections, from which this article is excerpted.