Cal. Codes » Penal Code / PART 2. OF CRIMINAL PROCEDURE / TITLE 7. OF PROCEEDINGS AFTER THE COMMENCEMENT OF THE TRIAL AND BEFORE JUDGMENT / CHAPTER 2. The Trial
California Codes

Cal. Penal Code § 1127b (2026)

The Trial

✓ current as of May 2026
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When, in any criminal trial or proceeding, the opinion of any expert witness is received in evidence, the court shall instruct the jury substantially as follows:

Duly qualified experts may give their opinions on questions in controversy at a trial. To assist the jury in deciding such questions, the jury may consider the opinion with the reasons stated therefor, if any, by the expert who gives the opinion. The jury is not bound to accept the opinion of any expert as conclusive, but should give to it the weight to which they shall find it to be entitled. The jury may, however, disregard any such opinion, if it shall be found by them to be unreasonable.

No further instruction on the subject of opinion evidence need be given.

Notes of Decisions
Cited in 45 cases (4 in the last 5 years), 1942–2026 · leading case: People v. Housley, 6 Cal. App. 4th 947 (Cal. Ct. App. 1992).
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People v. Housley, 6 Cal. App. 4th 947 (Cal. Ct. App. 1992). · cites it 2× “(Pen. Code, § 1127b.) Because juries may accord undue weight to an expert’s opinion, special care must be taken to insure the jury understands its duty to independently assess the expert opinion along with and in light of all other relevant evidence.”
People v. Brigham, 599 P.2d 100 (Cal. 1979). · cites it 2× “1939) when it enacted statutory instructions on expert testimony (Pen. Code, § 1127b) and evidence of flight by the defendant ( id.”
People v. Reeder, 65 Cal. App. 3d 235 (Cal. Ct. App. 1976). · cites it 2× “" (Pen. Code, § 1127b.) *242 It is impossible to assign a relative value even roughly equivalent to the impact upon the jury of the testimony of a particular witness.”
People v. Baker, 98 Cal. App. 4th 1217 (Cal. Ct. App. 2002). “303 ) and to disregard any expert’s opinion (Pen. Code, § 1127b). Second, even if the evidence presented was susceptible to a reasonable interpretation supporting Baker’s acquittal, we nonetheless must affirm because the evidence also was reasonably susceptible to an…”
People v. Modesto, 382 P.2d 33 (Cal. 1963). “80 ]), and an erroneous failure to instruct (Pen. Code, § 1127b) that the jury are not bound to accept the testimony of an expert witness as conclusive ( People v.”
People v. Wolff, 394 P.2d 959 (Cal. 1964). “Indeed, the code specifically requires that the jury be instructed (and they were so instructed in the ease at bench) that "The jury is not bound to accept the opinion of any expert as conclusive, but should give to it the weight to which they shall find it to be entitled.”
People v. Murray, 225 Cal. App. 3d 734 (Cal. Ct. App. 1990). “6 Finally, appellant contends, “What differentiates this case from all others of its genre is uncontradicted substantial evidence that appellant unknowingly suffered from an organic brain syndrome which would tend to (unwittingly) magnify the effect of intoxicants on his system.”
People v. Cole, 301 P.2d 854 (Cal. 1956). “Paxton to express his opinion. The jurors, of course, were not bound by the opinion of the witness but were free to determine the weight to which it was entitled and to disregard it if they found it to be unreasonable, and they were so instructed.”
People v. Horowitz, 161 P.2d 833 (Cal. Ct. App. 1945). “Under proper instructions (Pen. Code, § 1127b) the jury was not obliged to accept the opinions of the experts as the only evidence of the forgery or more than evidence.”
People v. Arguello, 244 Cal. App. 2d 413 (Cal. Ct. App. 1966). · cites it 2× “(Pen. Code, § 1127b.) [6a] The objection that the opinion of an expert coincides with the "ultimate issue" in the case is equally untenable as such.”
People v. Coogler, 454 P.2d 686 (Cal. 1969). “Having set forth a complete summary of the defense *166 case above, we hold that the trial court properly gave the first degree deliberate and premeditated murder instructions because the jury could properly have disregarded the expert testimony as to defendant’s diminished…”
People v. Patrick W., 84 Cal. App. 3d 520 (Cal. Ct. App. 1978). “The doctor’s opinion included a consideration of the circumstances surrounding the event, without which such evaluation would be of little or no value.”
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