On July 17, 2023, California Governor Gavin Newsom signed Senate Bill (SB) No. 652, adding Section 801.1 to the California Evidence Code. This section provides additional requirements for expert opinions relating to medical causation. In particular, it allows a party not bearing the burden of proof to offer a contrary expert in response to an expert proffered by a party bearing the burden of proof as to medical causation who is required to opine that causation exists to a reasonable medical probability. The contrary expert may only be proffered, however, if he or she is able to opine that an alternative medical causation is one that exists to a reasonable medical probability. Section 801.1, however, does not preclude an expert witness from testifying that a specific matter cannot meet a reasonable degree of probability in the applicable field.

With respect to medical causation, a “reasonable degree of probability” means that the expert is testifying that a particular event or source was more likely than not the cause of a person’s injuries.

Prior to the Bill’s signing, expert witnesses were permitted to testify in the form of an opinion as to medical causation, if (1) the opinion was sufficiently beyond the common experience such that it would assist the trier of fact; and (2) the opinion was based on a matter, whether or not admissible, that was known to the witness prior to offering testimony and that may reasonably be relied upon by an expert in forming his or her opinion on the matter. Put more simply, causation could previously be proven within a reasonable medical probability based upon competent expert testimony. However, recent cases have altered the standard as between parties who do and do not bear the burden of proof. A recent California Court of Appeal opinion found that the reasonable medical probability requirement only applies to the party bearing the burden of proof on the underlying issue. Kline v. Zimmer, Inc. (2022) 79 Cal. App. 5th 123 (Aug. 31, 2022).

In Kline, the plaintiff brought a personal injury action against a defendant medical device manufacturer for injuries sustained after implantation of an artificial joint that allegedly caused plaintiff’s injuries. After a verdict was entered in favor of the plaintiff, the defendant appealed the trial court’s denial of a motion for a retrial. There, the defendant argued that one basis for retrial was the trial court’s exclusion of expert testimony offered by the defendant for the purpose of undercutting the opinions provided by plaintiff’s expert to a reasonable medical probability. The appellate court differentiated expert testimony seeking to prove an actual alternative cause, rather than simply seeking to introduce causation opinions and other evidence to challenge the causation opinion of the plaintiff’s expert. In the latter situation, the court found that the same standard should not apply to both sides; the defendant need only show that the plaintiff’s evidence was insufficient to prove that the injuries were more likely than not caused by the defendant.

Some attorneys, including sponsors of the Bill, believe the Kline decision upends the credibility of expert witness testimony and allows defense experts to follow more lax standards when testifying as to medical causation. SB 652 presumably clarifies Evidence Code §801 to ensure that all experts must testify to a reasonable degree of probability based on their field of expertise, and codifies the standard that has been consistently relied upon for decades to ensure only reliable testimony is presented to juries.

Moving forward, expert witnesses offering medical causation opinions, on both sides, must be prepared to testify that the opinion is based on a reasonable degree of probability. Alternatively, an expert may simply opine that a matter cannot meet the reasonable degree of probability threshold. In effect, the Bill raises the standard for defense experts testifying as to medical causation relating to alternative causes. A defense expert must now be willing to state that the alternative cause also exists to a reasonable medical probability.