California Enacts New Expert Testimony Rule

Effective January 1, 2024, California Senate Bill 652 (codified as Evidence Code section 801.1) creates a new evidentiary standard for defendants proffering alternative causation evidence. Specifically, the defense must now demonstrate that the evidence supporting an alternative cause must rise to the level of reasonable medical probability.

Specifically, Section 801.1 states:

(a) In a general civil case, as defined in Rule 1.6 of the California Rules of Court, where the party bearing the burden of proof proffers expert testimony regarding medical causation and where that party’s expert is required as a condition of testifying to opine that causation exists to a reasonable medical probability, the party not bearing the burden of proof may offer a contrary expert only if its expert is able to opine that the proffered alternative cause or causes each exists to a reasonable medical probability, except as provided in subdivision (b).

(b) Subdivision (a) does not preclude a witness testifying as an expert from testifying that a matter cannot meet a reasonable degree of probability in the applicable field, and providing the basis for that opinion.

This new rule diverges from, and supersedes, case law discussed in Kline v. Zimmer Inc. (2022) 79 Cal.App.5th 123, which held that proving causation to a reasonable degree of medical probability in a personal injury action applied “only to the party bearing the burden of proof on the issue which is the subject of the opinion.” Since the plaintiff bears the burden to prove causation, the evidentiary threshold of “reasonable degree of medical probability” presumably only applied to plaintiff.

Now each side is held to the same evidentiary standard when there is a theory of alternative causation. Defense experts should be prepared to address this heighted standard when they have alternative causation opinions.