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.