California Code, Evidence Code - EVID § 646

(1) If the facts which would give rise to a res ipsa loquitur presumption are found or otherwise established, the jury may draw the inference from such facts that a proximate cause of the occurrence was some negligent conduct on the part of the defendant; and

(2) The jury shall not find that a proximate cause of the occurrence was some negligent conduct on the part of the defendant unless the jury believes, after weighing all the evidence in the case and drawing such inferences therefrom as the jury believes are warranted, that it is more probable than not that the occurrence was caused by some negligent conduct on the part of the defendant.