“Ordinarily, tort liability cannot be established unless plaintiffs can prove their injuries resulted from the tortious acts of a particular defendant or defendants. As a general rule, the burden of proof as to negligence (duty and breach) and causation rests upon plaintiff; and plaintiff does not meet this burden simply by demonstrating that one of several defendants could have been responsible. However, there are some exceptions to this rule. These are situations in which, for policy reasons, the burden effectively shifts to defendants to absolve themselves from responsibility or to show that some other defendant or event caused the injuries. Where an accident is of such a nature that, in light of past experience, it probably was the result of someone’s negligence and defendant is probably the one responsible, the law may permit the trier of fact to draw an inference of defendant’s negligence. This is the doctrine of “res ipsa loquitur.” It is an evidentiary rule. In California, the doctrine is defined as a presumption affecting the burden of producing evidence (Ev.C. § 646)—i.e., provided three preliminary fact conditions are met, defendant is presumed to have been negligent and the burden shifts to defendant to produce evidence either that defendant was not negligent or that any negligence on defendant’s part was not a proximate cause of the occurrence. Plaintiffs seeking to invoke res ipsa loquitur have the burden of establishing the following three conditions: That the accident is of a kind which ordinarily does not occur in the absence of someone’s negligence; That the accident was caused by an agency or instrumentality within the defendant’s (or defendants’) exclusive control; and That the accident was not due to any voluntary action or contribution on plaintiff’s part.
If plaintiff can prove that several defendants each owed him or her a duty of care, and that each of the defendant’s acts are possibly the sole cause of plaintiff’s injuries, the burden shifts to each defendant to prove that he or she did not cause the injury (i.e., that another defendant was the sole cause). To hold otherwise (requiring plaintiff to identify which defendant was the legal cause of the harm) might effectively leave an innocent victim without a remedy, since each wrongdoer could escape liability because of plaintiff’s lack of proof. Further, defendants in such a case are ordinarily in a “far better position” to offer evidence on the issue of causation. [Summers v. Tice, supra] But defendants’ superior access to causation evidence is not a prerequisite to shifting the burden of proof; indeed, liability may follow even if defendants are in no better position than plaintiff to prove who was at fault. The theory is that since defendants “brought about a situation where the negligence of one of them injured the plaintiff … it should rest with them each to absolve himself if he can.”
The so-called “market share liability” theory shifts the burden of proof to defendant manufacturers to absolve themselves of responsibility for injuries resulting from use of a dangerous fungible product they all manufactured using the same formula and which cannot be traced to any particular manufacturer through no fault of plaintiff. It suffices that each manufacturer’s liability is approximately equal to its “market share” of the dangerous product.”
[California Practice Guide: Personal Injury [certain citations omitted]]
