Holtz v. Holder
In Holtz v. Holder, 101 Ariz. 247, 248-49, 251, 418 P.2d 584, 585-86, 588 (1966), the Arizona Supreme Court addressed the plaintiff's burden of proving causation "where the plaintiff is unable to prove which defendant caused which injuries or whether all were caused by one defendant or the other." 101 Ariz. at 249, 418 P.2d at 586.
In such circumstances, the court reasoned that public policy favored adopting the so-called "'single injury' rule," thereby relaxing the plaintiff's burden of proof. Id. at 251, 418 P.2d at 588.
The court observed that it is more desirable, as a matter of policy, for an injured and innocent plaintiff to recover his entire damages jointly and severally from independent tortfeasors, one of whom could have to pay more than his just share, than to let two or more wrongdoers escape liability altogether, simply because the plaintiff cannot carry the impossible burden of proving the respective shares of causation or because the tortfeasors have not committed a joint tort. Id.