21.5.1 Cause-in-Fact.

• Jurisdiction: Arizona

21.5.1 Cause-in-Fact. Two related but distinct legal concepts are recognized in proving cause-in-fact. In a single cause event, Arizona courts apply the “but for” test. When multiple actors or causes contribute to the ultimate injury, Arizona courts resort to the “substantial factor” test. Cause-in-fact relies upon proof that an immediately preceding event caused injury. In its purest and simplest form, cause-in-fact is the event which is close in time and without which the accident would not have occurred.¹⁶⁵ The concept of “proximate cause” is enunciated in Nichols v. City of Phoenix .¹⁶⁶ For example, a defendant negligently crashed his airplane into the plaintiff’s apartment. “But for” the pilot’s negligent conduct the apartment would remain standing. This example typifies the foundation for the “but for” test.¹⁶⁷

The “but for” test is entrenched in American and Arizona jurisprudence.¹⁶⁸ What constitutes cause-in-fact is ordinarily a question for the finder of fact. It is neither a question of “science or of legal knowledge” but is determined in view of the circumstances of fact attending it.¹⁶⁹ The “but for” test of cause-in-fact is simple and effective when applied in single actor or where single cause events exist; however, its application is less effective when multiple actors or multiple causes contribute to the injury. The classic illustration of this deficiency focuses on a single property destroyed by two separate fires set independently by separate actors. Under the “but for” rule, each tortfeasor escapes liability by claiming that the property still would have been destroyed by the act of the other.¹⁷⁰

In response to the shortcomings of the “but for” rule, the Arizona Supreme Court in McDowell v. Davis ¹⁷¹ recognized the “substantial factor” test. Negligent conduct was a legal cause of harm if that conduct was a “substantial factor” in bringing about the harm.¹⁷² The court approved the notion that more than one cause of an event or subsequent injury was recognizable.¹⁷³ This allowed responsibility to be spread amongst multiple wrongdoers. While the McDowell court approved of the “substantial factor” test, it specifically disapproved of an instruction given by the court which included the language “substantial factor.” The court was concerned that the primary dictionary meanings of “non-imaginary nor illusive” would be disregarded and jurors would substitute more commonplace definitions such as “substantial,” “abundant,” or “plentiful” and thereby require a higher burden of proof.¹⁷⁴

When the conduct of two or more tortfeasors cause a common injury, there is widespread application of the “substantial factor” test.¹⁷⁵ A claimant must introduce evidence which affords a basis to reasonably conclude that the conduct of any tortfeasor is a “substantial factor” in bringing about a specific result.¹⁷⁶ The quantum of proof required to fulfill that burden has variously been described as “probable facts” from which negligence and causal relations may be inferred,¹⁷⁷ at least “a little”¹⁷⁸ evidence and more broadly as “any” cause which in a natural and continuous sequence produces the injury.¹⁷⁹

The court provided guidance to considerations properly taken into...