Summary
Section 431 addresses proximate (legal) cause, providing that the actor’s negligent conduct is a legal cause of harm to another if (a) the conduct is a substantial factor in bringing about the harm, and (b) there is no rule of law relieving the actor from liability because of the manner in which the negligence has resulted in the harm.
The “substantial factor” test replaced the older “but-for” test as the primary test for proximate causation in the Restatement. A factor is substantial if it is of sufficient significance that a reasonable person would regard it as a cause. This test is particularly useful in concurrent cause cases where multiple negligent acts combine to produce harm and neither alone would have been a but-for cause.
The second prong incorporates policy limitations on liability, including foreseeability, the directness of the causal chain, and superseding cause doctrines. Even when conduct is a substantial factor, liability may be cut off if an unforeseeable intervening cause breaks the chain of causation.
Key Elements
- 1Negligent conduct must be a substantial factor in causing harm
- 2No rule of law relieves the actor from liability
- 3Substantial factor test is broader than but-for causation
- 4Particularly useful in concurrent cause scenarios
- 5Policy limitations may cut off liability despite factual causation
Practical Application
Courts apply § 431 in complex causation cases, including toxic tort litigation, multi-vehicle accidents, and environmental contamination. The substantial factor test allows plaintiffs to recover even when their harm has multiple contributing causes. It is also important in medical malpractice cases where the patient had a pre-existing condition that contributed to the injury.
Exam Relevance
Proximate cause is one of the most frequently tested areas in torts. Look for fact patterns with intervening causes, concurrent causes, or attenuated causal chains. The key question: Was the defendant’s conduct a substantial factor? Then ask whether any superseding cause or policy limitation cuts off liability. The Palsgraf debate (foreseeability of plaintiff vs. foreseeability of harm) often appears in this context.