But-For Causation
What is the But-For Causation?
The plaintiff must prove that but for the defendant's negligent conduct, the injury would not have occurred. This actual cause requirement is a necessary element of every negligence claim.
Definition
But-for causation, also known as cause-in-fact or actual causation, requires the plaintiff to demonstrate that the injury would not have occurred absent the defendant's negligent act. The test is straightforward: but for the defendant's conduct, would the plaintiff have been harmed? If the answer is no, actual causation is established. If the harm would have occurred regardless of the defendant's conduct, but-for causation fails.
But-for causation can become complicated in cases involving multiple tortfeasors. When two independent acts each would have been sufficient to cause the harm (concurrent sufficient causes), the traditional but-for test breaks down because neither cause is a but-for cause — remove either one and the harm still occurs. In such cases, courts apply the substantial factor test, asking whether each defendant's conduct was a substantial factor in producing the harm. This approach, endorsed by the Restatement (Second) of Torts, prevents defendants from escaping liability simply because another independent cause was also sufficient.
Another important variation arises in cases of indeterminate causation, where multiple defendants acted negligently but only one caused the harm and the plaintiff cannot identify which one. The landmark case of Summers v. Tice addressed this by shifting the burden of proof to the defendants, each of whom must prove that their conduct was not the actual cause. Market share liability, developed in Sindell v. Abbott Laboratories for DES cases, is a further extension. Understanding these variations is essential because standard but-for analysis alone cannot resolve many real-world causation disputes.
Key Elements
- 1The defendant engaged in negligent conduct
- 2The plaintiff suffered an actual injury or harm
- 3The injury would not have occurred but for the defendant's conduct
- 4In concurrent cause cases, the defendant's conduct was a substantial factor in causing the harm
- 5The burden of proof rests on the plaintiff (unless shifted under Summers v. Tice or similar doctrine)
Landmark Cases
Summers v. Tice
33 Cal.2d 80 (1948)
Shifted the burden of proving causation to defendants when multiple negligent actors made it impossible for the plaintiff to identify which one caused the harm.
Sindell v. Abbott Laboratories
26 Cal.3d 588 (1980)
Created market share liability for indeterminate causation cases, holding DES manufacturers liable in proportion to their market share.
Kingston v. Chicago & N.W. Ry.
191 Wis. 610 (1927)
Applied the substantial factor test where two fires merged, each independently sufficient to cause the harm, so that but-for causation could not be proven for either.
Perkins v. Texas and New Orleans Ry. Co.
243 S.W.2d 527 (Tex. 1951)
Illustrated the application of but-for causation, finding the defendant's failure to slow the train was not the but-for cause of the collision.
Exam Tips
- Always apply the but-for test first. If it fails, consider whether the substantial factor test or burden-shifting applies.
- On exams, look for two simultaneous or concurrent causes — this signals the substantial factor test.
- When the plaintiff cannot identify which defendant caused the harm, discuss Summers v. Tice burden-shifting and potentially market share liability.
Common Mistakes to Avoid
- Stopping at but-for causation when two concurrent causes exist — you must pivot to the substantial factor test.
- Confusing but-for causation with proximate cause — but-for is a factual inquiry, while proximate cause is a policy-based limitation.
- Assuming that correlation establishes but-for causation — the plaintiff must show a direct causal link, not merely that harm followed the defendant's conduct.
Memory Aid
BUT FOR the D's act, P would be fine. If two causes merge, use SUBSTANTIAL FACTOR instead.