But-For Causation vs. Proximate Cause (Foreseeability)
A detailed comparison of these two torts rules, including key differences, exam strategies, and guidance on when to apply each.
Overview
But-for causation (also called actual causation or cause-in-fact) and proximate cause (also called legal cause) are the two components of the causation element in a negligence claim. Both must be established for liability, but they address different questions and serve different functions in the analysis.
But-for causation asks a purely factual question: would the plaintiff's injury have occurred but for the defendant's negligent conduct? If the injury would have happened anyway regardless of the defendant's actions, then but-for causation is not satisfied. This test is straightforward in most cases but can become complicated when multiple causes converge. In cases of concurrent independent causes (where either cause alone would have produced the harm), courts may apply the substantial factor test instead of strict but-for analysis.
Proximate cause, by contrast, is a legal and policy question: should the defendant be held liable for all the consequences that factually flowed from the negligent act, or should liability be cut off at some point? The dominant test is foreseeability, drawn from Palsgraf v. Long Island Railroad (Cardozo's majority opinion): the defendant is liable only for harms to foreseeable plaintiffs from foreseeable risks. Intervening causes can break the chain of proximate causation if they are unforeseeable (superseding causes), but foreseeable intervening causes do not. The egg-shell plaintiff rule is a notable exception, holding that a defendant takes the plaintiff as they find them even if the extent of injury was unforeseeable.
Key Differences
| Aspect | But-For Causation | Proximate Cause (Foreseeability) |
|---|---|---|
| Nature of the inquiry | Factual question: did defendant's conduct actually cause the harm? | Legal/policy question: should defendant be held liable for this harm? |
| Test | 'But for' the defendant's conduct, would the harm have occurred? | Was the type of harm a foreseeable result of defendant's conduct? |
| Multiple causes | Substantial factor test used when but-for test fails due to concurrent causes | Intervening/superseding cause analysis determines if the chain is broken |
| Key case | Perkins v. Texas & New Orleans Railway | Palsgraf v. Long Island Railroad (1928) |
| Policy role | Ensures a factual link between conduct and harm | Limits liability to prevent infinite chains of responsibility |
Exam Tips
Always analyze both types of causation separately on a torts exam. Start with but-for causation; if it fails, briefly consider the substantial factor test for concurrent causes. Then move to proximate cause and discuss foreseeability. The most common exam patterns involve intervening causes: determine whether the intervening cause was foreseeable (defendant remains liable) or unforeseeable and superseding (breaks the chain). Remember that the eggshell plaintiff rule applies to the extent of harm but not the type of harm. A fact pattern where the type of harm is bizarre or unforeseeable is testing proximate cause, not but-for causation.
When to Apply Which
Apply but-for causation whenever you need to establish the basic factual link between the defendant's conduct and the plaintiff's injury. This is the threshold causation requirement. Apply proximate cause to determine whether liability should extend to the particular harm that occurred. Proximate cause becomes the critical issue when there are intervening events, the harm occurred in an unusual way, or the plaintiff was unforeseeable. Both must be satisfied for liability, so analyze them sequentially.