butfor

But-for causation, or the but-for test, is a standard used in tort law to determine factual causation between a defendant’s conduct and a plaintiff’s harm. It asks whether the plaintiff’s injury would have occurred if not for the defendant’s actions. If the injury would have happened anyway, the defendant’s breach is not a factual cause; if the injury would not have occurred without the defendant’s conduct, that conduct is a but-for cause.

In practice, the test is applied by considering a hypothetical world in which the defendant’s conduct did not occur. If the plaintiff would have avoided the harm in that world, the defendant’s action is considered causally relevant. The test helps separate legal responsibility from mere correlation and is often the starting point for establishing liability in negligence and related torts.

The but-for test has limits, especially when there are multiple concurrent or alternative causes. If two independent acts each would have caused the harm, no single act may be a but-for cause, leading courts to use other approaches such as the substantial factor or material contribution tests, or to allocate liability among defendants in different ways. In some jurisdictions, special doctrines address harmful exposure from multiple tortfeasors or uncertain causation, including loss-of-chance or market-share liability theories.

Historically rooted in common-law reasoning, the but-for standard remains a foundational concept in determining factual causation. It is distinguished from proximate causation, which concerns policy-based limits on liability, and from legal causation, which governs outcomes in specific jurisdictions.