Causation

Causation is one of the four essential elements of a negligence claim in personal injury law, requiring the plaintiff to prove a direct link between the defendant's breach of duty and the plaintiff's injuries. Causation is analyzed in two parts: actual cause (also called cause-in-fact) and proximate cause (also called legal cause). Both must be established for a plaintiff to recover damages in a negligence case. Without causation, a defendant cannot be held liable even if they were clearly careless.

Actual causation is typically analyzed using the but-for test: but for the defendant's negligent act or omission, would the plaintiff's injury have occurred? If the answer is no — meaning the injury would not have happened without the defendant's conduct — then actual causation is established. In cases with multiple potentially causative factors, the substantial factor test may be used instead, asking whether the defendant's conduct was a substantial contributing cause of the plaintiff's harm.

Proximate cause, or legal cause, limits the scope of a defendant's liability to harm that was a foreseeable result of their negligence. Even if a defendant's conduct was the but-for cause of an injury, they may not be liable if the injury was caused by an unforeseeable intervening act or if the harm that resulted was so remote and unexpected that it would be unjust to hold the defendant responsible. Proximate cause questions often arise in complex accident scenarios where multiple parties and events contributed to the ultimate harm.

Causation is frequently contested in personal injury cases, particularly in medical malpractice and toxic tort litigation. Defendants commonly argue that the plaintiff's injuries were caused by a pre-existing condition, subsequent events, or factors unrelated to the defendant's conduct. Expert medical testimony is almost always required to establish causation in cases involving physical injury, and economists, accident reconstructionists, and engineers may also testify on causation issues depending on the type of case.