Also known as: but for causation · but-for causation · but-for causation standard · but for test · but-for test · sine qua non causation
Written by attorneys · grounded in primary & secondary sources — see below
A factual causation requirement under which a defendant's conduct is a cause of the plaintiff's harm if the harm would not have occurred without that conduct. The test supplies the necessary link between breach and injury in negligence and between act and result in homicide. It operates even when other contributing forces are present so long as the defendant's act remains essential to the outcome.
Eric injected Cole with a non-sterile needle during a lab prank. Cole developed an infection that combined with other medical factors and caused fatal organ failure eighteen months later. But for the injection the infection would not have occurred and Cole would not have died. The prosecution cannot charge homicide because the death fell outside the one-year-and-one-day window even though the but-for standard is satisfied.
A food-processing plant knowingly operated an aging conveyor with its emergency shutoff disabled. During Carla's shift the conveyor jammed and her hand became caught in the gears. But for the plant's decision to run the machine without the shutoff Carla's hand would not have remained caught. The but-for standard is therefore met and the plant's conduct is a factual cause of the injury.
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No. But-for causation is necessary but not sufficient. The plaintiff must also prove legal or proximate cause, which incorporates policy limits on responsibility such as the substantial-factor test and rules governing superseding causes.
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The year-and-a-day rule operates as a strict temporal limit that bars homicide liability even when but-for causation is clearly established. If the victim dies after the period expires the prosecution cannot charge homicide regardless of the strength of the causal chain.
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The court decides which legal causation doctrines govern the case and instructs the jury accordingly. The jury then applies those rules to the facts to determine whether but-for or substantial-factor causation has been shown.
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The plaintiff must prove that the defendant's breach was a legal cause of the loss. This requires showing both factual or but-for causation and that the breach satisfies the additional legal-cause limitations imposed by the governing rules.
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