Causation in criminal law is the required link between what a defendant did and the harm that followed, and prosecutors must prove it before any conviction for a result-based crime can stand. For charges like homicide or assault, where the offense is defined by a specific outcome, the state has to show two things: that the defendant’s conduct actually brought the harm about, and that the connection is close enough to justify holding the defendant responsible. Those two layers are called factual causation and legal causation, and they work together.
Factual Causation and the But-For Test
Factual causation is the starting point. Courts assess it with the but-for test: would the harm have happened if you removed the defendant’s conduct from the equation? If the answer is no, the defendant is a factual cause of the harm.1Legal Information Institute. But-For Test Someone poisons a drink, the victim drinks it, the victim dies. The death would not have happened but for the poisoning. Factual causation is established.
The test also catches a defendant who speeds up an outcome that was already on its way. If a victim has a terminal illness and an assault causes death sooner than the disease would have, the defendant still factually caused the death. Hastening an inevitable result counts the same as causing it outright.1Legal Information Institute. But-For Test A defendant cannot escape liability by arguing the victim was already dying.
When the But-For Test Breaks Down
The but-for test works well in straightforward cases, but it stumbles when two independent acts combine to cause the same harm. Picture two people who separately set fires, and both fires merge to destroy a building. Neither defendant can honestly claim “the building would have burned anyway because of the other fire,” even though that is technically true under a strict but-for analysis. Applied rigidly, the test would let both defendants walk free, which is the wrong result.2Legal Information Institute. Merged Causes
Many courts handle these situations with the substantial factor test instead. Rather than asking whether the harm would have occurred without the defendant’s act, this test asks whether the defendant’s conduct was a significant contributing factor in bringing about the result. The act need not be the only cause or even the primary one, but it has to be more than trivial.3Legal Information Institute. Substantial Factor Test Under this approach, both fire-setters are liable, because each fire was independently capable of destroying the building.
Legal Causation and Foreseeability
Factual causation alone is not enough. Legal causation, often called proximate cause, asks a different question: is the connection between the defendant’s act and the final harm close enough that holding the defendant responsible makes sense? The doctrine exists to limit liability to results reasonably proximate to the defendant’s conduct, rather than distant or freakish consequences.4Legal Information Institute. Proximate Cause
Foreseeability is the central concept. If the type of harm that occurred was a foreseeable consequence of the defendant’s actions, legal causation is satisfied. The exact way the harm unfolds doesn’t have to be predictable, but the general kind of harm does. A defendant who shoves someone off a ledge is the proximate cause of the victim’s injuries from the fall, even if the specific fractures weren’t predictable, because serious injury from a fall is foreseeable.4Legal Information Institute. Proximate Cause
How direct and necessary the defendant’s conduct was to the injury also matters. Remote or trivial factors might qualify as actual causes but will not satisfy the proximate cause standard.4Legal Information Institute. Proximate Cause Someone who sold a car to a driver who later drove drunk and killed a pedestrian might be an extremely remote but-for cause, but no court would treat the car sale as a proximate cause of the death.
Intervening and Superseding Causes
Events that fall between a defendant’s initial act and the final harm are called intervening causes, and they are where causation disputes get heated. An intervening cause is anything that enters the chain of events after the defendant acts but before the harm is complete. The critical question is whether that intervening event was foreseeable. If it was, the original defendant typically stays on the hook. If it was not, the intervening event may become a superseding cause that breaks the chain and absorbs the liability.
Some intervening events are treated as foreseeable almost automatically. When a defendant injures someone and a doctor provides negligent medical treatment that worsens the injury, that treatment is generally foreseeable because medical complications happen. The defendant remains liable. Extraordinary or highly unusual actions by medical professionals, or completely unrelated events like a lightning strike hitting the victim on the way to the hospital, are the kinds of unforeseeable events that courts treat as superseding.
The foreseeability line is not always clean. Courts look at the totality of the circumstances, but a few principles are well established. Rescue attempts are foreseeable, so a defendant who creates a dangerous situation is liable for injuries a rescuer sustains. Ordinary natural forces, like a rainstorm, are foreseeable. Truly extraordinary natural events and deliberate criminal acts by third parties are usually treated as unforeseeable superseding causes that shield the original defendant from liability for those consequences.
The Eggshell Skull Rule
A major exception to the foreseeability requirement is the eggshell skull rule, sometimes called the thin skull rule. Under this doctrine, a defendant who causes injury must take the victim as they find them. If a victim has an unusual pre-existing condition that makes the harm far worse than anyone could have predicted, the defendant is liable for the full extent of the injury anyway.5Legal Information Institute. Eggshell Skull Rule
The classic example gives the rule its name. A defendant punches someone who happens to have an abnormally thin skull. Most people would walk away with a bruise, but this victim suffers a fatal skull fracture. The defendant is fully liable for the death, even though a reasonable person would never have predicted it. Once you commit an unlawful act against someone, you accept responsibility for the consequences, including consequences amplified by conditions you did not know about.5Legal Information Institute. Eggshell Skull Rule The rule shows up regularly in homicide cases where a victim’s hidden medical vulnerability turns an assault into a killing.
The Model Penal Code Approach
The Model Penal Code, which many states have adopted in some form, uses its own framework. It starts with the but-for test: the defendant’s conduct must be an antecedent without which the result would not have occurred. The MPC then adjusts the legal causation analysis based on the defendant’s mental state.
When a crime requires that the defendant acted purposely or knowingly, the actual result must be within what the defendant intended or expected. The MPC makes an allowance: if the actual result differs from the intended result only because a different person was harmed or the injury was less severe than planned, causation still holds. The result also counts if it involves the same kind of harm the defendant contemplated and is not too remote or accidental to fairly bear on the defendant’s liability.1Legal Information Institute. But-For Test
For crimes based on recklessness or negligence, the MPC uses the same logic but measures the actual result against the risk the defendant was aware of (for recklessness) or should have been aware of (for negligence). If the actual harm is the same kind of harm within that risk and is not too remote or accidental, causation is established. For strict liability offenses, the standard is simpler: the result must be a probable consequence of the defendant’s conduct. The causation standard tightens or loosens depending on how culpable the defendant’s mindset was.
The Year-and-a-Day Rule
An older common-law rule still worth knowing is the year-and-a-day rule. Under this rule, a defendant could not be convicted of homicide if the victim died more than a year and a day after the defendant’s act.6Legal Information Institute. Year and a Day Rule Modern forensic evidence can now trace a death back to injuries sustained long before, and many states have abolished the rule through legislation or court decisions. Some states retain it, and a few have modified it by extending the time period rather than eliminating it. If a case involves a significant delay between an injury and a death, the rule may still matter depending on the jurisdiction.
Proving Causation at Trial
The prosecution carries the burden of proving both factual and legal causation beyond a reasonable doubt, the same standard that applies to every element of a criminal charge. A jury that believes the defendant probably caused the harm but is not sure beyond a reasonable doubt must acquit.
Prosecutors build causation through physical evidence, witness testimony, and expert analysis. Homicide cases frequently hinge on forensic pathology reports linking the cause of death to the defendant’s actions. Defense attorneys attack causation by pointing to alternative explanations, pre-existing conditions, or intervening events. If the defense can raise a reasonable doubt about whether the defendant’s conduct actually caused the harm, or whether a superseding cause broke the chain, that is enough for acquittal on any charge that requires a specific result. Causation is often the most contested element in serious felony trials.