Concept
Multiple Sufficient Causes
Actual causation (but-for cause)
Multiple Sufficient Causes
The but-for test examined so far errs by admitting too much: a necessary condition such as a killer's conception passes it even though no one would call the conception a cause of the killing. The test also errs in the opposite direction. A condition can be sufficient to produce a result without being necessary to it, and when two such conditions operate together, removing either one leaves the other in place to produce the result. Each can then truthfully say that the result would have happened anyway, so a mechanical application of the test concludes that neither caused it. The outcome is a paradox: two people who independently bring about an event would, between them, have caused nothing.1
Tort law supplies a clean picture of the problem. If A negligently starts a fire that is by itself enough to burn B's property, and at about the same time X, or lightning, starts a second fire that is also enough on its own, the two fires merge and the property burns. Delete A's fire and the second fire still burns the property, so one cannot say that but for A's conduct the property would have been saved. Yet A is held to have caused the injury.2
The two versions differ in a single fact, which is whether the second fire could have burned the property without the first. When it could not, deleting A's fire changes the outcome and the ordinary test works, as it does in almost every case in which conduct is held to cause a result.3 When it could, the test breaks down, because it measures necessity and a second sufficient cause makes the first one unnecessary. Sufficiency without necessity is therefore the signature of this problem, just as necessity without sufficiency was the signature of the over-inclusion problem in the last section.
The law refuses the literal answer for a reason that comes from the structure of the situation. If a second sufficient cause could erase the first, responsibility would shrink as the number of wrongdoers grew. When several people independently deliver blows that would each be fatal, every one of them could argue that the victim would have died without that blow, and the most dangerous situation, with multiple would-be killers acting at once, would be the one in which nobody answers. Treating each sufficient cause as a cause prevents the number of wrongdoers from being used to erase responsibility. The same reasoning shows why the exception has an outer edge, since a cause that never operated on the actual result did not produce it. On an exam, the tempting mistake is to accept the other sufficient cause as a defense, which is exactly the argument the exception rejects.
The Code's text does not supply the fix. Section 2.03(1)(a) states the but-for formula, treating conduct as a cause when the result would not have occurred without it, and it contains no separate clause for redundant causes.4 The answer comes instead from the common law: when two causes, each sufficient alone, operate together to produce a result, each is treated as a cause, and cause in fact is more accurately stated as whether the defendant's conduct was a substantial factor in bringing about the forbidden result.5 Every but-for cause is a substantial factor, but a substantial factor need not be a but-for cause, so the broader formulation keeps everything the strict test catches and adds the redundant causes it misses.6 An answer that reaches the exception should present it as general doctrine rather than as Code text.
Civil law meets neighboring puzzles with devices that should not be carried into a criminal answer. In Summers v. Tice, two hunters fired in the plaintiff's direction, a single shot struck the plaintiff's eye, and the plaintiff could not prove which hunter fired it; the California Supreme Court held both liable unless one proved that the other fired the shot.7 That is a problem of identifying which of two wrongdoers acted, with only one cause at work, so it differs from redundant sufficient causes. The burden shift is a tort device, adopted because but-for proof was impossible for the plaintiff and the defendants were better placed to know the facts.8 Criminal law offers the prosecution no comparable shortcut, because when causing a result is an element of the offense, the prosecution must prove it beyond a reasonable doubt.9
Employment discrimination law adds a civil variation that runs the other way. A plaintiff normally must show that an unlawful motive was a but-for cause of a discharge, and when a lawful reason would alone have produced the discharge, the unlawful motive is held not to have caused it, although Congress has by statute lowered the causation showing for some Title VII claims based on status.10 Loss of chance is a further civil device, giving partial recovery in medical cases where a wrong reduced the odds of avoiding harm but actual causation cannot be proved.11 Each of these answers a civil problem with civil purposes, so none of them states how a criminal court treats two sufficient causes.
The parts that follow take the criminal problem in the order in which it is easiest to learn. The first teaches the central exception, in which two causes operate together and each is enough alone. The second teaches the substantial-factor formulation that explains the exception. The third turns to preempted causes, where one sufficient cause arrives first and the exception stops. The last treats statutes whose causal wording demands but-for proof, where the substantial-factor shortcut is unavailable.
Simultaneous Sufficient Causes
The paradox just described has a standard criminal illustration, and the rule that resolves it is clearest on the facts that produced it. A stabs B, inflicting a fatal wound. At the same moment X, acting independently, shoots B in the head, also inflicting a fatal wound, and B dies from the combined effects of the two wounds. Each wound alone was enough to kill, so deleting either one leaves the other in place, and a literal but-for analysis would clear both attackers. The rule of concurrent causation says otherwise: A has caused B's death, and X, standing in exactly the same position, has caused it as well.13 That conclusion settles only the causal element of the offense. A is guilty of murder if the conduct included an intent to kill and of manslaughter if it amounted to recklessness, so the mental-state analysis still runs separately for each attacker.14
The reason the law refuses the literal answer can be read off from what the but-for test is for. The test is a device for detecting whether the defendant's conduct made a difference to the outcome, and ordinarily a real difference shows up when the conduct is deleted. When two sufficient forces overlap, each hides the difference the other makes, even though both were fully at work in the death that occurred. A wound that was capable of killing and that actually contributed to the killing is a cause under any sensible use of the word, and the but-for formula simply fails to register it. Courts therefore ask a different question, whether the defendant's conduct was a substantial factor in bringing about the result, and each mortal wound plainly was one. In Velazquez v. State, a Florida vehicular homicide prosecution, the court explained that the but-for test breaks down in the rare case where two defendants, acting independently and not in concert, each commit an act sufficient to produce the prohibited result, as when each inflicts a mortal wound, and that each defendant's conduct is then a cause in fact of the death.15
The same answer follows when both attackers intend death. If two gang members shoot a victim at the same time, each intending to kill, and each bullet inflicts a mortal wound, each is a cause of the death even though the victim would have died had either one held fire.16 The criminal law does not excuse one actor's intentional harm because another actor also produced the same harm.17 People v. Arzon shows the idea under more pressure. The defendant set a fire on the fifth floor of an abandoned building to keep warm, a second fire started independently on the second floor trapped the firefighters who responded, and one firefighter died from the smoke of both fires. The case is a closer one than the two mortal wounds, because the firefighter probably would not have died without the second fire, yet the defendant's fire also satisfied the but-for requirement and was one component of the forces that killed.18
Civil cases state the same principle with fire in place of bullets. In Kingston v. Chicago & N.W. Ry., sparks from a railroad's locomotive started a forest fire that merged with another fire of unknown origin, and the combined fires burned the plaintiff's property; because either fire alone would have burned it, the railroad's fire was held to be a cause in fact even though it was not a but-for cause. The second fire there came from an unknown source, and in the tort picture the other sufficient force may be innocent or purely natural, so the exception does not depend on the second cause being culpable.19 In Cipollone v. Liggett Group, the Third Circuit, applying New Jersey law, rejected an argument that a manufacturer's conduct could not be a cause because other conduct would have produced the same cancer anyway. It observed that the substantial-factor test has traditionally governed cases with two or more causes each sufficient to cause the injury, and that it is unfair to let a wrongdoer escape for conduct that was enough by itself merely because other conduct might also have been enough.20
The two scenes differ in one respect, which is whether both forces were working on B when death came. In the stabbing-plus-shooting case each wound contributed to the single death, and the redundancy only masks that contribution. In the second scene X's shot finished the work before A's wound reached the result, so nothing in the story supports saying that A's conduct helped bring about the death B actually suffered. The exception is framed for two causes that operate together to produce the result,21 and whether a cause that is overtaken before it operates counts at all is the preempted-cause question taken up below.
The order of the steps matters because the substantial-factor formulation is an alternative reserved for unusual situations in which the but-for test gives unsatisfactory results, and in ordinary cases the but-for showing is the one that must be made.24 Students lose points when they run the facts through both tests as though each had to be satisfied, since a defendant who passes the ordinary test needs no exception and a defendant caught in redundancy cannot pass it.
The Substantial Factor Test
The last part arrived at the substantial-factor formulation as the common-law answer to two independent causes that are each sufficient alone. This part examines what the formulation asks, how far it reaches, and where it stops. The ordinary question under § 2.03(1)(a) is whether the result would have occurred without the defendant's conduct.26 The alternative question is whether the conduct was a substantial factor in bringing about the forbidden result.27 Conduct that passes the first question passes the second, because a condition without which the result would not have happened cannot be a minor part of how it happened. The reverse is not true, since conduct can be a substantial factor in a result that another force would have produced anyway.28 The two questions therefore overlap without being identical, and the extra ground that only the broader one covers is the redundant cause that the but-for test fails to register.
The word substantial carries the weight in this formulation. The but-for test is all or nothing, since the result either would or would not have occurred without the conduct. Once the law relaxes that demand to reach redundant causes, it needs some other way to keep the link between conduct and result real, and the size of the contribution does that work. Without a size requirement, nearly any act at the scene of a death could be said to have added something to it, and the relaxed test would swallow the rule it was meant to supplement. The price of the standard is that it measures degree, so the factfinder must judge how much is enough, and that judgment leaves more room for intuition than the yes-or-no but-for question does.29
The two scenes differ in the fact that decides the answer. The fire at 99 Main Street was fully capable of destroying the house at 101 and was actually at work on it, while the graze could not have killed anyone and contributed nothing of weight to the death. Students go wrong here by treating any contribution at all as enough, when the standard asks for a substantial one. The criminal cases that apply the test give it a firm anchor: in Velazquez v. State the Florida court reserved it for the relatively rare case in which two defendants acting independently each commit an act sufficient to produce the result, as when each inflicts a mortal wound.30 Keeping that anchor in view prevents the standard from drifting into a general invitation to find causation wherever a defendant was somehow involved.
The inquiry into whether each defendant's own contribution was substantial drops out in one setting. The Florida court described the anomaly as arising when two defendants act independently and not in concert with one another.31 That framing has a corollary. When defendants are conspirators, the law attributes each member's acts within the plan to the others, so the fatal act already counts as the act of every member who agreed to it, and the question whether a particular member's own contribution was substantial never needs an answer.
The agreement is the only fact that moved the answer between the graze scene and this one. The attribution reaches acts within the common plan, so a stranger who happened to shoot the same victim would be an independent actor, and the graze analysis would return.
A final caution concerns the phrase itself. Courts use substantial factor to test cause in fact, to test proximate cause by asking whether a cause in fact is weighty enough to carry legal responsibility, or as shorthand covering both, and the Third Restatement of Torts disavows the term in every context.32 The idea survives without the label: that Restatement states the but-for rule in § 26 and treats multiple sufficient causes separately in § 27, where each is a factual cause.33 Usage varies even inside cause in fact. One Louisiana court, deciding whether a train's excessive speed caused a crossing collision, stated cause in fact as a substantial factor and then answered by asking whether the collision would have occurred without the speed, which makes the phrase a gloss on the but-for question.34 In Cipollone v. Liggett Group, by contrast, the Third Circuit collected New Jersey decisions that applied the label to a physician whose seven-month delay in operating on a tumor could have been a substantial factor in the later harm, even though the delay alone would not have produced it, because the delay substantially increased the probability of injury.35
Those civil usages show how far the label can stretch, and the Supreme Court has been wary of the stretch in criminal cases. In Burrage v. United States the Court, applying the rule of lenity to a federal penalty provision, held that where the distributed drug was not an independently sufficient cause of the death, the defendant could not be liable unless the drug was a but-for cause.36 The Court observed that the Government could not say how substantial a cause must be, and that uncertainty of that kind cannot be squared with proof beyond a reasonable doubt.37 That decision turned on statutory wording and is taken up in the last part below, but it explains why a criminal answer should anchor the test in independently sufficient causes rather than in contribution alone. An answer that uses the phrase should say which question it is answering, which is cause in fact by the redundant-cause route, with proximate cause analyzed separately.
Preempted Causes
The last part closed on a scene in which X's shot finishes the work before A's earlier wound reaches the result, and it left open whether A counts as a cause at all. A force that would have produced the result by itself but is overtaken by another force that produces it first is a preempted cause, and the question is how to treat the actor behind it. The actor who does the overtaking is the easy half. When A inflicts a mortal wound that would kill B in an hour and X then kills B instantly, X is a cause of the death, because X shortened B's life by about an hour and one who hastens a death causes it.39 A's position is harder. Delete A's wound and B still dies at that moment and in that way from X's act, so A does not satisfy the ordinary question of whether the result would have occurred without the conduct.40 The exception for simultaneous sufficient causes does not help either, because it reaches two forces that operate together on the death, and here X's act completed the death before A's wound could.
The authorities divide on whether A is nonetheless a cause in fact. Some decisions hold that A's conduct is not a cause of B's death, on the theory that X's act cut A's off, but there is authority to the contrary, and the prevailing academic view approves the first position.41 A standard illustration has two shooters: D1's shot enters the lung and would have killed in an hour, while D2's shot enters the heart and kills instantly. Only D2 has killed the victim, and D1 is guilty of an attempt.42 Attempt is the natural landing place for an actor in D1's position, because the intent and the dangerous act are both present while the death is attributed to someone else.
The reason for the majority position lies in what a result element asks the prosecution to prove. When causing a particular result is an element of the crime, the prosecution must establish beyond a reasonable doubt that the defendant caused the result that actually occurred.43 That death was produced at a particular moment by X's act, so the inquiry is directed at the forces that brought it about and not at the forces that were poised to. The same idea explains the redundant-cause exception: a wound that really contributed to a death is not erased by the presence of a second sufficient wound, because the question is what did happen and not what would have happened.44 Applied here, that question excludes A, because when death came A's wound had not yet produced it. Treating A as a cause would convict A of homicide for a death that A's conduct did not bring about, when the law already reaches A's culpable conduct and intent through attempt. Students lose this issue when they treat a wound that would have killed as though it did kill.
How much the later act displaces the earlier one separates the settled cases from the contested one. When A shoots B and leaves B dying, and C then stabs B and hastens the death, C is guilty of murder for intentionally shortening B's life, and most courts also hold A liable because both wounds were contributing to the life-shortening process at the time of death.45 If C had decapitated B, holding A responsible would be harder, as State v. Luster suggests, and A's liability would most likely be limited to attempted murder.46 The two scenes differ in one respect, which is whether A's wound was still working on B when death came. A stab leaves A's wound a participant in the dying, while a decapitation ends life by a route to which A's wound contributed nothing. A fallback theory remains available even then: A's intent to kill may have left B unable to defend against C, which would make A a proximate cause, and compare Henderson v. Kibbe, where defendants who robbed a thoroughly intoxicated man and abandoned him on an unlighted road on a cold night were guilty of second-degree murder after a truck struck him.47 That theory works through legal cause rather than cause in fact, so it leaves the actual-cause gap in place and belongs to the legal-cause analysis that follows once actual cause is settled.
The area stays contested because the facts can be altered so that the overtaking act does not itself kill. Henri sets out across a desert. Alphonse, intending to kill Henri, poisons the canteen. Gaston, who also intends to kill Henri and knows nothing of the poison, punctures the canteen, and Henri dies of thirst.48 Neither act appears to be a but-for cause. Henri never swallowed Alphonse's poison, and Gaston's puncture only deprived Henri of poisoned water that would have killed even faster than thirst, so the puncture prolonged Henri's life. Yet if neither had acted Henri would be alive, which is why the case resists a tidy answer. Each candidate carries a pull and a problem. Alphonse had the intent and created the lethal danger but never touched the result, while Gaston's act is nearest the death but made it later than it would otherwise have come. The preemption view ties causation to events that actually happened and not to intent or to the danger an actor created, so it gives uncomfortable answers on these facts, and it has been questioned on somewhat different facts, including a desert-poison variation.49 The puzzle is best presented as unresolved.
On an exam, a mortal wound overtaken by an instantly fatal act calls for a hedged but definite answer. State that the overtaken actor is not a but-for cause and, on the view most decisions and the prevailing academic writing favor, not a cause in fact of the death, so attempt is the likely charge, and then acknowledge that some authority points the other way.50 The overtaking actor is a cause of the death because that actor hastened it. Where the facts show the first wound still contributing when death came, as with a stab added to a gunshot or two slow mortal wounds, return to the simultaneous-sufficient-causes rule. For the canteen pattern, explain why neither act is a but-for cause and lay out the competing accounts without announcing a settled rule.
Statutory But-For Language
The preceding parts treated cause in fact as a judge-made inquiry that begins with the but-for question and, where two independent forces were each sufficient, relaxes into the substantial-factor test. That account assumes the statute defining the offense is silent about causation. When the legislature writes its own causal wording into an offense or a penalty provision, the first task is to read those words, and the reading can close the substantial-factor route. The baseline remains the question whether the result would have occurred without the conduct.52 The Model Penal Code leaves room for more, because it requires but-for causation together with any other causal requirement that the Code or the law defining the offense imposes, and it lets legislatures write traditional causal elements into a statute if they wish.53 The question here is how courts translate causal verbs such as results from and because of when a criminal penalty depends on them.
The leading criminal illustration is Burrage v. United States, which construed a federal penalty enhancement. The statute imposed a mandatory minimum sentence on a defendant who unlawfully distributed a drug when “death or serious bodily injury results from the use of such substance,” and the supplied drug in the case contributed to the death or injury without being a but-for cause of it.54 Applying the rule of lenity, which resolves genuine ambiguity in a criminal statute in the defendant’s favor, the Court read “results from” to impose a requirement of but-for causation.55 State statutes that require only that the supplied drug “contribute to” the death or injury stand in contrast, since they track the view that a substantial or contributing factor is enough.56 The wording of the provision therefore decided whether a contribution short of but-for causation could support the enhancement.
The Court framed its holding with a limit that connects to the earlier parts of this chapter on redundant causes. A defendant cannot be liable under the enhancement unless the drug use was a but-for cause, at least where the drug was not an independently sufficient cause of the death or injury.57 The two-fatal-wounds cases involve causes that are each independently sufficient, and that is the setting in which the substantial-factor test earned its place. A drug that was merely one contributor and could not have produced the death alone sits outside that setting, so the substantial-factor label gives a prosecutor in that position nothing to work with. The Court also separated the substantial-factor approach from the traditional view, under which one of several causes counts as a cause in fact only if it could have brought about the harm by itself.58
Two reasons support reading the verb this way, and they operate at different levels. The first is ordinary meaning: results from and because of customarily carry the same sense in the Court’s decisions, and both call for a showing that the harm would not have occurred without the conduct.59 A court that gives the words their usual sense reaches but-for causation without any special criminal doctrine. The second reason combines the rule of lenity with the reasonable-doubt standard. The Government could not specify how substantial a cause must be, and it was unclear whether a drug that made death fifty percent more likely would qualify, or fifteen percent, or five. A threshold that no one can state cannot be reconciled with proof beyond a reasonable doubt or with the need to write criminal laws in terms ordinary people can understand.60 The incentive problem is easy to see: a substantial-contribution standard would leave each jury to draw the line after the fact, so two defendants who did the same thing could receive very different sentences depending on how a jury weighed the word substantial. The but-for inquiry produces a yes-or-no answer that the prosecution either proves or fails to prove, which is a sturdier footing for a mandatory minimum. Neither reason stops a legislature from choosing a looser standard; it simply has to write one, as the contribute-to statutes do.
Reading a statute to require but-for causation does not demand that the defendant be the only cause. This matches the earlier point that one result usually has several but-for causes. A predicate act can combine with other factors and still qualify, so long as those factors alone would not have produced the result. The Court’s own illustration was poison given to a man weakened by several diseases: the poison is a but-for cause of his death even though the diseases played a part, provided that without the poison’s incremental effect he would have lived.61
A civil statute shows the same translation of a causal phrase, and a short parallel is enough here. In Gross v. FBL Financial Services, Inc. the Age Discrimination in Employment Act forbade adverse action “because of” an individual’s age, and the Court read those words to mean “by reason of,” so that the plaintiff must prove age was the but-for cause of the employer’s decision.62 A plaintiff who shows only that age was one motivating factor does not shift the burden of persuasion to the employer, because the statute’s text creates no such shift and the plaintiff keeps the ordinary burden of proving the claim.63 The contrast with Title VII shows that wording drives the result: Congress amended Title VII in 1991 to allow liability when a protected trait was a motivating factor, did not extend that language to the ADEA, and the Court treated the omission as intentional.64
Courts apply the same reading to criminal motive elements. Construing the federal hate-crimes provision that reaches bodily injury caused “because of” a victim’s actual or perceived religion, the Sixth Circuit concluded that the victim’s religion must have been a but-for reason for the defendant’s act, and it rejected an instruction that a “significant motivating factor” would suffice.65 The court noted that the Supreme Court has insisted on but-for causality for statutes using that phrase in criminal and civil cases alike, whether the phrase attaches to a prohibited act or to a prohibited motive.66 Its objection to the looser instruction repeats the one raised in the drug case: a jury cannot tell whether a motive that was one of three reasons for acting, or one of ten, was significant enough, and uncertainty of that kind cannot be squared with the reasonable-doubt standard.67
The usual error on an exam is to carry the substantial-factor test into every causation problem. Students lose this issue when they apply substantial factor to a statute whose own causal verb already sets the standard.68 If the provision says results from or because of, run the but-for question and accept that a modest contribution that cannot be shown to have made the difference will not do. If the statute says only that the conduct contributes to the harm, the looser reading is the one the text invites. Either way, legal cause remains a separate inquiry that follows once actual cause is settled.