The substantial factor test is a causation rule in tort law that holds a defendant liable when their conduct played a meaningful role in causing an injury, even if other forces also contributed. Courts turn to it when the ordinary “but-for” causation standard would produce an unjust result, typically because two or more independent causes combined to produce a single harm that cannot be divided between them. The test asks the jury a straightforward question: was the defendant’s conduct important enough in bringing about the injury that a reasonable person would call it a cause?
Why Courts Need an Alternative to But-For Causation
The default rule in negligence law is that a plaintiff must prove the injury would not have happened but for the defendant’s conduct. In most cases, that works. A driver runs a red light and hits a pedestrian; without the negligence, no injury.
The rule falls apart when two independent forces are each sufficient to cause the entire harm. The classic illustration is two fires. Suppose a railroad engine negligently throws sparks that start Fire A, and a separate fire of unknown origin becomes Fire B. The two merge and burn down a house. Fire A alone would have destroyed the house. So would Fire B. Neither is a but-for cause, because the house would have burned down anyway from the other fire. Under strict but-for reasoning, the railroad walks away from clearly negligent conduct and the homeowner recovers nothing.
The Minnesota Supreme Court confronted exactly this problem in Anderson v. Minneapolis, St. Paul & Sault Ste. Marie Railway and refused to accept that outcome. The court instructed the jury that if the railroad’s fire was “a material or substantial factor” in causing the damage, the railroad was liable regardless of the other fire’s contribution. That phrasing stuck, and it became the foundation of the test courts still use.
What Counts as a Substantial Factor
The Restatement (Second) of Torts provided the framework most courts adopted. Section 431 states that a defendant’s conduct is a legal cause of harm if it is “a substantial factor in bringing about the harm” and no rule of law otherwise relieves the defendant of liability. Section 433 then lists what juries should weigh in making that call: how many other forces contributed to the harm, whether the defendant’s conduct created a force that remained active up to the moment of injury, and whether the contribution was significant enough that a reasonable person would regard it as a cause.1H2O. Torts: Basic Fluency in a Fundamental Legal Language (Revised) – Restatement Second, Section 433, On Substantial Factor
The threshold is deliberately flexible. The defendant’s act does not have to be the primary cause, or even the largest cause. A defendant whose conduct accounted for 20% of the force behind an injury can still be a substantial factor. What falls outside the test is conduct so minor or remote that no reasonable person would connect it to the harm. The Restatement treats this as a de minimis floor: casual or minimal contact is not enough.
Juries decide this question on the totality of the evidence, weighing both the size of the contribution and its role in the actual sequence of events. A small act that triggered a chain reaction might qualify. A larger contribution that had dissipated before the injury occurred might not. The focus is always on whether the defendant’s role was real and significant in context.
When the Test Applies: Concurrent and Indivisible Harm
The substantial factor test is for cases involving concurrent causes, meaning two or more separate acts or forces that combine to produce one injury that cannot be split by source. Divisibility is what matters, not timing. If one driver breaks your arm in one crash and a different driver breaks your leg in another, those are separate injuries with individual liability attached to each. But if both drivers hit you at the same intersection and your spinal injury cannot be traced to one impact rather than the other, the harm is indivisible and the causes are concurrent.
Even negligent acts separated by days or weeks can be concurrent causes if the final injury cannot be apportioned among them. The test turns on whether there is a reasonable basis for separating each defendant’s contribution. When there isn’t, each defendant who contributed meaningfully faces potential liability for the whole injury.
Applying the Test in Toxic Exposure Cases
Toxic tort litigation is where the substantial factor test gets its heaviest workout. A worker exposed to asbestos from a dozen manufacturers over a 30-year career develops mesothelioma. Each product may have contributed, but no medical test can identify which fibers triggered the disease. Courts needed concrete criteria for deciding when a plaintiff has shown enough to reach a jury.
The Fourth Circuit supplied one in Lohrmann v. Pittsburgh Corning Corp., establishing the “frequency, regularity, and proximity” test. To survive summary judgment, a plaintiff must show exposure to the specific defendant’s product “on a regular basis over some extended period of time in proximity to where the plaintiff actually worked.” The court explicitly described this as a de minimis rule requiring “more than a casual or minimum contact with the product.”2CaseMine. Lohrmann v. Pittsburgh Corning Corp.
The Lohrmann framework has been widely adopted in asbestos cases and extended to other toxic exposure contexts. A plaintiff who worked alongside a particular insulation product daily for five years has a much stronger case than one who walked past the same product once during a building tour. Expert testimony usually carries the weight here. A general opinion that asbestos causes mesothelioma is not enough; the expert must connect this plaintiff’s actual exposure history to this defendant’s specific product.
What the Restatement Third Changed
The Restatement (Third) of Torts abandoned the “substantial factor” language entirely. The drafters concluded that the phrase had become a source of confusion, sometimes used to expand liability beyond what the evidence supported, other times used to narrow it inappropriately. The Third Restatement returned to but-for as the default rule under Section 26, with a specific carve-out for the overdetermination problem.
Section 27 handles multiple sufficient causes directly: when multiple acts each would have independently caused the same harm at the same time, each act is treated as a factual cause of the harm.3H2O. Restatement (3d.) (Liability for Physical and Emotional Harm) 27 – Multiple Sufficient Causes This produces the same answer as the substantial factor test in the two-fire scenario without relying on the vaguer phrase.
The practical effect is to narrow the universe of cases where courts use anything other than but-for causation. Under the Second Restatement, some courts stretched “substantial factor” into a general replacement for but-for across all kinds of cases. The Third Restatement limits the exception to true overdetermination, where each act independently would have been sufficient to cause the entire harm. Many jurisdictions still apply the Second Restatement formulation, however, so which framework governs depends on where the case is filed.
A Different Problem: Unknown Which Defendant Caused the Harm
The substantial factor test should not be confused with the burden-shifting rule from Summers v. Tice. In that California case, two hunters simultaneously fired toward the plaintiff and one pellet struck him in the eye. The evidence could not establish whose gun fired the injuring shot. The court shifted the burden to the defendants, requiring each to prove his shot was not the cause. If neither could, both were liable.4Justia Law. Summers v. Tice
That doctrine addresses a different problem: not multiple causes combining to produce one injury, but uncertainty about which of several negligent defendants actually caused the harm. It applies only when all potential defendants are before the court, each acted negligently, and the plaintiff genuinely cannot identify which one caused the injury. The substantial factor test, by contrast, assumes multiple defendants did contribute and asks whether each contribution was meaningful enough to count.
What a Substantial Factor Finding Means for Damages
Being found a substantial factor in an indivisible injury can carry heavy consequences under joint and several liability. Each defendant found to be a substantial factor can be held responsible for the entire judgment. If a plaintiff wins $2 million against three defendants and two are bankrupt, the remaining solvent defendant pays the full $2 million. The reasoning is that the plaintiff did nothing wrong and should not bear the risk that some defendants cannot pay.
Many states have modified this through tort reform, limiting joint and several liability to defendants above a certain fault threshold or replacing it entirely with proportional liability. In a proportional jurisdiction, a defendant found 15% at fault pays only 15% of the damages, regardless of whether the co-defendants can cover their shares. The risk of a co-defendant’s insolvency shifts to the plaintiff.
Comparative fault adds another layer. If the plaintiff bears some responsibility for the injury, recovery is reduced by the plaintiff’s assigned percentage, and in some states barred entirely once that percentage crosses a threshold. In a concurrent causation case, the jury may end up assigning fault across the plaintiff and multiple defendants in a single verdict. Whether any one defendant can be forced to cover another defendant’s share depends on the state’s liability rules.
This is why defendants in concurrent causation cases fight the substantial factor question so hard. A finding that their conduct cleared the threshold can mean liability far beyond their actual share of fault, with only a theoretical right to collect contribution from co-defendants who may have nothing worth pursuing. For plaintiffs, the combination of the substantial factor test and joint and several liability remains the strongest route to full compensation when several wrongdoers combined to cause a single harm.