To win a negligence case, a plaintiff has to connect the defendant’s conduct to the injury on two separate levels, and causation in negligence law is the element that tests both. First, the plaintiff must show that the defendant’s act was a real, physical cause of what happened. Second, the plaintiff must show that the harm was the kind of consequence the law is willing to hold the defendant responsible for. A claim that clears the first hurdle can still fail at the second, and many do.
The Two Layers Every Plaintiff Must Prove
Courts split causation into actual cause (also called cause-in-fact) and proximate cause. Actual cause is mechanical: did this defendant’s conduct set in motion the events that hurt the plaintiff? Proximate cause is a policy filter: even if the physical chain is intact, was the injury a reasonably foreseeable result of the defendant’s conduct, or is it too remote to justify liability?
The plaintiff carries the burden on both, by a preponderance of the evidence — meaning it must be more likely than not that the defendant caused the harm.1Legal Information Institute. Preponderance of the Evidence That is a lower bar than the criminal standard, but it still requires evidence rather than argument, and in complex cases it usually requires expert testimony.
The But-For Test
The but-for test is the default way courts decide actual causation. Mentally remove the defendant’s negligent act from the sequence of events and ask whether the injury still would have occurred. If the harm disappears in that hypothetical, the defendant’s conduct was a necessary cause. If the harm happens anyway, the defendant did not cause it.2Legal Information Institute. but-for test
A driver runs a red light and hits a pedestrian in the crosswalk. But for the driver running the light, would the pedestrian have been struck? No. The driver is the cause-in-fact. That subtraction exercise strips away background facts and isolates whether the defendant’s specific mistake made the difference.
The but-for test handles most cases cleanly. It breaks down in one recurring situation: when two independent forces each would have been enough on their own to cause the same harm.
The Substantial Factor Test
The classic illustration involves two separate fires, set independently, that merge and destroy a house. Apply but-for logic to either fire and you get an absurd answer: remove one and the other still burns the house down, so neither “caused” the loss. Under strict but-for reasoning, both defendants escape.3Legal Information Institute. Substantial Factor Test
The substantial factor test fixes this by asking whether the defendant’s conduct was a significant contributor to the outcome, even if other forces were operating at the same time. The defendant’s act does not have to be the only cause, or even the main one. It just has to be more than trivial. Both fire-starters were substantial factors, so both are liable.
This test does most of its work in cases with multiple defendants whose independent actions converge on a single injury. A defendant cannot escape by pointing to someone else who also contributed.
Proximate Cause and the Scope of the Risk
Actual causation is only half the answer. Even when the physical chain is clear, the law limits liability to consequences that were reasonably foreseeable results of the defendant’s conduct. Without that limit, a single careless act could theoretically trigger responsibility for an endless cascade of downstream events.
The foundational case is Palsgraf v. Long Island Railroad Co. (1928), which held that a defendant’s duty of care extends only to people within the foreseeable range of risk created by the negligent act.4New York State Unified Court System. Palsgraf v Long Island Railroad
The important distinction is between the type of harm and its severity. Proximate cause asks whether the general kind of injury was foreseeable, not whether the exact sequence of events was predictable. A driver who rear-ends another car can foresee neck injuries to the occupants. The driver does not need to foresee the precise medical complications that follow. When the injury falls within the range of risks that made the conduct negligent in the first place, proximate cause is satisfied.
Many courts and the Restatement (Third) of Torts now frame this as a “scope of risk” analysis rather than proximate cause. The question becomes whether the harm that actually occurred is the same general type of harm that made the defendant’s conduct risky. A factory that dumps chemicals is liable when neighbors develop respiratory illness, because respiratory illness is exactly the risk that made the dumping negligent. If instead a delivery truck swerves to avoid the spill and crashes a mile away, that harm sits outside the original risk, even though a physical chain of events connects the two. The outcomes tend to look similar to traditional foreseeability analysis; the framing just keeps the jury focused on the specific risk the defendant created.
The Eggshell Skull Rule
There is one major boundary on foreseeability limits. Under the eggshell skull rule, once some injury to the plaintiff was foreseeable, the defendant is liable for the full extent of the harm, even if the actual injuries turned out far worse than anyone could have predicted. The defendant takes the victim as they find them.5Legal Information Institute. Eggshell Skull Rule
Bump into someone with an unusually thin skull and cause a catastrophic head injury that a healthy person would have shrugged off, and you are on the hook for the severe outcome. You had no way of knowing about the condition; it does not matter.
This rule trips up a lot of people because it looks like it contradicts proximate cause. It does not. Proximate cause requires the general type of harm to be foreseeable. The eggshell skull rule says the severity of that harm does not. A defendant can argue that a brain injury was not a foreseeable result of a fender bender. A defendant cannot argue that the plaintiff’s brain injury should have been minor instead of catastrophic.
Intervening and Superseding Causes
Between the defendant’s negligence and the final injury, other events sometimes intervene. Whether those events break the chain depends on how foreseeable they were.
An intervening cause is any new force that enters the picture after the defendant’s act. If the new event was a normal, foreseeable consequence of the situation the defendant created, the defendant is still liable. A superseding cause is an intervening event so unexpected or extraordinary that it severs the legal connection, shifting liability off the original defendant entirely.
Some intervening causes are treated as foreseeable almost automatically. Negligent medical treatment is the leading example. If a defendant injures someone and the hospital worsens the injury through a surgical error, the original defendant remains liable, because the risks of medical care are a predictable consequence of hurting someone badly enough to need it. Only extraordinary medical misconduct or an intentional act by the treating physician might qualify as superseding. Rescue attempts follow the same pattern. When a defendant’s negligence puts someone in danger, it is foreseeable that others will try to help, and injuries to those rescuers remain the defendant’s responsibility.
Genuinely unforeseeable criminal acts are the most common superseding causes. Leaving a warehouse door unlocked, followed by a random arsonist burning the building down, may sever liability, because arson was not a predictable consequence of the door being open. Context changes the answer. If the warehouse sits in an area with a known arson problem, or if the defendant was hired specifically to prevent unauthorized entry, the criminal act becomes foreseeable and liability survives.
Harder Causation Problems
A few contexts strain the standard tests enough that courts have developed special rules.
Toxic Exposure Cases
Toxic tort claims split causation into two questions. General causation asks whether the substance is capable of causing the type of illness the plaintiff has, typically shown through epidemiological studies. Specific causation asks whether this plaintiff’s illness was actually caused by this defendant’s substance at the dose received. A plaintiff who cannot prove both loses.6Legal Information Institute. Toxic Tort Some courts add a dose-response requirement on top, demanding proof that the plaintiff’s individual exposure was high enough to cause harm. The layers make these among the most expensive cases in civil litigation.
When the Specific Defendant Is Unknown
Sometimes the plaintiff can prove that one of several defendants caused the injury but cannot say which. In Summers v. Tice, two hunters negligently fired toward the plaintiff and one pellet struck him, with no way to tell whose gun fired the shot. The doctrine of alternative liability flips the burden: once the plaintiff shows all defendants acted negligently and one of them caused the harm, each defendant must prove it was not the one responsible, and any defendant who cannot make that showing is liable.7Legal Information Institute. Doctrine of Alternative Liability Market share liability applies a related idea to fungible products where the manufacturer cannot be identified, most notably the drug DES, but courts have kept the doctrine narrow.
Loss of Chance in Medical Cases
Standard causation rules produce a harsh outcome when a doctor’s negligence reduces, but does not clearly eliminate, a patient’s odds. If a missed cancer diagnosis costs a patient a 40 percent chance of survival, the patient was more likely than not going to die anyway, and but-for causation fails. Roughly half the states have adopted a loss of chance doctrine that lets the plaintiff recover damages proportional to the lost chance — 40 percent of the total in that example. The doctrine redefines what counts as the injury: the compensable harm is the lost opportunity, not the death itself.
Proving Causation at Trial
In simple cases, causation gets proved through eyewitness testimony, police reports, and photographs. Medical malpractice, product defect, and chemical exposure cases almost always require expert witnesses — doctors, engineers, toxicologists, accident reconstructionists — to connect the defendant’s conduct to the plaintiff’s specific injury. Expert fees commonly run several hundred dollars per hour, and a single case may require multiple experts.
The Daubert Gatekeeper
Before the jury hears any of it, the trial judge decides whether the expert testimony is admissible. The Supreme Court established this gatekeeping role in Daubert v. Merrell Dow Pharmaceuticals (1993), directing judges to evaluate whether an expert’s methodology is scientifically sound.8Justia US Supreme Court. Daubert v Merrell Dow Pharmaceuticals Inc, 509 US 579 Federal Rule of Evidence 702 codifies the requirement: the court must find it more likely than not that the expert’s knowledge will help the jury, that the testimony rests on sufficient facts, that the methods are reliable, and that the expert applied them properly.9Legal Information Institute. Rule 702 – Testimony by Expert Witnesses
Judges look at whether the theory has been tested, whether it has been published and peer-reviewed, its known error rate, whether standards govern its application, and whether it has gained acceptance in the relevant scientific community. When a plaintiff’s causation expert gets excluded under Daubert, the case almost always ends in summary judgment, because there is no longer any admissible evidence bridging the defendant’s conduct and the injury.10Legal Information Institute. Federal Rules of Civil Procedure Rule 56 – Summary Judgment
This is where causation cases most often turn. Compelling facts, a sympathetic client, and clear evidence of negligence are not enough if the expert testimony on causation does not survive the judge’s scrutiny. Nothing that never reaches the jury can persuade one.