To prove causation in a personal injury case, you have to show two things by a preponderance of the evidence: that your injury would not have happened without the defendant’s conduct (actual cause), and that your type of injury was a reasonably foreseeable result of that conduct (proximate cause). Fail either half and the claim collapses, even if the defendant was plainly careless. Courts routinely grant summary judgment against plaintiffs who cannot connect the dots with enough evidence to let a reasonable jury find causation more likely than not. The work of proving causation is partly legal argument and mostly evidence: medical records, scene documentation, witness accounts, and expert testimony that survives an admissibility challenge.
Actual Cause: The But-For Test
The starting point is a counterfactual question. Would the injury have occurred if the defendant had not acted the way they did? If the answer is no, the defendant’s conduct is an actual cause. If the answer is yes, the causal chain breaks and the claim fails on that element alone.
This looks clean in the abstract and gets messy fast. Picture a crash where the other driver ran a red light and hit you, but your dashcam shows you were speeding. Would the collision have happened at all if the other driver had stopped at the light? Probably not. Running the light is still a but-for cause, even though your speed may have made the impact worse. Your speed becomes a comparative fault question, not a causation defeater for the other driver.
The but-for test has a well-known weak spot: it breaks down when two independent forces each would have been enough to cause the harm by themselves. The classic example is two separate fires, each from different negligent acts, that merge and destroy a building. Under a strict but-for reading, neither defendant caused the loss, because each could point to the other fire. Courts refuse that result and switch to a different test.
Substantial Factor: When Multiple Causes Combine
When the but-for test fails because several forces combined, many courts apply the substantial factor test. You show that the defendant’s conduct was a significant contributor to the harm, even if other factors also played a role.
The test comes from the Restatement (Second) of Torts, which treats a defendant’s negligent conduct as a legal cause of harm if it was “a substantial factor in bringing about the harm.” Courts weigh how many other factors contributed and how significant each was, whether the defendant’s conduct created a force still actively operating when the injury happened, and how much time passed between the act and the harm.1Open Casebook. Restatement Second, Section 433, On Substantial Factor
The Restatement (Third) of Torts later addressed “multiple sufficient causes” directly: where two or more acts would each independently have caused the same harm, each is treated as a factual cause even though neither one was strictly necessary.2Open Casebook. Restatement Third, Section 27, On Multiple Sufficient Causes
Proximate Cause and Foreseeability
Actual cause is necessary but not sufficient. Proximate cause limits liability to the kinds of harm a reasonable person could have anticipated. The law refuses to hold someone responsible for every strange downstream consequence of their actions, no matter how attenuated.
The guiding principle, from the landmark railroad platform case, is that “the risk reasonably to be perceived defines the duty to be obeyed.”3New York Courts. Palsgraf v Long Island Railroad If your injury was a bizarre, unforeseeable chain reaction, proximate cause can fail even though the but-for test was clearly satisfied.
In practice, foreseeability usually becomes a jury question. The judge decides whether a reasonable jury could find the harm foreseeable. If reasonable people could disagree, the issue goes to trial. If the chain of events is so extraordinary that no reasonable person would have predicted it, the judge may dismiss the claim before trial.
What Breaks the Chain: Intervening and Superseding Causes
Even after you establish actual and proximate cause, the defendant can argue that something else came along afterward and broke the causal chain. These arguments turn on the distinction between intervening and superseding causes.
An intervening cause is any new event between the defendant’s original conduct and your injury. Not all intervening causes free the defendant. If the intervening event was foreseeable, the defendant stays liable. A driver who caused a crash is not relieved of liability because the ambulance took a long time to arrive and the delay worsened your injuries. Delayed medical treatment after an accident is a foreseeable risk.
A superseding cause is an intervening event so unforeseeable and independent that it becomes the real explanation for the injury. The Restatement (Second) of Torts specifically excludes criminal or negligent acts by third parties from being superseding causes when those acts were a foreseeable consequence of the defendant’s negligence, or when the risk of those acts was the very reason the defendant’s conduct was negligent in the first place.4Open Casebook. Torts – Class 20 – Proximate Cause The genuine superseding cause tends to be something extraordinary and disconnected from the original negligence.
Defendants raise this defense constantly, and it usually fails. Most intervening events that follow a negligent act are foreseeable enough that courts refuse to treat them as superseding. The defense succeeds mainly when the intervening event is so detached from the original conduct that holding the first defendant liable would feel arbitrary.
The Eggshell Skull Rule
One doctrine works in the plaintiff’s favor. Under the eggshell skull rule (sometimes called the thin skull rule), a defendant takes the plaintiff as they find them. If you have a pre-existing condition that makes you more vulnerable to injury, the defendant is liable for the full extent of your harm, even if the same conduct would barely have affected a healthier person.
This matters because defendants frequently argue that your injuries came from the pre-existing condition rather than from their conduct. The rule limits that argument. If the defendant’s negligence aggravated a pre-existing back problem and turned a manageable condition into one requiring surgery, the defendant is responsible for the surgical outcome. The rule applies to both physical and, in many jurisdictions, psychological conditions. Proving causation here often requires expert testimony to separate the pre-existing baseline from the new, aggravated condition.
Evidence That Builds the Causal Link
Legal standards matter, but cases are won on evidence. The strongest causation arguments tie specific, contemporaneous documentation to the defendant’s conduct in a chain a jury can follow step by step.
Medical Records and Diagnostic Imaging
Medical records are the backbone of causation evidence. Your initial emergency room visit creates a timestamp showing when symptoms first appeared and what the treating physician observed. Follow-up records, specialist referrals, and diagnostic imaging document how the injury developed over time. The most valuable entries are the ones where a doctor, without prompting, notes the mechanism of injury and attributes the condition to the incident in question.
Gaps in treatment undermine causation badly. If you wait three weeks after an accident to see a doctor, the defense will argue that something else caused or worsened your injury during that gap. Consistent, timely care creates a paper trail that is difficult to attack.
Scene Documentation and Official Reports
Time-stamped photographs capture conditions that change quickly: skid marks, weather, debris patterns, traffic signals, and property damage. These images help reconstruct how the incident happened. Law enforcement reports add another layer, typically including the responding officer’s observations, a diagram of the scene, and sometimes a preliminary fault determination. You can usually obtain these from the agency that responded.
Witness Statements
Eyewitness accounts collected shortly after an incident carry significant weight because they are less susceptible to memory distortion. A witness who saw the defendant run a stop sign, or who noticed hazardous conditions before anyone was hurt, provides narrative evidence bridging the defendant’s conduct and your injury. The sooner statements are recorded, the more credible they tend to be.
Expert Testimony and the Admissibility Fight
In most causation disputes, raw evidence alone is not enough. You need an expert to interpret the evidence and explain, in language a jury can follow, how the defendant’s conduct produced the injury. Many cases turn here, because an expert whose testimony gets excluded leaves a causation gap the plaintiff usually cannot fill.
What Experts Do
Medical experts review your treatment records, examine you, and offer an opinion on whether the defendant’s conduct caused or substantially contributed to your condition. Courts generally require medical experts to state their opinion to a “reasonable degree of medical certainty,” which despite its imposing sound usually means more likely than not. The phrase prevents speculation while keeping the threshold consistent with the civil burden of proof.
Accident reconstructionists tackle the physics of how an incident happened. They examine vehicle damage patterns, roadway markings, and physical evidence to calculate speeds, angles of impact, and forces involved. Their analysis converts static evidence into a dynamic explanation. Other specialists fill niche roles: toxicologists in chemical exposure cases, biomechanical engineers in product liability disputes, vocational experts who quantify lost earning capacity.
Daubert, Frye, and Rule 702
Federal courts and a majority of states evaluate expert testimony under Federal Rule of Evidence 702, which requires the proponent to show that the expert’s specialized knowledge will help the jury, that the testimony is based on sufficient facts, that it reflects reliable principles and methods, and that those methods have been properly applied to the case.5Legal Information Institute. Rule 702 Testimony by Expert Witnesses The judge acts as a gatekeeper, filtering out unreliable opinions before they reach the jury.
The Supreme Court’s decision in Daubert v. Merrell Dow Pharmaceuticals identified factors courts may consider: whether the expert’s theory has been tested, whether it has been subjected to peer review, its known or potential error rate, whether it follows established standards, and whether it has gained acceptance in the relevant scientific community.6Legal Information Institute. Daubert v. Merrell Dow Pharmaceuticals, 509 US 579 (1993) These are guidelines, not a rigid checklist. Courts focus on the expert’s methodology rather than the conclusions it produces.
A handful of states still follow the older Frye standard, which asks only whether the expert’s methodology is generally accepted by specialists in the field. In a Frye jurisdiction, scientific acceptance carries even more weight. Either way, hiring an expert whose methodology cannot survive a challenge is one of the costliest mistakes a plaintiff can make.
General vs. Specific Causation in Toxic Exposure Cases
Toxic exposure and pharmaceutical injury cases split causation into two distinct questions. General causation asks whether the substance is capable of causing the type of harm you allege. Specific causation asks whether the substance actually caused your particular injury.
The Restatement (Third) of Torts explains that general causation typically relies on group-based studies comparing disease rates in exposed versus unexposed populations. When those studies show that exposed individuals develop the disease at more than twice the rate of unexposed individuals, courts treat that as sufficient evidence to let a jury decide specific causation. The reasoning: if the disease is more than twice as common in the exposed group, any individual member of that group more likely than not developed it because of the exposure.7Open Casebook. Restatement Third of Torts on General v. Specific Causation
Specific causation is usually the harder fight. Even after you prove the substance can cause the disease, you still need to show it caused yours. Defendants point to alternative explanations: genetics, lifestyle factors, unrelated environmental exposures. You need expert testimony tying your exposure history, dosage, and timeline to your diagnosis while ruling out competing causes.
Comparative Negligence and Your Recovery
Proving causation does not guarantee full recovery if the defendant can show you were partly at fault. Most states follow some version of comparative negligence, which reduces damages by your percentage of responsibility. Where states draw the cutoff varies.
Under pure comparative negligence, you can recover something even if you were 99% at fault; your damages simply get reduced by your share. Under modified comparative negligence, which most states use, you are barred from recovering anything once your fault reaches a threshold. In roughly half of those states the bar kicks in at 50% fault, in the rest at 51%. The difference is real: in a 50% bar state, a plaintiff found equally at fault with the defendant recovers nothing, while in a 51% bar state the same plaintiff still recovers half their damages. A small number of jurisdictions still follow contributory negligence, which bars all recovery if the plaintiff is at fault to any degree.
Comparative negligence intersects with causation because the defense will try to reframe your conduct as a contributing cause. If you were not wearing a seatbelt, texting while walking, or ignoring a warning label, expect that argument. Building a strong causation case means anticipating it and being prepared to show that the defendant’s conduct, not yours, was the dominant cause.
The Burden of Proof
In a civil lawsuit, you prove causation by a preponderance of the evidence: your version of events is more likely true than not. If the evidence tips even slightly in your favor, you have met the burden. If the evidence is perfectly balanced, you lose.8United States District Court District of Vermont. Burden of Proof – Preponderance of Evidence
This standard sits well below the “beyond a reasonable doubt” threshold used in criminal cases. You do not need to eliminate every other possible explanation. You need to show that the defendant’s conduct is the most probable one. Where causation cases fail the preponderance test, the failure usually comes from mundane gaps: a missing medical record, an expert who overstated conclusions, a timeline with a hole in it. Courts grant summary judgment against plaintiffs who cannot produce enough evidence to let a reasonable jury find causation more likely than not.
Preserve Evidence Before It Disappears
The duty to preserve relevant evidence arises the moment litigation is reasonably anticipated, not when a lawsuit is actually filed. Failing to preserve can result in sanctions, adverse inference instructions where the jury is told to assume the missing evidence would have been unfavorable, or in extreme cases dismissal of claims or defenses.
Skid marks fade, surveillance footage gets overwritten, electronic records are routinely purged. If you anticipate a claim, act now:
- Photograph the scene, your injuries, property damage, and any conditions that contributed to the incident.
- Request medical records, police reports, incident reports from businesses, and maintenance logs for equipment or property involved.
- Send a written preservation letter notifying potential defendants and third parties that they must retain relevant documents, surveillance footage, and electronic data.
- Save your own text messages, emails, receipts, and anything else that documents the timeline of events and your resulting harm.
Evidence that looks insignificant early on sometimes becomes decisive later. A maintenance log showing that a property owner knew about a hazard months before your fall, or a text message proving the defendant was distracted at the time of the accident, can be the piece that tips the preponderance scale. The worst time to realize you need a document is after it has been destroyed.