Determining Causation in Personal Injury Cases

The insurer says your injury was pre-existing, or that the accident didn’t cause it. If that feels unfair, it’s understandable: you know how you felt before and how you feel now. Here’s the plain answer. Causation is the link between the other side’s carelessness and your injury, and you have to show it’s more likely than not. An old condition doesn’t sink your claim: you can recover for the harm the accident caused, including an old problem it made worse. What decides it is proof: records from before and after, and, when the link is medical, a doctor’s opinion connecting them.

The hard part is usually the claims process, not the law: the other side’s insurer handles these arguments every day, and the burden of proof is yours. Michigan’s rules, current as of September 2026, are in the boxes marked “In Michigan.”

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What do you have to prove in an injury claim?

A negligence claim generally requires four things (Cornell Law School’s Legal Information Institute, or LII, which splits causation in two):

1. Duty: the other side had to use reasonable care toward you.
2. Breach: they didn’t.
3. Causation: that failure caused your injury.
4. Damages: you were actually harmed.

The standard in a civil trial is a preponderance of the evidence: you must convince the jury there’s a greater than 50% chance your claim is true (LII).

Duty and breach are about fault; see how fault is decided, state by state. Causation is a separate fight: in Wilkinson v Lee (2000), the other side admitted responsibility for a rear-end crash, and the fight over causation went to the Michigan Supreme Court. For the whole claims process, see the guide to personal injury claims.

What’s the difference between cause in fact and proximate cause?

Courts ask two questions: did the careless act actually cause the injury, and is it the kind of harm the law holds that person responsible for?

Cause in fact is decided with the but-for test: but for the careless act, would the injury have happened (LII)? The Restatement (Third) of Torts, a summary of the law by the American Law Institute that courts often cite but aren’t bound by (LII), says conduct is a factual cause “when the harm would not have occurred absent the conduct” (§ 26, quoted by the Texas Supreme Court in Bostic v. Georgia-Pacific Corp., 2014).

A made-up example: A homeowner leaves a pool gate unlatched, and a neighbor’s child pushes it open, falls in and drowns. The unlatched gate is a cause in fact. If the child had climbed the fence instead, the unlatched gate wouldn’t be a cause in fact, because a latched gate wouldn’t have stopped the climb. But the homeowner could still be liable on other grounds, such as an inadequate pool barrier or the attractive nuisance doctrine.

An injury can have more than one cause; if two acts would each have caused the same harm at the same time on its own, each counts (§ 27, quoted in Bostic). The burden of proving factual cause normally stays with you. Under § 28, it shifts to the defendants in one narrow situation: you sue everyone whose careless conduct exposed you to the risk, prove that one or more of them caused your harm, and can’t reasonably be expected to prove which one did (quoted in Burks v. Abbott Laboratories, 2013).

Proximate cause, or legal cause, is “an actual cause that is also legally sufficient to support liability,” often judged by foreseeability (LII). The Restatement calls it the “scope of liability” (June v. Union Carbide Corp., 2009) and limits liability to “harms that result from the risks that made the actor’s conduct tortious,” or wrongful (§ 29, quoted by the Iowa Supreme Court in Royal Indemnity Co. v. Factory Mutual Insurance Co., 2010).

A made-up example: A store leaves a mop bucket in an aisle, a tripping risk. A shopper stops to step around it just as a ceiling tile loosened by a roof leak falls on her. The bucket is a but-for cause, but a falling tile isn’t one of its risks, so under the Restatement’s test the bucket doesn’t make the store liable. The leak might.

How do you prove the accident caused your injury?

With evidence that makes it more likely than not that the accident caused your injury. In Michigan, a causation theory needs “some basis in established fact,” and one that is, “at best, just as possible as another theory” isn’t enough (Skinner). You don’t have to rule out every other cause, but the evidence must “exclude other reasonable hypotheses with a fair amount of certainty” (Craig). And timing alone may not carry a medical link, because “correlation is not causation” (Craig).

Proof that ties an injury to an accident can include:

• Records from the first days that mention the accident and your symptoms.
• Exam findings and tests, such as imaging.
• Records from before the accident, showing your baseline.
• One consistent account of how you were hurt and when each symptom started.
• People who saw the change, and a doctor’s opinion on whether the accident caused the injury or made an old one worse.

See herniated disc claims, whiplash and soft-tissue claims and symptoms that show up days later.

What if the insurer says your injury was pre-existing?

Then the question becomes what the accident added. You can’t recover for the condition you already had, but it doesn’t erase your claim. Under the common-law eggshell skull rule, a defendant is liable for the full harm their wrongful act proximately caused, even if most people wouldn’t have been hurt as badly: a defendant must “take the victim as they find them” (LII).

A made-up example: A driver with years of on-and-off back stiffness, but no leg pain, is rear-ended. Now pain runs down one leg, and an MRI shows a disc pressing on a nerve. The insurer points to old chiropractor notes and calls it degenerative. Those notes also show the before: stiffness, no leg pain, no missed work. The claim is for the difference.

For a worn disc on an MRI report, see herniated disc claims; for how an old injury affects value, see how injury claims are valued.

How do insurers use old records and gaps in treatment?

Old records. Expect the other side to see them. They can also be your best proof of what you were like before, so tell every provider about earlier injuries.

Gaps in care. A late first visit or a long break in treatment lets the other side argue that something else caused your symptoms or that you weren’t badly hurt. That’s an argument, not a rule of law, but an unexplained gap makes it easier. You’re also expected to use ordinary care to limit your losses, which can include getting and following reasonable medical treatment; when there’s evidence you may not have, Michigan jurors, for example, are told not to compensate damages that resulted from failing to do so (M Civ JI 53.05 and comment). Gaps happen for ordinary reasons; what helps is a record of the reason, made at the time.

A made-up example: A warehouse worker waits six weeks for physical therapy after a crash because every opening is during her shifts. At her first visit, the therapist notes why she waited and that the pain never went away. The gap is still there, but so is the reason.

For what else goes into an adjuster’s file, see how insurance companies evaluate injury claims.

Is an insurer blaming an old injury or a gap in your care? Get a free case review. Four quick questions, no cost, no obligation.

When do you need a medical expert?

When causation is a medical question, such as whether a crash herniated a disc or only revealed an old one, it takes a medical answer. Three Michigan Supreme Court cases show what courts look for:

• A mechanism. In Craig (2004), the Court overturned a jury verdict for a child with cerebral palsy because no expert testified that the injury he claimed to have suffered during labor could cause it.
• Facts, not assumptions. In Skinner (1994), the experts’ causation theories failed because each “lacked a basis in established fact.”
• Doctors who know the history. In Wilkinson (2000), the treating physician testified that the crash precipitated the symptoms, and even the defense’s expert said the trauma “probably contributed to those symptoms and perhaps caused them.” That let the jury find causation.

Two kinds of cases add rules of their own:

• Medical malpractice. In Michigan, you must prove an injury that “more probably than not was proximately caused by the negligence of the defendant,” and you can’t recover for a lost chance to survive or to get a better result unless “the opportunity was greater than 50%” (MCL 600.2912a(2)). The Michigan Supreme Court has held that this 50% limit applies only to lost-opportunity claims, not to a traditional malpractice claim, in which you prove the negligence more probably than not caused your injury (O’Neal v St John Hospital, 2010). The committee that drafts Michigan’s model jury instructions deleted its instruction on the 50% rule in 2011, citing “a lack of consensus among the courts” on how to apply it (M Civ JI 30.20). See medical malpractice claims.
• Toxic exposure. Some courts require proof that the substance can cause the disease (general causation) and that it caused yours (specific causation), often through experts (LII). See toxic exposure lawsuits and suing over chemical exposure.

What to do next

1. Keep getting care, and give every provider the whole history: the accident, any earlier problem with the same part of your body, and what’s different now.
2. Build a before-and-after file: your earlier records, a dated symptom journal and the names of people who’ve seen the change.
3. Close any gap, and explain it. Tell your doctor why you stopped, and keep proof, such as a denial letter or a work schedule.
4. Be careful with the other side’s insurer. You have no contract with it (Texas Department of Insurance), so you don’t have to give it a recorded statement, and it’s safer not to before you’ve had advice. Read any medical authorization before you sign it. See what to say when an adjuster calls.
5. Get a free case review before your deadline. Get a free case review, or in Michigan, compare injury lawyers near you. Get any fee agreement in writing.

What waiting can cost

• Your deadline. Most Michigan injury lawsuits must be filed within three years (MCL 600.5805(2)), and PIP benefits need written notice to your insurer within a year of the accident unless it has already paid (MCL 500.3145(1)). For other states, see injury deadlines by state.
• The gap. Every week without care is one more to explain.
• An early release. A settlement release generally gives up your right to sue over the injury (LII), even if it turns out worse than you thought.

What a good outcome looks like

Your records tell one story: what you were like before, what changed after, and the care that followed, with any gap explained. A doctor who knows your history can say what the accident added, and if the other side still blames an old condition, the dispute is decided on that evidence. From being told your injury was “pre-existing” to knowing what the accident changed, what the law lets you recover for and what your case needs.

Updated September 28, 2026

This guide is general information, not legal advice, and laws change. For advice about your situation, talk to a lawyer licensed in your state. Reading this page or contacting us does not create an attorney-client relationship.