# Causation

> Causation is the link between someone's careless act and your injury. In a crash claim you must show the crash caused the harm you are claiming.

Canonical page: https://accidentclaimfacts.com/glossary/causation/  
Last reviewed: October 29, 2026 by the Accident Claim Facts editorial team  
General information, not legal advice. Talk to a licensed attorney in your state.

## Quick answer

Causation is the link between a careless act and the harm it did. In an injury claim, you must show that the other driver's conduct caused the crash, and that the crash caused the injuries and losses you are claiming. Courts look at two parts: actual cause and proximate cause.

## In a car accident claim

Causation is one of the elements of a negligence claim, along with a duty, a breach of that duty and damages. The [car accidents](https://accidentclaimfacts.com/accidents/car-accidents/) guide walks through all four. Actual cause asks whether the harm would have happened without the careless act. Proximate cause, in the words of Cornell's Legal Information Institute, is "an actual cause that is also legally sufficient to support liability."

The fault part is often clear in a rear-end crash. The harder fight is usually about injuries. Insurers may argue that pain came from an old condition, a later event or something else entirely. Records made soon after the crash, doctors' opinions and a steady treatment history are the usual proof. When a crash makes an earlier condition worse, the eggshell plaintiff rule and the line between old and new harm come into play; see [back and neck injuries](https://accidentclaimfacts.com/injuries/back-and-neck-injuries/) and the [injuries hub](https://accidentclaimfacts.com/injuries/).

## Example

> **Example:** A driver is rear-ended and has neck pain the next day. Years earlier, the same driver hurt their neck playing sports. The insurer says the pain comes from the old injury; the driver's doctor writes that the crash made the condition worse. How much of the current problem the crash caused becomes the main question in the claim.

## Related terms

- [Negligence](https://accidentclaimfacts.com/glossary/negligence/)
- [Duty of care](https://accidentclaimfacts.com/glossary/duty-of-care/)
- [Pre-existing condition](https://accidentclaimfacts.com/glossary/pre-existing-condition/)
- [Damages](https://accidentclaimfacts.com/glossary/damages/)

## FAQ

### What is the difference between actual cause and proximate cause?

Actual cause means the harm would not have happened without the act. Proximate cause adds a legal limit: the harm must be a close enough result of the act for the law to hold the person responsible.

### Who has to prove causation?

The person making the claim. In civil cases, the plaintiff generally has the burden of proving their case, including that the crash caused the injuries.

## Sources

- [Cornell Law School, Legal Information Institute, Wex: proximate cause](https://www.law.cornell.edu/wex/proximate_cause) — actual and proximate cause
- [Cornell Law School, Legal Information Institute, Wex: negligence](https://www.law.cornell.edu/wex/negligence) — the elements of a negligence claim
- [United States Courts, Glossary of legal terms](https://www.uscourts.gov/glossary) — burden of proof