Causation
Causation is a required element of negligence — the injured person has to show that the defendant's conduct actually caused the harm. It's where many injury claims get contested.
On this page
The essentials
- Causation has two parts: cause in fact (but-for) and proximate cause (foreseeability).
- Insurers often contest causation even when fault is clear.
- The eggshell plaintiff rule means defendants take the injured person as they find them.
- Medical expert testimony is usually needed to prove causation in serious cases.
General U.S. background. State rules and individual facts can change the answer. Find your jurisdiction →
Two kinds of causation
Courts usually split causation into two questions:
- Cause in fact — would the harm have happened without the defendant's conduct? (The 'but-for' test)
- Proximate cause — was the harm a foreseeable result of the conduct, not a remote or bizarre chain of events?
Why insurers focus on it
Causation is often the softest spot in an injury claim. Even if fault is clear, the insurer can argue the claimed injury wasn't actually caused by this event. Pre-existing back pain, degenerative disc disease, prior car accidents, prior falls — any of these can be used to argue the current complaints would have happened anyway. Medical records that predate the injury become central.
The eggshell plaintiff rule
A long-standing rule says a defendant takes the injured person as they find them. If the person had a fragile neck and an otherwise mild crash caused serious injury, the defendant is still responsible for the full harm — even if a healthier person would have walked away. The rule softens the 'it would have happened anyway' argument, though insurers still argue about how much damage was really caused versus pre-existing.
Medical causation evidence
In serious cases, proving causation often requires medical expert testimony — a doctor saying, within a reasonable degree of medical certainty, that the injury was caused or aggravated by the incident. Without that testimony, complex causation questions are difficult to win.
Sources and review scope
This legacy reference article has not received a new claim-by-claim source or licensed-attorney review. Its content update date is not a legal verification date. Check current official sources for rules relevant to your claim. How we distinguish content dates and source checks.
Put the information to use.
General information only. This page explains common concepts in plain language. It is not legal advice and does not create an attorney-client relationship. Laws vary by state and change over time. For any specific situation, consult a licensed attorney in your jurisdiction.