Proximate cause
The link between the careless act and your injury — in California, whether it was a substantial factor.
Causation is where defence medical experts do most of their work. It is the argument that you were hurt, but not by this. This is general information, not legal advice.
What it means
California asks whether the conduct was a substantial factor in causing the harm. It does not have to be the only cause, or even the main one. Conduct that is remote or trivial does not count.
More than one cause can be a substantial factor at the same time. That is the answer to the argument that because something else also contributed, the defendant is off the hook.
In practice the fight is usually medical rather than mechanical: not whether the collision happened, but whether it produced the disc injury, or whether the disc was already there.
Where this is dealt with properly
This entry is the short answer. These pages own the subject.
Common questions
- I had a bad back before the crash. Does that end my claim?
- No. A defendant takes you as they find you. If the collision made an existing condition worse, the worsening is what is claimed — and there is a jury instruction that says so.
Start a case review call
On a case review call, I go through the facts with you: what happened, when, whether you were hurt, whether anyone represents you, and how to reach you. It is not legal advice, and I will not put a value on your claim.
Sources: CACI No. 430, Judicial Council of California Civil Jury Instructions (2026 edition) (causation: substantial factor); CACI No. 431 (causation: multiple causes); CACI No. 3927 (aggravation of a pre-existing condition). CACI references after the first use the short form.