How is causation proved in a California negligence case?

California · updated 2026-09-01 · 673 words · 7 authorities cited

The rule

A defendant's negligence is a cause of harm if it played any part, no matter how small, in bringing about the harm, even if other factors also contributed to the harm 4. If a defendant's negligence was a substantial factor in causing the plaintiff's harm, then the defendant is responsible for that harm 1. Responsibility is not avoided simply because another person, condition, or event was also a substantial factor 1.

How it applies

The "substantial factor" test for causation means that the defendant's negligence need only play "any part, no matter how small," in causing the harm 4. This standard applies even when other factors also contributed to the harm 4. For instance, if a defendant's negligence combines with another factor to cause harm, and the defendant's negligence is found to be a substantial factor, the defendant remains responsible 1.

In specific situations, the method of proving causation can vary. For example, if a violation of a statute, regulation, or ordinance has been established, and that violation was a substantial factor in harming the plaintiff, then the violator is deemed negligent 2. This is known as negligence per se, where the focus shifts to whether the violation caused the harm 2.

The doctrine of res ipsa loquitur can also be used to prove causation 3. Under this doctrine, a plaintiff may prove that the defendant's negligence caused harm by demonstrating three elements:

1. The harm would not ordinarily have happened unless someone was negligent 3.
2. The harm was caused by something only the defendant controlled 3.
3. The plaintiff's voluntary actions did not cause or contribute to the event that harmed them 3.

If these elements are proven, a jury may, but is not required to, find that the defendant was negligent or that their negligence was a substantial factor in causing the harm, or both 3. If the plaintiff does not prove one or more of these elements, the jury must still decide whether the defendant was negligent based on other instructions 3. If, after weighing all evidence, it is more probable than not that the defendant was negligent and that their negligence was a substantial factor in causing the harm, the jury must decide in favor of the plaintiff 3.

In medical malpractice cases, a specialized form of res ipsa loquitur applies 5. The elements are similar, but with specific considerations:

1. The harm would not ordinarily have occurred unless someone was negligent, and in deciding this, only expert testimony should be considered 5.
2. The harm occurred while the plaintiff was under the care and control of the defendant 5.
3. The plaintiff's voluntary actions did not cause or contribute to the event that harmed them 5.

As with general res ipsa loquitur, if these elements are proven, the jury may, but is not required to, find negligence or that negligence was a substantial factor 5.

Once causation is established, meaning the defendant's negligence was a substantial factor in causing the harm, the amount of damages is then determined 7. The defendant is responsible for each item of harm caused by their wrongful conduct, even if the particular harm could not have been anticipated 7.

What this does not cover

This article does not cover the specific elements required to prove negligence itself, beyond the causal link 6. It also does not address comparative negligence principles or other defenses that might reduce or eliminate a defendant's liability even if causation is established. The sources do not detail the specific types of evidence required to prove causation in various scenarios, nor do they discuss the nuances of proving causation in cases involving multiple tortfeasors or complex chains of events beyond the "substantial factor" test.