Proximate Cause

Proximate cause answers the question: is the connection between the defendant's negligence and the plaintiff's injury close enough, and foreseeable enough, to impose liability?

Proximate Cause in Personal Injury Cases

Causation is the third element of negligence — after duty and breach. The plaintiff must prove both actual cause (but-for causation) and legal cause (proximate cause). California uses the substantial factor test for proximate cause, as set out in CACI No. 430.

Actual cause vs. proximate cause. Actual cause (cause-in-fact) asks: but for the defendant's conduct, would the injury have occurred? Proximate cause (legal cause) asks: is the connection between conduct and injury legally sufficient to impose liability, taking into account foreseeability?

The substantial factor test. Under California law (Mitchell v. Gonzales, 1991), the test for causation is whether the defendant's conduct was a "substantial factor" in causing the harm. This replaced the older but-for test in California and is designed to address situations involving multiple causes.

Foreseeability. Proximate cause requires that the type of harm that occurred was reasonably foreseeable as a result of the defendant's conduct. A defendant is not liable for bizarre, highly improbable consequences of their negligence that no reasonable person would have anticipated.

Superseding causes. A superseding cause is an independent intervening act that breaks the chain of causation between the defendant's negligence and the plaintiff's injury. If the intervening act was foreseeable, it does not break the chain — the defendant remains liable. If it was unforeseeable, it may sever the causal connection.

How Proximate Cause Works in Practice

In a straightforward car accident case, causation is typically not disputed — the defendant ran a red light, struck the plaintiff's vehicle, and caused fractures. The causal chain is short and obvious.

Causation becomes contested in more complex cases: a rear-end collision aggravates a pre-existing cervical condition; a slip and fall triggers a cardiac event; a delayed diagnosis allows a treatable cancer to progress. Defense arguments typically focus on challenging whether the defendant's conduct was the substantial factor, or whether a pre-existing condition or intervening act was the true cause.

The eggshell plaintiff rule. California follows the eggshell plaintiff doctrine — a defendant takes the plaintiff as found. If the plaintiff had a pre-existing vulnerability that made them more susceptible to injury, the defendant is still liable for the full extent of the injury, even if a healthy person would have suffered less harm. The eggshell rule applies to damages, not duty or causation — the defendant must still have been a substantial factor in causing harm.

Expert testimony is frequently needed to establish causation in cases involving medical conditions, aggravations, or complex injury patterns.

California Causation Standards

California formally adopted the substantial factor test for causation in Mitchell v. Gonzales (1991) 54 Cal.3d 1041, explicitly rejecting the "but for" test as the sole standard. This makes California's causation analysis slightly more plaintiff-friendly in multiple-cause situations.

CACI No. 430 is the standard jury instruction on causation. It tells jurors that conduct is a cause of harm if it is a substantial factor in bringing about the harm, and that a substantial factor is something more than a remote or trivial factor but does not need to be the only cause.

California courts also apply the "lost chance" doctrine in some medical malpractice cases — allowing recovery where a defendant's negligence reduced the plaintiff's probability of a better outcome, even without proof of what would have happened absent negligence.

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