CACI No. 430: what a substantial factor in causing harm means

Most California instructions listing the elements of a tort claim end with the same requirement: the defendant's conduct must have been a substantial factor in causing the plaintiff's harm. CACI No. 430 is the instruction that tells the jury what that phrase means. It is short, and most of the hard questions sit in its Directions for Use (CACI No. 430).

The definition in plain terms

CACI No. 430 gives jurors four points to apply:

• The conduct must be something a reasonable person would see as playing a part in the harm.
• Its role must go beyond the remote or the trivial.
• Other causes can share in the harm; this one need not stand alone.
• An optional sentence adds the but-for idea: if the harm would have happened anyway, the conduct does not count.

The word substantial can mislead. The California Supreme Court decisions quoted in the Sources and Authority describe the standard as broad and warn against putting too much weight on that one word. A cause with only an infinitesimal or theoretical role falls short, while even a small contribution that actually produces harm can qualify. The court has said this keeps the test consistent with comparative fault, where each party answers for its own share.

How the but-for test fits inside

The but-for test asks whether the harm would have happened anyway if the defendant had not acted as it did. According to the Directions for Use, the substantial factor definition already contains that test, and the optional last sentence spells it out. The directions add that leaving the sentence out can be error in some cases, citing the California Supreme Court's decision in Soule v. General Motors (1994).

California adopted the substantial factor approach from the Restatement Second of Torts. The case law gathered with the instruction says it usually reaches the same result as the but-for rule but also handles cases with independent or concurrent causes, where but-for reasoning breaks down.

When the last sentence comes out

Concurrent independent causes are the main exception. The Directions for Use describe them as separate forces operating at the same time, each enough by itself to cause the same harm. Two fires set by different people that merge and burn a house are the classic picture. Under a strict but-for test, each person could argue the house would have burned anyway because of the other fire. The directions say not to read the but-for sentence in that kind of case.

Asbestos cases follow their own rule. When a claim is based on disease from asbestos exposure, the directions say to give CACI No. 435 in place of CACI No. 430. Courts have explained that describing a long-ago exposure as remote could mislead jurors in a disease that can take many years to appear after exposure. The directions also point to an appellate decision holding it was not error to give both instructions where the defendants included a product maker and a property owner.

Conduct, not coincidence

The instruction asks about conduct, and the Directions for Use explain what that word means here: the culpable acts or failures to act that the claim of fault rests on, such as negligence, a product defect or a breach of contract. A chance event in the chain, like an alarm clock that failed and put someone at the crash scene at the wrong moment, is not the kind of conduct the test measures.

Related causation instructions

CACI No. 430 has neighbors. The directions say that in cases of multiple causes that depend on one another, CACI No. 431 on multiple causes should also be given. CACI Nos. 432 and 433 are affirmative defenses for a superseding cause, where a third person's conduct, or an intentional or criminal act, breaks the chain. Because the July 2026 supplement updated the source notes for CACI No. 431, anyone relying on that instruction should read the supplement as well.

Questions to sort a causation dispute

• What conduct is the claim based on? Name the act or failure, not the whole event.
• Would the harm have happened without it? If yes, the but-for sentence may defeat the claim.
• Were there other causes? List them and whether each depended on the others or acted alone.
• Is the contribution more than trivial? When causation is beyond common experience, expert testimony is needed.
• Is a special rule in play? Asbestos disease and superseding cause have their own instructions.

This page explains California's official jury instructions for general readers and is legal information, not legal advice. For a case going to trial, work from the official text and consult a lawyer licensed in California.