The “Substantial Factor” Test – How Does It Work?

The substantial factor test is used to determine whether someone else’s conduct was the proximate cause of your injury. It asks whether the defendant’s conduct was a substantial factor leading to your injuries.

In a personal injury case, you generally must show that there was causation, including proximate causation, between the defendant’s conduct and your injuries in order to succeed.

What is causation?

Causation is the connection between one event and another. In personal injury law, it is the requirement that the defendant’s negligence is sufficiently connected to your injuries.

There are two legal elements to causation:

1. actual cause, also called cause-in-fact, which focuses on whether the defendant’s conduct was connected at all to your injuries; and
2. proximate cause, also called legal cause, which focuses on whether the defendant’s conduct was adequately connected to your injuries to impose liability.

As the victim, you have the burden of proving both in a personal injury case for compensation.

For example: While boarding a train, a passenger drops a bag carrying fireworks, which explodes. The explosion knocks over a statue, which falls on Helen. Unable to find the passenger who dropped the bag, Helen sues the railroad company. The railroad company claims that it was too far removed to be the proximate cause of the statue falling and hurting her.¹

Courts have used several tests for proximate cause, but there are problems with all of them. Critics have pointed out that they generally fail to instruct juries on what to look for, and that they tend to morph into analyzing factual causation, instead.²

Some jurisdictions, like California, claim to have combined actual and legal causation, though they still require evidence that satisfies both.³

How does the substantial factor test work?

California Civil Jury Instruction “CACI” 430 defines a substantial factor in causing harm as a factor that a reasonable person would consider to have contributed to the harm.⁴

If the defendant’s acts or omissions were a substantial factor, then proximate cause is satisfied. Factors that are remote from the injuries or that were trivial are not substantial.⁵ There can be more than one substantial factor.⁶

For example: Abe’s alarm clock is defective and does not wake him up in time to go to work. When he finds out that he is going to be late, Abe gets in his car and speeds to work. His unsafe driving causes an accident with Brittany. Abe’s bad driving was a substantial factor in causing Brittany’s injuries. The defective alarm clock, however, was probably not a substantial factor.

The substantial factor test is often used when:

• there are multiple defendants (“tortfeasors”), or
• there were independent or concurrent causes for the accident.

Any defendant who was a substantial factor in your injuries can be held liable.⁷

This test is also used frequently in products liability cases and toxic torts, like when a worker suffers an occupational disease from asbestos and dies. If the workplace exposure was enough of a contributing factor or a sufficient cause of the injury, the trial court can determine that it caused the wrongful death.

Are there other tests for determining proximate cause?

Yes, there are several other tests for determining whether the defendant’s conduct was the proximate cause of your injuries.

Foreseeability is a frequently used test for proximate causation.⁸ If it was foreseeable that the defendant’s acts or omissions would cause your injuries, the defendant can be held liable.

Remoteness is sometimes used to test whether proximate cause is not satisfied. However, this also focuses on:

• the foreseeability of the cause and
• its effect in causing your injuries.

For example, the defendant’s negligent conduct can happen years before or thousands of miles away from your injuries. While extremely remote, they can still clearly be the cause.

For example: A psychiatrist negligently withdraws a psychotic patient’s medication. Several months later, the patient kills his parents during a mental episode.⁹

What about the “but-for” test?

The but-for test is a very common test for causation. This test asks the question, “but-for the defendant’s conduct, would the victim have gotten hurt?”

• If the answer is “no,” there is causation and the defendant can be held liable.
• If the answer is “yes,” then the victim would have gotten hurt, anyway. Therefore, the defendant was not the cause of the victim’s injuries.

The but-for test is generally accepted to be the test for actual cause, not proximate cause. If only the but-for test is used, then responsibility for an accident can stretch extremely far and lead to strange and unfair results.

For example: Abe’s defective alarm clock does not wake him up on time. He speeds to work and his reckless driving causes an accident that hurts Brittany. But-for the defective alarm clock, Brittany would not have gotten hurt. The alarm clock manufacturer would be held liable for her injuries.

Because the but-for test and actual cause can stretch unfairly far, proximate cause puts a limit on how far it goes. It does this by requiring the actual cause to also be the proximate cause due to the injuries it produced being:

• foreseeable,
• not too remote, or
• the substantial factor in the victim’s injuries.

What else do I have to prove to win my case?

Causation is only one element in a personal injury case. In order to win a personal injury negligence case, you have to prove all of the following 4 elements:

1. the defendant had a duty of care to keep the plaintiff free from harm,
2. the defendant’s action or inaction breached that duty,
3. the breach of that duty was the cause of the plaintiff’s injuries, and
4. the plaintiff was, in fact, injured.

A personal injury attorney from a reputable law firm can help you prove all of them.

Additional Reading

For more in-depth information, refer to the following scholarly articles:

• Causation, Valuation, and Chance in Personal Injury Torts Involving Preexisting Conditions and Future Consequences – Yale Law Journal.
• Actual Causation in the Second and Third Restatements: Or, the Expulsion of the Substantial Factor Test – Cardozo Legal Studies.
• Proximate Cause in Civil Damages Act Cases – William Mitchell Journal of Law and Practice.
• Substantial Factor Test – West Reserve Law Review.
• In Second Collision Cases, a Manufacturer’s Liability is Established by a Showing That the Design Defect Was a Substantial Factor in Producing Damage over and above that Caused by the Original Impact; If the Damage Consists of an Indivisible Injury, the Manufacturer and Negligent Driver Are Liable as Joint and Several Tortfeasors – University of Cincinnati Law Review.

---