Determining Causation in a Personal Injury Case

Learn about the “but-for test” and other tests used to determine causation

In a personal injury lawsuit, you typically have to prove that the defendant was negligent. One of the key elements in a negligence claim is causation.

Though this might seem simple enough, the legal concept of causation involves two different types of causation: actual cause and legal cause. To win a negligence lawsuit, you need to prove both types of causation in addition to the other elements of negligence.

Let’s take a closer look.

Actual cause (also called “cause-in-fact”)

Actual cause refers to what you might consider the factual cause of the accident. Without this cause, the accident that resulted in your injury couldn’t have happened.

Let’s look at an example:

In the above example, Linda’s actions (running the red light) clearly caused the accident. In this case, actual cause can be established.

However, in some cases, causation isn’t as clear cut. For example:

Did Marcy’s actions (swerving the vehicle) cause the accident, or did the pedestrian’s actions (stepping into the middle of the road) cause the accident?

Most states use 1 of 2 tests to determine actual case:

• But-for test
• Substantial factor test

But-for test

The but-for test says that an action is a cause of an injury if, but for the action, the injury wouldn’t have occurred. In other words, would the harm have occurred if the defendant hadn’t acted in the way they did? If the answer is NO, then the action caused the harm.

In most cases, the but-for test is sufficient. But, the test isn’t perfect.

Let’s take a look at a problematic example:

Under the but-for test, neither vehicle caused the accident legally speaking.

How can this be?

Remember, under the but-for test we must ask:

So, but for the vehicle traveling west, would the harm have occurred?

The answer is YES. Jill still would have been injured because she would have been hit by the vehicle traveling east. So, the vehicle traveling west is not negligent. The same holds true for the vehicle traveling west.

Substantial factor test

Because of unfair results such as the one above, some states apply the substantial factor test.

Under the substantial factor test, the court considered whether the defendant’s actions were a substantial factor in causing the injury. This test gives the court more leeway to find that multiple parties caused an accident. 

Legal cause (also called “proximate cause”)

In some cases, a defendant’s actions may have technically caused an injury, but the injury was so unforeseeable that it would be unfair to hold the defendant liable for the injury.

Let’s look at a couple of examples.

In the above example, it’s technically true that the retired police neighbor Bill caused the car accident (if Bill hadn’t hotwired the car, the accident wouldn’t have occurred). However, most people would agree that it wouldn’t be fair to hold Bill liable for the car accident.

This is where legal cause comes into play.

For there to be legal cause, the injuries in question must have been foreseeable. In other words, it must be true that the defendant should have reasonably anticipated that their actions could result in the injuries that actually occurred.

In the above example, Bill couldn’t have reasonably anticipated that the action of hotwiring his neighbor’s car would result in a car accident.

Let’s look at another example:

In the above scenario, John could not have foreseen that running a red light would cause fume-related injuries. As a result, he’s not liable for those injuries (though he’s certainly liable for any injuries to Samantha). 

Has all of this causation talk got you confused?

Lawyers are skilled at identifying all possible defendants and arguing that causation exists (or doesn’t exist).

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