used the substantial factor test to do an increasing variety of things it was never intended to do and for which it is not appropriate.
In nearly all of these cases, the courts conceive of the test as an instantiation of the Restatement (Second) of Torts Section 431's substantial factor test of causation.
Misused in this way, the substantial factor test "undermines the principles of comparative negligence, under which a party is responsible for his or her share of negligence and the harm caused thereby." (69) This passage is not directly applicable to the question of what is substantial for purposes of contributing to risk, because it addresses but-for causation.
Under the substantial factor test, it is difficult to say.
(80) Sometimes when courts say they are using a substantial factor test in this situation, they then proceed to provide a definition of substantial factor that is the same as the but-for test.
As we have seen, the only multiple causation cases that are legitimately solved by the substantial factor test are the "combined force" situations in which we are morally certain that the but-for test stubbornly persists in yielding the wrong answer.
The second situation in which the Second Restatement envisions a separate role for the substantial factor test is where but-for causes are sufficiently trivial that they should be ignored:
Section 26 of the Third Restatement returns foursquare to the but-for test and explicitly rejects the substantial factor test. (87) Comment j explains why the Third Restatement abandons the substantial factor test, noting that it leads to both a too rigorous standard, i.e., when it requires more than a but-for cause to satisfy factual causation, and a too lenient standard, i.e., when something less than a but-for cause is permitted to satisfy the requirement of factual causation.
(98) We think that helps explain why the Rutherford court could be as sanguine as it was about emphasizing the insubstantiality of the substantial factor test it adopted with regard to risk contribution.
Like foreseeability, however, the substantial factor test can easily be misapplied.
The substantial factor test can be illustrated by a small, easily controlled fire that joins a second larger fire and ultimately burns down a house.
The substantial factor test allows the court to decide, as a matter of law, the issue of proximate cause.