The substantial factor test is often used in criminal law and torts to assess whether the defendant’s acts or omissions were a substantial factor in causing the harm, especially when there are multiple causes.
In criminal law, the substantial factor test is used to establish whether a defendant’s acts or omissions led to the result of the crime. The prosecution must prove that the defendant’s conduct was a substantial factor in bringing about the result. It does not need to be the sole or primary cause, but it must be more than trivial or insignificant, thus a “significant” factor. For example, if A and B independently and fatally shot C, leading to C’s death, both A and B’s actions could be considered a substantial factor in C’s death because both shots significantly contributed to the outcome.
Because the but-for test requires a much stricter proof of causation, the substantial factor test is often applied in some jurisdictions when the but-for test is too restrictive. For example, A and B would not be found for C’s death because the harm would have occurred even if either A or B did not shoot C. One way to still apply the but-for test without resorting to the substantial factor test is to elaborate the “but-for condition”: but for A or B’s voluntary act of shooting C, would C have died at the time and in the way C died?
The substantial factor test is also different from the proximate cause test in that the former focuses more on the defendant’s contribution to the harm while the latter focuses more on the foreseeability of the harm.
See: CALCRIM No. 620. Causation: Special Issues - Judicial Council of California Criminal Jury Instructions (2024 edition); People v. Pike (1988)
In torts, the substantial factor test is defined from the Second Restatement of Torts as follows:
“Except as stated in subsection (2), the actor’s negligent conduct is not a substantial factor in bringing about harm to another if the harm would have been sustained even if the actor had not been negligent. (2) If two forces are actively operating, one because of the actor’s negligence, the other not because of any misconduct on his part, and each of itself is sufficient to bring about harm to another, the actor’s negligence may be found to be a substantial factor in bringing it about.”
Thus, the substantial factor test in torts requires proof that the defendant’s actions or omissions were sufficient to cause the harm on their own right, even if there was another cause that might have led to the result. In a sense, the “but-for” concept is embedded in the substantial factor test in that the defendant’s conduct alone would be sufficient to bring about the harm.
See: Mavroudis v. Pittsburgh-Corning Corp. (1997); Roemmich v. 3M Co. (2022); Washington Pattern Jury Instructions--Civil: WPI 15.02 Proximate Cause - Substantial Factor Test and CACI No. 430. Causation: Substantial Factor - Judicial Council of California Civil Jury Instructions (2024 edition).
[Last updated in August of 2024 by the Wex Definitions Team]