California Civil Jury Instructions (CACI)

430. Causation: Substantial Factor

A substantial factor in causing harm is a factor that a reasonable person would consider to have contributed to the harm. It must be more than a remote or trivial factor. It does not have to be the only cause of the harm.

[Conduct is not a substantial factor in causing harm if the same harm would have occurred without that conduct.]

New September 2003; Revised October 2004, June 2005, December 2005, December 2007

Directions for Use

As phrased, this definition of “substantial factor” subsumes the “but for” test of causation, that is, “but for” the defendant’s conduct, the plaintiff’s harm would not have occurred. (Mitchell v. Gonzales (1991) 54 Cal.3d 1041, 1052 [1 Cal.Rptr.2d 913, 819 P.2d 872]; see Rest.2d Torts, § 431.) The optional last sentence makes this explicit, and in some cases it may be error not to give this sentence. (See Soule v. GM Corp. (1994) 8 Cal.4th 548, 572—573 [34 Cal.Rptr.2d 607, 882 P.2d 298]; Rest.2d Torts, § 432(1).)

“Conduct,” in this context, refers to the culpable acts or omissions on which a claim of legal fault is based, e.g., negligence, product defect, breach of contract, or dangerous condition of public property. This is in contrast to an event that is not a culpable act but that happens to occur in the chain of causation, e.g., that the plaintiff’s alarm clock failed to go off, causing her to be at the location of the accident at a time when she otherwise would not have been there. The reference to “conduct” may be changed as appropriate to the facts of the case.

The “but for” test of the last optional sentence does not apply to concurrent independent causes, which are multiple forces operating at the same time and independently, each of which would have been sufficient by itself to bring about the same harm. (Viner v. Sweet (2003) 30 Cal.4th 1232, 1240 [135 Cal.Rptr.2d 629, 70 P.3d 1046]; Barton v. Owen (1977) 71 Cal.App.3d 484, 503—504 [139 Cal.Rptr. 494]; see Rest.2d Torts, § 432(2).) Accordingly, do not include the last sentence in a case involving concurrent independent causes.

In cases of multiple (concurrent dependent) causes, CACI No. 431, Causation: Multiple Causes, should also be given.

In asbestos-related cancer cases, Rutherford v. Owens-Illinois, Inc. (1997) 16 Cal.4th 953, 977 [67 Cal.Rptr.2d 16, 941 P.2d 1203] requires a different instruction regarding exposure to a particular product. Give CACI No. 435, Causation for Asbestos-Related Cancer Claims, and do not give this instruction.

Sources and Authority

“The test for joint tort liability is set forth in section 431 of the Restatement of Torts 2d, which provides: ‘The actor’s negligent conduct is a legal cause of harm to another if (a) his conduct is a substantial factor in bringing about the harm, and, (b) there is no rule of law relieving the actor from liability because of the manner in which his negligence has resulted in the harm.’ Section 431 correctly states California law as to the issue of causation in tort cases.” (Wilson v. Blue Cross of So. Cal. (1990) 222 Cal.App.3d 660, 671—672 [271 Cal.Rptr. 876].)
“California has definitively adopted the substantial factor test of the Restatement Second of Torts for cause-in-fact determinations. Under that standard, a cause in fact is something that is a substantial factor in bringing about the injury. The substantial factor standard generally produces the same results as does the ‘but for’ rule of causation which states that a defendant’s conduct is a cause of the injury if the injury would not have occurred ‘but for’ that conduct. The substantial factor standard, however, has been embraced as a clearer rule of causation—one which subsumes the ‘but for’ test while reaching beyond it to satisfactorily address other situations, such as those involving independent or concurrent causes in fact.” (Rutherford, supra, 16 Cal.4th at pp. 968—969, internal citations omitted.)
“The term ‘substantial factor’ has not been judicially defined with specificity, and indeed it has been observed that it is ‘neither possible nor desirable to reduce it to any lower terms.’ This court has suggested that a force which plays only an ‘infinitesimal’ or ‘theoretical’ part in bringing about injury, damage, or loss is not a substantial factor. Undue emphasis should not be placed on the term ‘substantial.’ For example, the substantial factor standard, formulated to aid plaintiffs as a broader rule of causality than the ‘but for’ test, has been invoked by defendants whose conduct is clearly a ‘but for’ cause of plaintiff’s injury but is nevertheless urged as an insubstantial contribution to the injury. 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.’ ” (Rutherford, supra, 16 Cal.4th at pp. 968—969, internal citations omitted.)
“The substantial factor standard is a relatively broad one, requiring only that the contribution of the individual cause be more than negligible or theoretical. Thus, ‘a force which plays only an “infinitesimal” or “theoretical” part in bringing about injury, damage, or loss is not a substantial factor’, but a very minor force that does cause harm is a substantial factor. This rule honors the principle of comparative fault.” (Bockrath v. Aldrich Chemical Co. (1999) 21 Cal.4th 71, 79 [86 Cal.Rptr.2d 846, 980 P.2d 398], internal citations omitted.)
“The text of Restatement Torts second section 432 demonstrates how the ‘substantial factor’ test subsumes the traditional ‘but for’ test of causation. Subsection (1) of section 432 provides: ‘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.’ . . . Subsection (2) states that if ‘two forces are actively operating . . . 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.’ ” (Viner, supra, 30 Cal. 4th at p. 1240, original italics.)
“A tort is a legal cause of injury only when it is a substantial factor in producing the injury. If the external force of a vehicle accident was so severe that it would have caused identical injuries notwithstanding an abstract ‘defect’ in the vehicle’s collision safety, the defect cannot be considered a substantial factor in bringing them about. [¶] The general causation instruction given by the trial court correctly advised that plaintiff could not recover for a design defect unless it was a ‘substantial factor’ in producing plaintiff’s ‘enhanced’ injuries. However, this instruction dealt only by ‘negative implication’ with [defendant]’s theory that any such defect was not a ‘substantial factor’ in this case because this particular accident would have broken plaintiff’s ankles in any event. As we have seen, [defendant] presented substantial evidence to that effect. [Defendant] was therefore entitled to its special instruction, and the trial court’s refusal to give it was error.” (Soule, supra, 8 Cal.4th at p. 572—573, original italics, footnote and internal citations omitted.)
“The first element of legal cause is cause in fact … The ‘but for’ rule has traditionally been applied to determine cause in fact. The Restatement formula uses the term substantial factor ‘to denote the fact that the defendant’s conduct has such an effect in producing the harm as to lead reasonable men to regard it as a cause.’ ” (Mayes v. Bryan (2006) 139 Cal.App.4th 1075, 1095 [44 Cal.Rptr.3d 14], internal citations omitted.)
“ ‘Whether a defendant’s conduct actually caused an injury is a question of fact . . . that is ordinarily for the jury …’ ‘[C]ausation in fact is ultimately a matter of probability and common sense: “[A plaintiff] is not required to eliminate entirely all possibility that the defendant’s conduct was not a cause. It is enough that he introduces evidence from which reasonable [persons] may conclude that it is more probable that the event was caused by the defendant than that it was not. The fact of causation is incapable of mathematical proof, since no [person] can say with absolute certainty what would have occurred if the defendant had acted otherwise. If, as a matter of ordinary experience, a particular act or omission might be expected to produce a particular result, and if that result has in fact followed, the conclusion may be justified that the causal relation exists. In drawing that conclusion, the triers of fact are permitted to draw upon ordinary human experience as to the probabilities of the case.” ’ . . . ‘ “A mere possibility of . . . causation is not enough; and when the matter remains one of pure speculation or conjecture, or the probabilities are at best evenly balanced, it becomes the duty of the court to direct a verdict for the defendant.” ’ ” (Raven H. v. Gamette (2007) 157 Cal.App.4th 1017, 1029—1030 [68 Cal.Rptr.3d 897], internal citations omitted.)
“[E]vidence of causation ‘must rise to the level of a reasonable probability based upon competent testimony. [Citations.] “A possible cause only becomes ‘probable’ when, in the absence of other reasonable causal explanations, it becomes more likely than not that the injury was a result of its action.” [Citation.] The defendant’s conduct is not the cause in fact of harm “ ‘where the evidence indicates that there is less than a probability, i.e., a 50—50 possibility or a mere chance,’ ” that the harm would have ensued.’ ” (Bowman v. Wyatt (2010) 186 Cal.App.4th 286, 312 [111 Cal.Rptr.3d 787].)
“However the test is phrased, causation in fact is ultimately a matter of probability and common sense.” (Osborn v. Irwin Memorial Blood Bank (1992) 5 Cal.App.4th 234, 253 [7 Cal.Rptr.2d 101], relying on Rest.2d Torts, § 433B, com. b.)