Understanding Causation in Negligence Cases: The Substantial Factor Test
In negligence cases, proving causation is crucial. One of the most widely used legal standards to establish causation is the substantial factor test. This test helps determine whether a defendant’s conduct was significantly responsible for a plaintiff’s harm. Unlike the rigid "but-for" test, the substantial factor standard provides a broader, more practical approach to causation.
What is a Substantial Factor?
A substantial factor is a factor that a reasonable person would consider to have contributed to the harm. It must be more than trivial or remote but does not have to be the sole cause. Mitchell v. Gonzales (1991) 54 Cal.3d 1041, 1052 [1 Cal.Rptr.2d 913, 819 P.2d 872].
In Mitchell, "Twelve-year old Damechie Mitchell, the son of James and Joyce Mitchell (plaintiffs), was invited to a lake by his friend Luis and Luis’s parents, Jose and Matilde Gonzales (defendants). Joyce allegedly told Matilde that Damechie could not swim and should be kept ashore. Matilde denied the conversation. Without Jose or Matilde’s knowledge, Luis and Damechie took a paddleboard onto the lake. Damechie told Luis he could not swim and asked for help if he fell into the water. Luis began rocking the paddleboard and tipped it, causing both boys to fall off. When Damechie tried to grab onto Luis, Luis pushed him away. Damechie drowned. The Mitchells sued the Gonzaleses for negligence in a California state court. At trial, the parties requested different jury instructions on causation."
"The Gonzaleses requested Book of Approved Jury Instructions (BAJI) No. 3.75: 'A proximate cause of [injury] . . . is a cause which, in natural and continuous sequence, produces the [injury] . . . and without which the [injury] . . . would not have occurred.” The Mitchells requested BAJI No. 3.76, which defined the cause of injury as “a cause which is a substantial factor in bringing about the [injury].' The trial court gave BAJI No. 3.75 as an instruction. In closing argument, counsel for the Gonzaleses emphasized Damechie’s knowing inability to swim. The jury found, by an overwhelming margin, that the defendants were negligent. They also concluded, however, by an overwhelming margin, that the defendants’ negligence did not cause Damechie’s death. The Mitchells appealed." (see here).
The California Supreme Court therein held that the substantial factor standard "subsumes the ‘but-for’ test of causation," meaning that a defendant’s conduct is a cause of harm if it was a substantial factor in bringing it about.
The standard applied is standard is critical in cases where multiple contributing factors exist. In Rutherford v. Owens-Illinois, Inc. (1997) 16 Cal.4th 953, 968-969 [67 Cal.Rptr.2d 16, 941 P.2d 1203], the court emphasized that a cause-in-fact must be a substantial factor in the harm. It further explained that the test addresses independent and concurrent causes more effectively than the traditional "but-for" causation standard.
Substantial Factor vs. But-For Causation
The "but-for" test asks whether the harm would have occurred if the defendant had not acted negligently. However, the substantial factor test broadens the scope of liability, making it more applicable in complex cases where multiple causes may exist. In Viner v. Sweet (2003) 30 Cal.4th 1232, 1240 [135 Cal.Rptr.2d 629, 70 P.3d 1046], the California Supreme Court explained that the substantial factor test is particularly useful when multiple independent forces operate simultaneously to cause harm.
However, in cases involving multiple sufficient causes—where each cause alone could have caused the harm—the "but-for" test is inappropriate. In Yanez v. Plummer (2013) 221 Cal.App.4th 180, 187 [164 Cal.Rptr.3d 309], the court clarified that "a defendant cannot avoid responsibility just because some other person, condition, or event was also a substantial factor in causing the plaintiff’s harm."
Application in Toxic Exposure and Product Liability Cases
The substantial factor test has been particularly significant in cases involving toxic exposure and product liability. In asbestos-related litigation, for example, plaintiffs often face challenges in proving which exposure caused their illness. Rutherford held that causation in asbestos cases requires showing that the defendant’s product contributed substantially to the risk of developing the disease. The court in Lopez v. The Hillshire Brands Co. (2019) 41 Cal.App.5th 679, 688 [254 Cal.Rptr.3d 377] reinforced that an exposure's remoteness in time does not necessarily preclude causation under the substantial factor test.
Comparative Fault and the Substantial Factor Test
The substantial factor test aligns with comparative negligence principles, ensuring that defendants are held responsible only for the portion of harm they caused. In Bockrath v. Aldrich Chemical Co., Inc. (1999) 21 Cal.4th 71, 79 [86 Cal.Rptr.2d 846, 980 P.2d 398], "Thomas Bockrath (plaintiff) worked for Hughes Aircraft Company (Hughes) from 1973 to 1994. Bockrath developed cancer while working for Hughes. Bockrath sued at least 55 defendants, including the manufacturers of many common products like WD-40 and rubber cement."
"The complaint alleged that: (1) Bockrath had been exposed to most and perhaps all the products, (2) all the products were used or improperly stored at the Hughes plant, (3) Bockrath had inhaled or had skin contact with all the products, and (4) the chemicals and chemical ingredients in the products produce carcinogenic effects. Two previous complaints were dismissed, and Bockrath filed a second amended complaint."
"The defendants demurred or moved to dismiss the second amended complaint as well, alleging that Bockrath had not adequately explained how the products caused Bockrath’s cancer. The trial court granted the motion to dismiss without leave to amend, and Bockrath appealed. The California Court of Appeal affirmed, and Bockrath petitioned the California Supreme Court for review." (see here)
The Bockrath court emphasized that even minor but significant contributions to harm qualify as substantial factors, reinforcing the principles of proportionate liability.
Jury Consideration and Legal Instructions
Courts often instruct juries on causation using CACI No. 430, which states that a substantial factor is one that is more than a remote or trivial cause of harm. However, courts have recognized the importance of tailoring jury instructions based on case specifics. For instance, in Petitpas v. Ford Motor Co. (2017) 13 Cal.App.5th 261, 299 [220 Cal.Rptr.3d 185], the court upheld the decision to provide both CACI No. 430 and CACI No. 435 in a case involving both product liability and premises liability.
Additionally, in Soule v. GM Corp. (1994) 8 Cal.4th 548, 572-573 [34 Cal.Rptr.2d 607, 882 P.2d 298], the court held that if a plaintiff’s injuries would have occurred regardless of the defendant’s conduct, then that conduct is not a substantial factor in causing the harm.
Limitations of the Substantial Factor Test
While broad, the substantial factor test is not limitless. In Modisette v. Apple Inc. (2018) 30 Cal.App.5th 136, 152 [241 Cal.Rptr.3d 209], the court held that when a plaintiff’s allegations fail to establish a reasonable link between the defendant’s conduct and the harm, the court may decide the causation issue as a matter of law.
Similarly, in Bowman v. Wyatt (2010) 186 Cal.App.4th 286, 312 [111 Cal.Rptr.3d 787], the court reinforced that causation must rise to the level of probability. A "mere possibility" that a defendant’s conduct caused harm is not sufficient to establish liability.
Final Thoughts
The substantial factor test is a cornerstone of California tort law, providing a flexible and fair approach to causation in negligence cases. Whether applied in toxic exposure, product liability, or personal injury cases, this test ensures that responsibility is assigned based on a defendant’s meaningful contribution to the harm.
For plaintiffs, understanding the substantial factor standard is crucial in building a strong negligence case. For defendants, it serves as a tool to argue against tenuous claims of causation. Ultimately, the test ensures that courts fairly assess liability by focusing on whether a defendant’s actions were a significant and meaningful factor in causing harm.
If you are involved in a negligence case and need guidance on how causation laws apply, consulting with an experienced attorney can help clarify your legal standing and options.