California's design defect framework remains one of the most cited in the country, and its structure still drives how plaintiffs plead and try product cases in state court and in federal diversity cases applying California law. Barker v. Lull Engineering Co., 20 Cal.3d 413 (1978), established two alternative ways to prove a design defect. Soule v. General Motors Corp., 8 Cal.4th 548 (1994), then limited when the first one can be used. Knowing the boundary determines whether you ask the jury a simple question or a complicated one.
The Two Prongs
Under Barker, a product is defective in design if it fails to perform as safely as an ordinary consumer would expect when used in an intended or reasonably foreseeable manner. That is the consumer expectations test. Alternatively, a product is defective if the plaintiff proves that the design proximately caused the injury and the defendant fails to prove that the benefits of the design outweigh its risks. That is the risk-benefit test, and the burden shifts to the defendant on the balancing question once causation is shown.
The burden shift is the distinguishing feature of the second prong. The jury weighs factors such as the gravity of the danger, the likelihood that it will occur, the feasibility of an alternative design, the cost of that alternative, and the adverse consequences to the product and consumer. A plaintiff does not need to prove that the alternative design was practical in the abstract; the defendant must justify its choice.
What Soule Changed
In Soule, the California Supreme Court held that the consumer expectations test is not appropriate in every case. It applies when the everyday experience of the product's users permits a conclusion that the product's design violated minimum safety assumptions, and the product's failure is within the common knowledge of lay jurors. It does not apply when the question depends on technical, complex tradeoffs among competing design considerations, or when the product's performance in the circumstances is beyond the ordinary consumer's experience.
In Soule itself, the plaintiff's claim about how a car's wheel assembly and floor pan performed in a particular type of frontal collision involved complex engineering questions, so the court held the consumer expectations instruction should not have been given. The ruling does not limit the risk-benefit prong, which remains available in every design case.
When the First Prong Is Worth Pursuing
The consumer expectations test remains valuable when the failure is so basic that a jury needs no engineering to evaluate it. Examples practitioners commonly cite include a product that catches fire during ordinary use, a ladder that collapses under normal weight, a kitchen appliance that explodes in routine operation, or a children's product that fails at its core function. In those cases, expert testimony is still helpful, but the test lets the jury rely on its common sense and avoids a technical dispute where defense experts can generate doubt.
It is less promising for products involving crash dynamics, drug or chemical effects, software behavior, or sophisticated machinery. Judges in those cases often grant motions to exclude the instruction, so plan the case on the risk-benefit theory and treat the first prong as an addition rather than the foundation. Our product liability coverage discusses how these choices interact with recall evidence and regulatory standards.
Pleading and Instruction Strategy
Plead both theories and request both instructions, but be prepared for the pretrial fight. A few practical points:
- Brief the common-knowledge argument early, with evidence of ordinary use and what a lay user would assume. Declarations from users, marketing materials that set expectations, and the manufacturer's own warnings can help.
- Prepare the alternative design package for the risk-benefit prong. Identify a feasible design, cost data, prototypes or existing competitor models, and testing, since the defendant will use the balancing factors to argue that the plaintiff's design is impractical.
- Consider whether the defendant's own documents show the tradeoffs were considered. Internal design reviews, failure mode analyses, and cost-benefit memoranda are strong evidence, and they matter to the burden on the defendant.
- Protect the verdict form. Ask for a special verdict that separates theories if there is risk that one instruction will be found erroneous on appeal.
Evidence on the Defendant's Burden
Because the defendant carries the burden of showing that the design's benefits outweigh its risks, discovery should aim at the materials the defendant will rely on to carry it. Request design history files, hazard analyses, testing protocols and results, warranty and complaint data, and prior incident reports. Depose the engineer responsible for the decision, and ask directly what alternatives were considered and why they were rejected. If the rejection rested on cost, ask for the number.
Industry standards and regulatory compliance are relevant but not dispositive. Compliance with a standard is evidence that the design was reasonable, yet it does not establish the absence of a defect. Be ready for the defense to emphasize compliance, and respond with evidence that the standard is a minimum, or that the manufacturer knew of risks the standard did not address.
Failure-to-Warn and Interaction With Design
Design and warning claims are distinct, and a warning generally cannot cure a design defect that could have been engineered out. That distinction matters in closing: a defense that the product carried a clear label is not a response to a claim that a safer design existed. For related practice in cases involving prescribers and medical devices, see our medical malpractice section, which addresses how product and professional claims are often pleaded together.
Takeaway
Treat risk-benefit as the main road and consumer expectations as a shortcut that is sometimes closed. Build the alternative-design case thoroughly, pursue the defendant's cost-benefit documents, and confirm current California Supreme Court and appellate decisions before briefing, since lower courts continue to refine how Soule applies to modern products.