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I have a question about negligence and cause-in-fact. In a hypo I was reading over, a bicycle helmet broke when a plaintiff crashed and she sues the store she bought it from and the manufacturer. All other issues aside about their duty and if they breached it, when trying to prove cause-in-fact would you treat the store and defendant as one since they would be jointly liable and use the but-for test? or would you use multiple sufficient causes because you could not tell who caused the harm, since they are negligent for the faulty helmet?

We have not really addressed product liability in class, but I want to understand this in case it shows up on the exam.

You would have two separate claims.

I. Negligence against shop
-I think this is claim dies with negligence as the shop owner is not a learned intermediary. Even so, if you could prove that shop owner should have know about the defected helmet, you would ask "but-for the shop negligently selling the helmet after a failed inspection, would P still crash?". You would then need more evidence on P's speed on his bike. For example, if he were riding so quickly that a non-defective helmet would break, then there is no cause-in-fact.

II Design Defect against manufacturer
-You would argue consumer expectation + risk-utility/alternative design tests. As mentioned above, you would need more evidence on P's actions. You would only establish cause-in-fact if the helmet would not have broken, but-for the defective design.

Hope this helps.