Market share liability

Market share liability is a legal doctrine that allows a plaintiff to establish a prima facie case against a group of product manufacturers for an injury caused by a product, even when the plaintiff does not know from which defendant the product originated. The doctrine is unique to the law of the United States and apportions liability among the manufacturers according to their share of the market for the product giving rise to the plaintiff's injury.

Market share liability was introduced in the California case Sindell v. Abbott Laboratories. In Sindell, the plaintiffs were injured by DES, a drug prescribed to prevent miscarriage. The mothers of the plaintiffs had taken DES while pregnant, and expert testimony showed this to be a proximate cause of reproductive tract cancers in the plaintiffs years later. The plaintiffs, however, could not ascertain which drug company distributed the DES taken by their mothers. The court responded by allowing the plaintiffs to apportion liability among the defendant drug companies according to their respective shares in the DES market.

Sindell laid out the requirements for applying the doctrine of market share liability:

First, the defendants in court must constitute substantially all of the market. This is a distinguishing factor from alternative liability, which requires that all of the defendants be in court (See Summers v. Tice). Having "substantially all" of the market makes it more likely that the actual wrongdoer will be in court. A main reason for not requiring all of the relevant market is that as time passes, some manufacturers drop out of the market, and it would raise the bar for the plaintiff too high. Also if all defendants were present, then market share liability would be unnecessary, because the plaintiff would be able to apply the doctrine of alternative liability to put the burden of proving causation onto the defendants.

Second, the products must be fungible (i.e. interchangeable—they must be of the same composition). For example, in Skipworth v. Lead Industries Association, the court found the lead paint sold by the defendants to not be fungible because the paints had lead pigments containing different chemical formulations, different amounts of lead, and differed in potential toxicity.

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