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The market share theory of liability is just one of several legal theories of collective responsibility espoused today. Collective responsibility comes into play when a product injures someone, but the identification of a definite culpable defendant is very difficult, if not impossible. For example, if a uniform product is manufactured by more than one company and causes an injury due to a defect, collective responsibility seeks to impose liability on all the manufacturers in the absence of identifying the manufacturer of the precise unit that caused the injury. The market share version would apportion the collective liability among the defendant manufacturers in accordance with their respective percentage shares of the sales market.

The genesis of the market share argument can be found in a California supreme court decision, Sindell v. Abbot Laboratories, 607 P.2d 924 (1980), a case dealing with DES. The court decided that the inability to identify a certain defendant was not fatal to the plaintiff's cause of action, provided that the plaintiff join as defendants a substantial share of manufacturers who made or supplied DES for the California market. Since that 1980 case, the market share theory has had a checkered pattern of acceptance in both state and federal courts; however, only two state supreme courts (Missouri and Iowa) have unqualifiedly rejected the market share theory (and in fact, all the theories of collective responsibility).

Many courts have accepted the market share theory based on the simple fact that a defective product caused an injury; it is held that the injured party should not have to suffer the consequences of that defect without compensation because the precise manufacturer of the defective product cannot be identified. All the manufacturers in that particular market share the rewards of the product and so, in theory, it is equitable that all should share the penalties. Those who oppose the market share theory counter that if all manufacturers have to pay damages even though only one or two made a defective product, then the public as a whole suffers since manufacturers will stop producing the product, curtail research and development into new products, and even go out of business. Furthermore, if a program of compensation for victims of defective products is to exist, it should be a matter for the legislative branch of the government (for example, the National Childhood Vaccine Injury Act of 1986) and not the courts.

Your client should know that Massachusetts has accepted the market share theory of collective liability. For a sample of the reasoning behind the acceptance, see McCormack v. Abbot Laboratories, 617 F. Supp. 1521 (1985).