I. Introduction

Market share liability is a doctrine within products liability law that apportions liability against a set of defendants according to their respective market shares of sales of a harmful product during the period that the harm occurred. The concept of market share liability stands in contrast to the traditional and paradigmatic tort principle that assigns liability only with respect to harm that was directly and identifiably caused by a single defendant or multiple defendants.


The concept of market share liability was first adopted as grounds for recovery 30 years ago by the California Supreme Court in Sin-dell v Abbott Laboratories,1 with respect to harms caused by the ingestion by claimants’ mothers of the generic drug diethylstilbestrol (DES), a product and an industrial context possibly uniquely appropriate for the concept. In succeeding years, several other state courts adopted the concept in DES cases—though many rejected it2—and many plaintiffs have urged adoption of the concept to harms alleged to have been caused by other products. Commentators appear to agree, however, that courts have found that there are very few products that share the product-and market-characteristics of DES—in particular, that all units of the product are identically harmful—and, therefore, have adopted market share liability in very small fraction of cases in which it has been proposed.3 Following Sindell, courts have accepted the concept in principle4 in a scattering of cases involving mineral spirits; DPT vaccines; blood clotting agents; asbestos brake pads; MBTE, a gasoline additive; airplane aluminum beverage carts; and rowing exercise machines.5 This scattering of cases suggests the weak conceptual grounds for the doctrine.


The basic rationale for market share liability derives from the quasi-economic principle of achieving appropriate market incentives as among product manufacturers by internalizing the costs of harms caused by a product to the manufacturer that produced the harmful product.6 The central idea is that, as among different products, to create appropriate market signals for product purchase, other things equal, those products that cause greater harm should be priced higher than those products that cause lesser harm so to direct purchases to the product best suited for the individual consumer. Tort liability judgments that place injury costs on the manufacturer of the product that caused the injury serve to internalize the costs of harm by building injury costs into the price of the product. Cost internalization through liability judgments will serve both to create incentives for manufacturers to avoid harm in the future and to cause manufacturers to provide consumers with a form of injury insurance whose costs will be passed along in the product price.7


This basic concept formed the foundation for the adoption of market share liability in Sindell. Sindell dealt with claims that the pharmaceutical DES—a drug taken by pregnant women to reduce the chance of miscarriage—was defective in a way that contributed to cancers in the daughters of women who had taken the drug. Because some 200 manufacturers sold DES on a

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