Ordinary product liability law requires a plaintiff to prove that a specific defendant's product caused the plaintiff's injury. Market share liability relaxes that identification requirement in a narrow category of cases: where a fungible, generically manufactured product injured the plaintiff, many manufacturers produced essentially identical versions of it, and the passage of time or the nature of the product makes it impossible, through no fault of the plaintiff, to identify which manufacturer's product was actually used.

The doctrine originated in litigation over a generic pharmaceutical taken by pregnant women decades before the resulting injuries in their children became apparent, where the fungibility of the drug and the long latency period made ordinary identification impossible for reasons entirely outside the plaintiffs' control. Courts recognized that without some adjustment, an entire class of provably injured plaintiffs would recover from no one, while an entire industry that collectively caused identical harm would face no liability at all.

Jurisdictions that recognize market share liability generally require the plaintiff to show: the product was fungible, meaning the various manufacturers' versions were chemically and functionally identical such that risk did not vary by manufacturer; identification of the specific manufacturer is impossible despite the plaintiff's reasonable efforts, often because of a long latency period between exposure and injury; the plaintiff joins a substantial share of the manufacturers who produced the product in the relevant market and time period; and each named defendant is liable only for a proportionate share of damages corresponding to its share of that market, rather than being jointly and severally liable for the whole.

Unlike ordinary joint and several liability, market share liability typically caps each defendant's exposure at its market share percentage. A defendant that can prove it did not manufacture the specific product that caused the plaintiff's injury, or that its true market share was lower than assumed, can reduce or eliminate its share; some formulations shift the resulting shortfall to the remaining defendants, while others leave it unrecovered by the plaintiff.

Market share liability has been adopted, and adopted in materially different forms, in only a minority of states, and has been expressly rejected in others. Courts that decline to adopt it generally emphasize that it detaches liability from actual causation of a specific plaintiff's injury, that market definition is inherently imprecise, and that the doctrine has proven difficult to extend beyond its original fungible-drug context to products with more variation in formulation, warning, or design across manufacturers.

Juricratic treats market share liability as a claim path where causation-against-a-specific-defendant is replaced by a proportionate-liability dial driven by market share evidence rather than by individualized proof. A user can war-game how total exposure shifts as named defendants drop out (through a market-share rebuttal) or as the plaintiff's total identified market coverage changes, making visible the mechanical relationship between market definition and each defendant's simulated exposure without treating any resulting figure as a predicted outcome.