Private Law Theory

ABSTRACT
The but-for test for actual causation in tort law fails in a well-documented and practically significant class of cases: those involving overdetermination, where multiple sufficient causes operate simultaneously, and preemption, where one sufficient cause overtakes another before it operates. Courts have responded with two doctrines – the substantial factor test and market share liability – that are admittedly circular or ad hoc. This Article proposes a principled alternative grounded in structural causal model theory. The framework is introduced as a three-element modified counterfactual test that systematically reproduces widely accepted doctrinal outcomes in the core case classes demonstrated here and translates into specific jury instructions derivable from expert causal models, placing a concrete demand on trial judges at the instruction derivation stage in return for a more explicit and auditable record for appellate review than the substantial factor test provides. For probabilistic harm cases, the Article applies the Fenton-Glynn probabilistic extension of the framework as a proof of concept; this is a distinct formal system from the deterministic three-condition test and remains methodologically contested. The Restatement Third of Torts §27 already declares the substantive standard; the modified counterfactual test is one principled decision procedure for implementing it.

Drewien, Seth, Actual Causation in Overdetermined Harm: A Structural Framework for the Cases the But-For Test Cannot Solve (March 26, 2026).