Private Law Theory

ABSTRACT
The but-for test has bedeviled the law for decades now. Over the last few years, however, the Supreme Court has cemented it as the centerpiece of the law’s model for factual causation. This spells trouble in a landscape brimming with disputes about disparate treatment, equal protection, Executive Branch action, and other constitutional rights whose vindication turns on defendants’ motives. For even where motives matter – where reasons for action mark the difference between permitted or forbidden hiring and firing decisions, or school admissions procedures, or coverage for medical services – the Roberts Court has insisted on adopting a counterfactual model of causation. And while courts and commentators have long known that it sometimes yields the wrong results, prevailing wisdom is that we can get by with minor variations on the model while preserving its overall approach.

This is a mistake. The traditional model splits the causal inquiry in two, independently testing for a defendant’s intentions and actions. But intentions and actions are intertwined; they are not self-standing pieces to be factored in and out of a modular, plug-and-play structure. Assuming otherwise reduces causation to dependence, leaving no room for a notion that is much more important for the law’s purposes: explanation. The result is not just a test that over- or under-generates; more importantly, it fails to satisfy constitutionally protected explanatory obligations to both plaintiffs and defendants in a wide variety of contexts, from civil rights violations to insider-trading allegations to hate crimes. The but-for test’s flaws are thus not cabined to corner cases. To the contrary, there is a deeper, structural mismatch between the law’s aims in testing for causation, on the one hand, and the tools with which it tries to do so, on the other.

After identifying this mismatch, the Article argues that the traditional model should be replaced with one that focuses on formal rather than efficient causation, centered around an inquiry into defendants’ intentions. Using equal-protection and disparate-treatment claims as test cases, the Article shows how this shift will better align with both constitutional and statutory motive-based liability regimes while avoiding the explanatory deficits of deviant causation. The result is a more accurate, practicable model—not just in corner cases fit for theory, but everywhere the law must account for the effects that we have, by accounting for the reasons on which we act.

Azmat, Ahson, Formal Causation in the Law (February 12, 2025).