110 Iowa L. Rev. 1247 (2025)
Abstract
As trademark law is currently litigated and understood, a plaintiff may succeed on an infringement claim by showing that (1) it owns a valid trademark, and (2) the defendant used a mark in commerce that is likely to confuse consumers into thinking that the plaintiff’s and defendant’s products come from the same source. We argue that this conventional understanding of the cause of action is missing an element: The plaintiff should also be required to show that the confusion arises from protectable features of the plaintiff’s trademark. Without this “causal tracing” requirement, plaintiffs can effectively claim exclusive rights to features that the law allows anyone to use, such as functional features or generic terms. Eliding the link between protectable features and confusion can hinder competition and undermine the purposes of trademark law.
Beyond theoretically demonstrating the need to fill this gap in the law, we show how to do it in practice. We focus on consumer surveys—an increasingly critical component of trademark litigation—and explain how to design surveys that address whether the missing element is satisfied. In doing so, we unearth and challenge an implicit assumption in current survey methodology: that a feature of a trademark must be a but-for cause of a consumer’s confusion in order for that consumer to count toward the rate of confusion attributable to that feature. We draw on broader debates about causation to argue that sufficient causes should count as causes for purposes of trademark confusion. We then lay out how to design surveys to take account of multiple sufficient causes. Finally, we analyze several sets of hypothetical results for a survey we designed based on a real case, comparing them to results under existing survey methods and showing that our approach is more helpful to courts. In short, we identify a fundamental problem in trademark law, and we offer a concrete, practical solution.