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This Article discusses the relationship between custom and law to highlight the phenomenon of endogeneity that arises when empirically evaluating the effects of laws. An important literature evaluates the roles of laws in motivating behaviors, including investigations of whether or how laws influence customs and social norms. Traditional economic analysis, for example, posits that codified laws influence behaviors by for- mally incentivizing a particular action, and social norm theories assert that the laws also communicate values.1 Enhancing this strand of thought, an increasing number of works employing historical or empirical analyses have linked laws to broader societal changes over time.2 Meanwhile, a valuable discourse examines how customs may deter- mine both de facto laws and formally enacted laws, including the court precedents that are rendered.3 Whether they are directly codified into a legal test or informally referenced, customs can influence formal laws that are adopted in a community and beyond. Indeed, some scholars have argued that evolving customs and norms have influenced the Supreme Court in its decisions.4
The subsequent effects of these formal laws and court decisions are of tremendous interest to policymakers and judges. With policy concerns in mind, we argue that one must not ignore the endogenous feedback between aggregate behaviors, customs, and laws. That is, while customs may shape or influence laws, laws can also shape customs through their effects on behaviors or norms in the aggregate. The endogeneity that custom produces suggests that simply by observing a correlation between law and behavior is not enough to assert that a law in itself is effective or to assert that social trends and evolving customs are driving legal change.5
Importantly, legal scholars from historical to international to political perspectives recognize that the interplay between custom and law involves feedback between both.6 Individuals endorse customs partly by learning from the apparent beliefs of others and partly by distorting their public responses in the interest of maintaining social acceptance. "Availability entrepreneurs" manipulate the content of public discourse and strive "to trigger availability cascades likely to advance their own agendas."7 Public discourse, in turn, influences law. Other scholars suggest that customary inter- national law, having no binding power on states, does not influence rational actors who only follow custom insofar as it suits their best interest.8 Theorists have formally modeled this relationship.9 Moreover, whether court decisions respond to the public's policy preferences is important in the larger context of understanding how courts may rely on dominant customs for public legitimacy.10 However, no empirical study of
custom and law sufficiently addresses this endogeneity in its causal claims, and we are not aware of previous studies that empirically distinguish between law and custom. It remains an open empirical question whether appellate precedent in practice is moti- vated by customs that vary across time and space. To what extent are customs actually driving some laws in real life?
We contribute original empirical evidence highlighting the role of custom within court-made law in the United States. To do so, we use a narrower conception of the relationship between custom and law.11 We focus on law as represented by court-made appellate precedent, and to illustrate our points, we use two distinct doctrinal examples in the United States. The first is obscenity law, where custom is directly referenced in the law through the Miller community standards test.12 The second example is takings law, where we argue that entrenched expectations about what constitutes just compen- sation and public use in the Takings Clause of the Fifth Amendment play significant roles in shaping the law though they are not explicitly codified. …