What About the Precedent: An Information-Theoretic Analysis of Common Law

Proceedings of the 2021 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies, pages 2275–2288 June 6–11, 2021. ©2021 Association for Computational Linguistics 2275 What About the Precedent: An Information-Theoretic Analysis of Common Law Josef ValvodaD Tiago PimentelD Niklas StoehrQ Ryan CotterellD,Q Simone TeufelD DUniversity of Cambridge, QETH Z¨urich jv406@cam.ac.uk, sht25@cam.ac.uk Abstract In common law, the outcome of a new case is determined mostly by precedent cases, rather than by existing statutes. However, how ex- actly does the precedent influence the out- come of a new case? Answering this ques- tion is crucial for guaranteeing fair and con- sistent judicial decision-making. We are the first to approach this question computationally by comparing two longstanding jurispruden- tial views; Halsbury’s, who believes that the arguments of the precedent are the main deter- minant of the outcome, and Goodhart’s, who believes that what matters most is the prece- dent’s facts. We base our study on the cor- pus of legal cases from the European Court of Human Rights (ECtHR), which allows us to access not only the case itself, but also cases cited in the judges’ arguments (i.e. the prece- dent cases). Taking an information-theoretic view, and modeling the question as a case out- come classification task, we find that the prece- dent’s arguments share 0.38 nats of informa- tion with the case’s outcome, whereas prece- dent’s facts only share 0.18 nats of informa- tion (i.e., 58% less); suggesting Halsbury’s view may be more accurate in this specific court. We found however in a qualitative anal- ysis that there are specific statues where Good- hart’s view dominates, and present some evi- dence these are the ones where the legal con- cept at hand is less straightforward. 1 Introduction Legal systems around the world can be divided into two major categories (Joutsen, 2019): civil law systems, which rely predominantly on the rules written down in statutes, and common law systems, which rely predominantly on past judicial decisions, known as the precedent. Within common law systems, jurisprudential scholars have pondered over the nature of precedent in law for at least a century (Halsbury, 1907). Is it the judges’ argumentation in the precedent, or is it the Figure 1: The text of ECtHR cases can be divided into facts, arguments and outcome. Arguments cite relevant cases, also known as the precedent. claimants’ specific individual circumstances that are the deciding factor in what becomes the law? Here, we present a new information-theoretical methodology that helps answer this question. In common law countries, statutes establish the general idea of the law, but the actual scope of the law is determined by the courts during a trial. To keep case outcomes consistent and predictable in subsequent cases, judges are forced to apply the rea- soning developed in prior cases with similar facts (precedent), to the facts of the new case under the doctrine of stare decisis (Duxbury, 2008; Lamond, 2016; Black, 2019). This is done by identifying the ratio decidendi (the reasons for …

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