Study of the structure and evolution of citation networks has potential to advance understanding of human activity in fields such as science, law, and patenting [1-3]. A main area of research in this vein has been the application of community detection (clustering) methods to citation networks [4]. While these methods are capable of grouping the nodes of a citation network according to known areas of science (e.g. physics, chemistry, mathematics, etc.) [5] or law (e.g. taxation, contracts, human rights, etc.) [6], past studies have not yet shown how citation network clustering can be applied by social science researchers as a tool to study phenomena occurring within the scientific or legal enterprise. Here, we apply the map equation clustering method to legal citation networks using higher-resolution data than previously studied, in which the judgments are tagged with legal topics within a given area of law (e.g. custody or child support, within the area of family law). We find that, in some cases, the method sorts multi-topic judgments into groups such that, for each judgment, one topic can be identified as dominant and the others as secondary. It appears that the dominant topic is the central driving motivation of the litigation, whereas the secondary topics are other factors at play in the litigation, which may include tactics used by the litigants for leverage or compensation. For example, in family law, in the cluster in which the dominant topic is child support payments, the main secondary topic after spousal support is custody of the children, consistent with the often-noted reality that custody is used as a tactic in securing financial advantage in family law litigation. We propose this sorting of multi-topic judgments as an application that may allow legal researchers – who are only able to analyze, using content analysis, a relatively small number of cases [7] – to select sub-groups of judgments focused on a phenomenon of interest, which may include the use of particular tactics or the appearance of other types of secondary characteristics in litigation that has a particular dominant topic.
