Animal and human societies exhibit extreme diversity in the size, composition and cohesion of their social units, in the patterning of sex-specific reproductive skew, in the nature of parental care, in the form and frequency of cooperation and in their competitive regime, creating a diversity of socially complex societies. However, there is an ongoing debate about whether social complexity is a real, emergent property of a society or whether it only provides a conceptual framework for studying the diversity and evolution of societies. In this introduction to our topical collection, we identify three areas of current research addressing relevant challenges in the study of social complexity. First, previous studies have suffered from a lack of a common conceptual framework, including shared definitions, and existing measures of social complexity do not acknowledge its multiple components and dimensions. Second, most previous studies have ignored intraspecific variation, and the proximate and ultimate determinants of variation in social complexity, as well as their interactions, remain poorly known. Third, comparative studies of social complexity offer opportunities to explore its biological causes and correlates and but it is frequently difficult to identify the causal relationships involved and the development of general insights has been hampered by conceptual and methodological difficulties. In this paper, we briefly characterize these three challenges and offer guidance to the other contributions to this topical collection on social complexity by placing their key results in the context of these three topics. Significance statement In this paper, we address the main current challenges in the study of social complexity and combine our brief overview with a synopsis of the other contributions to this topical collection on various aspects of social complexity.
