Calculating Relatedness: A Pedigree of Definitions

Biology can be viewed from both an organismal and a genic perspective. A good example is W.D. Hamilton's work on inclusive fitness and kin selection, which puts relatedness at the heart of our understanding of social behavior. Relatedness mediates how much an actor should value a specific behavior's effect on a relative compared to the cost incurred to itself. Despite its key explanatory role, relatedness is also a concept marred with misunderstanding. Part of the problem has been that the term has been used in different ways by different people. To help address this, we survey the history of how relatedness has been formally modeled, paying particular attention to how it is conceptualized from both a gene-centric and an organism-centric point of view.