The Cultural Evolution of National Constitutions

We introduce a hybrid of approaches, inspired by biology and genetics, to analyze patterns of cultural inheritance and innovation through the study of the diffusion of ideas through a corpus of 591 national constitutions spanning 1789-2008. We extract information from a topic modeling of the complete corpus and construct cultural diffusion trees of topics (in the topic modeling sense) in order to characterize constitutions as cultural recombinants borrowing from ancestral constitutions back to the Last Universal Common Ancestor of Constitutions (LUCAC), the US Constitution of 1789. We discover constitutions cluster into distinct epochs within which legal concepts are frequently shared. We find constitutions vary systematically in their patterns of borrowing from ancestral texts -- from asexual copying through to polysexual borrowing. Most constitutions are very similar and have only a short term influence on descendant constitutions but a few are surprisingly innovative with very many offspring with a long lasting influence. These highly influential constitutions tend to be the oldest. We find that constitutions behave "biologically" in that their patterns of inheritance follow a characteristic negative-binomial distribution of "offspring" arising through a preferential-attachment process. These findings support a principled definition of memes in which the particulate inheritance of topics reproduces regularities in both constitutional statistics and dynamics.