Every now and then the cultural paradigm of a society changes. Human history can be regarded as a sequence of long periods of cultural stasis punctuated by paradigm shifts that transform culture upside-down over the turn of a few generations. We propose here a population dynamics model devised to analyse paradigm shifts. In this model individuals are defined by a vector of cultural traits that can change mainly through imitation of other individuals' traits. The novelty of the model is that cultural traits may interact reinforcing or hindering each other. Imitation is then biased by the 'cultural fitness' landscape thus defined. Our main result is that abrupt paradigm shifts occur, as a response to weak changes in the landscape, only when cultural traits do interact -whereas adaptation is smooth if there is no interaction. Borrowing the genetic term, this interaction is called 'cultural epistasis'. The result is robust to the way that epistasis is implemented, to whether imitation is biased by homophily, or to changes in other model parameters. Finally, a relevant consequence of this dynamics is the irreversible nature of paradigm shifts: the old paradigm cannot be restored even if the external changes are undone. Our model puts the phenomenon of paradigm shifts in cultural evolution in the same category as catastrophic shifts in ecology or phase transitions in physics.
Epistasis between cultural traits drives paradigm shifts in cultural evolution -2/12It has been suggested that traits act as facilitators or inhibitors of other traits in modelling the appearance and accumulation of innovations [4,8]. The idea that traits affect each other holds in a wider context. Language evolution is driven by interaction of its specific traits. For instance, although n and m are phonetically distinct, the presence of a subsequent p inhibits the n in favour of the m [9]. Semantics is strongly affected by a network of close concepts, to the point that the meaning of a word can shift as a consequence of a change in this network [10]. Also, the acquisition of additional languages is facilitated by prior knowledge of two or more languages, and brings about effects in other aspects of the individuals' personal lives [11]. Other examples are the correlation between right-wing authoritarianism belief and low openness to experience [12], religious beliefs and health practices [13], or animal ethical profiles and diet choices [14].Although biological and cultural evolution do not share the same microscopic mechanisms [3], they are deeply related [15] and, often, the former have inspired the latter. Most models are suitable adaptations of those of population genetics, incorporating variants of the standard mechanisms of replication, mutation, and drift [16,17], but also-building on [4]-of branching and recombination [18]. The concept of epistasis in genetics (i.e., the mutual dependence between two genes or two positions in a sequence) also has its counterpart: cultural epistasis has been used to refer to the association between two idea...