Identifying Cases of Social Contagion Using Memetic Isolation: Comparison of the Dynamics of a Multisociety Simulation with an Ethnographic Data Set

A simulation is presented of a grid of connected societies of reproducing agents. These agents are capable of horizontal and vertical transmission of non-genetic cultural traits (memes). This simulation exhibits the theoretically predicted effect that horizontally transmitted memes are less likely, overall, to be encountered in geographical isolation than strictly vertically transmitted ones. Furthermore, when horizontal memes are under cultural selection, and thus behave ‘contagiously’, their likelihood of geographical isolation is virtually eliminated. By contrast, natural selection has far weaker effects than cultural selection in reducing geographical isolation. Thus it should be possible to identify contagious memes by an examination of their geographical distribution. The degree of geographical isolation of 17 categories of postulated cultural traits in an ethnographic data set of 863 societies is then examined, and compared with the simulations, using z-tests. Using this method, the empirical data can be sorted into four broad categories, each with a different spectrum of probabilities of mode of transmission and contagion.