Contact-tracing in cultural evolution: a Bayesian mixture model to detect geographic areas of language contact

Abstract

When speakers of two or more languages interact, they are likely to influence each other: contact leaves traces in the linguistic record, which in turn can reveal geographic areas of past human interaction and migration. However the complex, multi-dimensional nature of contact has hindered the development of a rigorous methodology for detecting its traces. Specifically, other factors may contribute to similarities between languages. Inheritance (a property is passed from an ancestor to several descendant languages), and universal preference (a property is universally preferred), may both overshadow contact signals. How can we find geographic contact areas in language data, while accounting for the confounding effects of inheritance and universal preference? We present sBayes, an algorithm for Bayesian clustering in the presence of confounding effects. The algorithm learns which similarities in a set of features are better accounted for by confounders, and which are due to contact effects. Contact areas are free to take any shape or size, but an explicit geographic prior ensures their spatial coherence. We test the clustering method on simulated data and apply it in two case studies to reveal language contact in South America and the Balkans. Our results are supported by ---mostly qualitative--- findings from previous studies. While we focus on the specific problem of language contact, the method can also be used to uncover other traces of shared history in cultural evolution, and more generally, to reveal latent spatial clusters in the presence of confounders.

Spatial evolution of human cultures inferred through Bayesian phylogenetic analysis

“…Horizontal transmission is one of the factors underlying the emergence of spatial or geographical patterns of many cultural traits, adding a layer of complexity to the question of how these patterns have formed; spatial distribution that we observe today can be explained by the past divergence of populations and spatial interaction between populations [4]. Regarding the former, when a small group of people splits from a large population and migrates to an uninhabited area to form a new population, the cultural traits of the new and parent populations become gradually different as they accumulate mutations independently (i.e.…”

“…Since human contact is more intense between geographically closer individuals, traits are likely to diffuse gradually from nearby populations to farther areas. The interplay of these two factors complicates the spatial evolution of cultural traits [4,7].…”

The social lives of isolates (and small language families): the case of the Northwest Amazon

“…To this end, we developed a database in which the languages of the sample are scored for 73 structural features. The features cover a broad range of grammar, ranging from phonology to syntax, and are easily connected to global distribution data available in [68] or [69], allowing for comparisons of regional NWA patterns to global patterns. For a more detailed description of the variables in the linguistic database, as well as for the language sample, see the electronic supplementary material.…”