The effect of cultural transmission on shared sign language persistence

In this paper, we revisit a mathematical model of sign language persistence by Aoki and Feldman (Theor Popul Biol 39(3):358–372, 1991), which investigates the evolution of genes causing deafness, affected by an assortative mating parameter, and the cultural transmission of sign language. To assess their model, we reimplement it as an agent-based simulation to be able to easily represent structured relationships in a finite population. We study the persistence of shared sign languages, a categorization of sign languages, which are typically shared by deaf and hearing members of a small community with a high incidence of hereditary deafness (Nyst, 2012. Shared sign languages. Sign language: An international handbook, pp. 552–574). We observe how shared sign language persistence is affected by hearing signers, marriage patterns, and various modes of sign language transmission: vertical, horizontal, oblique, and grandparental transmission. In contrast to Aoki and Feldman’s (Theor Popul Biol 9(3):358–372, 1991) finding that modes of transmission other than vertical are negligible, in the agent-based model we find that adding modes of transmission helps to ensure shared sign language persistence. A better understanding of sign language persistence has relevance for processes of cultural evolution, (sign language) linguistics, and language endangerment.