Sign languages can be categorized as shared sign languages or deaf community sign languages, depending on the context in which they emerge. It has been suggested that shared sign languages exhibit more variation in the expression of everyday concepts than deaf community sign languages (Meir, Israel, Sandler, Padden, & Aronoff, 2012). For deaf community sign languages, it has been shown that various sociolinguistic factors condition this variation. This study presents one of the first in-depth investigations of how sociolinguistic factors (deaf status, age, clan, gender and having a deaf family member) affect lexical variation in a shared sign language, using a picture description task in Kata Kolok. To study lexical variation in Kata Kolok, two methodologies are devised: the identification of signs by underlying iconic motivation and mapping, and a way to compare individual repertoires of signs by calculating the lexical distances between participants. Alongside presenting novel methodologies to study this type of sign language, we present preliminary evidence of sociolinguistic factors that may influence variation in the Kata Kolok lexicon.

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.

As evidence from sign languages is increasingly used to investigate the process of language emergence and evolution, it is important to understand the conditions that allow for sign languages to persist. We build on a mathematical model of sign language persistence (i.e. protection from loss) which takes into account the genetic transmission of deafness, the cultural transmission of sign language and marital patterns (Aoki & Feldman, 1991). We use agent-based modeling techniques and draw inspiration from the wealth of genetic and cultural data on the sign language Kata Kolok to move towards a less abstract model of sign language persistence. In a set of experiments we explore how sign language persistence is affected by language transmission types, the distribution of deaf alleles, population size and marital patterns. We highlight the value of using agent-based modeling for this type of research, which allows for the incorporation of real-world data into model development.

It has been suggested that social structure affects the degree of lexical variation in sign language emergence. Evidence from signing communities supports this, with smaller, more insular communities typically displaying a higher degree of lexical variation compared to larger, more dispersed and diverse communities. Though several factors have been proposed to affect the degree of variation, here we focus on how shared context, facilitating the use of iconic signs, facilitates the retention of lexical variation in language emergence. As interlocutors with the same background have similar salient features for real world concepts, shared context allows for the successful communication of iconic mappings between form and culturally salient features (i.e., the meaning specific to an individual based on their cultural context). Because in this case the culturally salient features can be retrieved from the form, there is less pressure to converge on a single form for a concept. We operationalize the relationship between lexical variation and iconic affordances using an agent-based model, studying how shared context and also population size affects the degree of lexical variation in a population of agents. Our model provides support for the relationship between shared context, population size and lexical variation, though several extensions would help improve the explanatory power of this model.

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