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The Effects of Individual and Social Learning on the Evolution of Cognitive and Communicative Aspects of Language Abilities

Recently, it is recognized that human linguistic abilities and human language are likely to have been shaped by their coevolutionary processes, in other words, the interplaying process of the biological evolution of the language faculty and the cultural evolution of language (Smith, 2018). In a general context of co-evolution of biological and cultural processes, including that of language (Christiansen and Kirby, 2003), effects of individual and social learning have been regarded as important factors. Higashi et al. (2018) constructed a computational model of evolution on a rugged fitness landscape which has many local optima, assuming the evolution of a complex and adaptive ability composed of mutually interacting traits. They showed that individual learning and social learning could work complementarily in the course of adaptive evolution on the rugged fitness landscape. That is, individual learning can find new adaptive phenotypes thanks to the diversity of genetic expressions created by social learning. Social learning also can keep a new adaptive phenotype found by individual learning in the population. We consider the human language faculty as one of such abilities at least in part because it is composed of various sub-faculty traits of language (e.g., brain structures, vocal organs). There are thought to be two sorts of primary abilities of human language (Harman, 1975). One is a cognitive ability or capacity of thoughts (e.g., building language hierarchy). The other is a communicative ability (e.g., intention sharing). However, they only assumed mutual interactions among traits within an individual, which could be regarded as the traits related to the cognitive ability of language. Thus, in order to deal with the evolution of communication ability, the existence of a communication partner and inter-individual interactions of their traits must be assumed. The purpose of this study is to clarify how cognitive and communicative abilities of language can evolve in terms of the co-evolution of linguistic abilities, based on social and individual learning. For this purpose, we extended the model of Higashi et al. (2018) by incorporating inter-individual interactions among traits into it.