فشرده سازی و ارتباطات در تکامل فرهنگی ساختار زبانشناختی

Language exhibits striking systematic structure. Words are composed of combinations of reusable sounds, and those words in turn are combined to form complex sentences. These properties make language unique among natural communication systems and enable our species to convey an open-ended set of messages. We provide a cultural evolutionary account of the origins of this structure. We show, using simulations of rational learners and laboratory experiments, that structure arises from a trade-off between pressures for compressibility (imposed during learning) and expressivity (imposed during communication). We further demonstrate that the relative strength of these two pressures can be varied in different social contexts, leading to novel predictions about the emergence of structured behaviour in the wild.

Language is unique among the communication systems of the natural world in exhibiting rich combinatorial and compositional structure. Our species can productively construct novel signals on the fly by recombining reusable meaningless elements (speech sounds) to form meaning-bearing units (morphemes and words) which are further recursively combined. Furthermore, the meanings of these complex utterances are derivable in a predictable way from the composition of their subparts. The precise way in which this combinatorial and compositional structure is realised differs from language to language and is part of the knowledge that each language learner must acquire. Nevertheless, the existence of this kind of systematicity is both universal to all languages – it is one of the fundamental design features of human language ( Hockett, 1960) – and largely absent in the communication of other species. 1 Understanding the origins of this structure is a central goal of cognitive science. A recent productive approach treats it as a consequence of cultural evolution (Christiansen & Chater, 2008). Languages, in common with many other human behaviours, persist through a repeated cycle of learning and production: individuals learn a language by observing the linguistic behaviour of their speech community, and the linguistic behaviour they subsequently produce shapes learning in others. Languages potentially change and evolve as a result of their transmission, adapting to the biases inherent in the processes of language learning and language use.

Our experimental results confirm the predictions of our model exactly. A pressure for expressivity or compressibility alone does not lead to the emergence of structure: only when both pressures are at play does structure reliably emerge. Crucially, we have shown that cultural evolution is a mechanism that can deliver a structured linguistic system where these two pressures from communication and learning interact. In this paper, we have equated the expressivity pressure with communication and compressibility with learning. The bias we have used in our model is grounded in the general, domain-independent, principle that cognition favours simplicity (Chater & Vitanyi, 2003). An alternative approach might be to include the expressivity pressure as part of the bias of learners, e.g. as preferences for clarity (Slobin, 1977), transparency (Langacker, 1977), or isomorphism (Haiman, 1980). However, the experimental data we review in Section 1.2 shows that iterated learning in human participants leads to the emergence of degenerate languages (i.e., languages which are highly compressible but not highly expressive). This suggests that learning biases favouring expressive languages are weak relative to the biases in favour of compressibility, at least in the types of task which are amenable to iterated learning designs with human participants. Compressibility pressures could also apply during language use. For example, Piantadosi et al. (2012) note that frequent forms are easier to process; highly compressible languages have fewer, more frequent items than less compressible languages, and are therefore more useable in this sense. Again though, the experimental data we present in this paper (specifically, that from closed groups) suggests that the compression pressures which apply during use are relatively weak compared to the expressivity pressures enforcing distinctiveness. Finally, it is worth noting that compressibility and expressivity biases can push in the same direction in some