Linguistic structure is an evolutionary trade-off between simplicity and expressivity

Linguistic structure is an evolutionary trade-off between simplicity and expressivity Kenny Smith (kenny@ling.ed.ac.uk), Monica Tamariz & Simon Kirby Language Evolution and Computation Research Unit, School of Philosophy, Psychology & Language Sciences, University of Edinburgh, Dugald Stewart Building, 3 Charles Street, Edinburgh, EH8 9AD, UK Abstract Language exhibits structure: a species-unique system for ex- pressing complex meanings using complex forms. We present a review of modelling and experimental literature on the evo- lution of structure which suggests that structure is a cultural adaptation in response to pressure for expressivity (arising dur- ing communication) and compressibility (arising during learn- ing), and test this hypothesis using a new Bayesian iterated learning model. We conclude that linguistic structure can and should be explained as a consequence of cultural evolution in response to these two pressures. Keywords: language; structure; cultural evolution; learning; communication Introduction Human language is unique among the communication sys- tems of the natural world in that it is exhibits a rich combina- torial and compositional structure: language provides a gen- erative system for productively combining meaningless ele- ments (e.g. speech sounds) to form meaning-bearing units (morphemes), which are further recombined to yield com- plex units (phrases) whose meaning is derived in a predictable manner from the meaning of their component parts and their manner of composition. This allows us massive expressive potential: at least at a first approximation, anything you can think you can express in language. No other species has a communication system providing anything approaching this expressive power: why do humans? One explanation for the presence of structure 1 in human language appeals to biological evolution under natural selec- tion (Pinker & Bloom, 1990): language is fundamentally a biological trait, being underpinned by some innate language- specific apparatus; the ability to communicate propositions which a structured language provides is adaptive, since it facilitates social interaction and ultimately increases fitness; therefore, structure in language represents a biological adap- tation to facilitate communication. A second account explains structure in language as a consequence of cultural, rather than biological, evolution (Christiansen & Chater, 2008). Rather than language structure reflecting an evolved domain-specific learning apparatus, the idea is that languages have adapted over repeated episodes of learning and production (a process sometimes called iterated learning) in response to weaker, domain-general constraints arising from the biases of lan- guage learners. We have previously termed this evolutionary process cultural selection for learnability (Brighton, Kirby, & Smith, 2005). A range of models and experiments show 1 We use the term structure as a shorthand for combinatoriality and/or compositionality. that cultural selection for learnability leads to the evolution of structure, under certain assumptions about the nature of transmission and the biases of language learners. Under a strong interpretation of this account, language’s function for communication could be seen as an epiphenomenon: struc- tured language provides a powerful medium for communica- tion, but language structure is not ‘for’ communication. Here we present a new model of iterated learning, moti- vated by recent experimental work, which goes some way to reconcile these two viewpoints. We draw from the biologi- cal account the insight that the alignment between language’s apparent function as a system for expressing propositions and its structure, tailor-made for just such a purpose, is unlikely to be a fortuitous coincidence. We draw from cultural evo- lutionary account two insights: 1) biological evolution is not the only evolutionary process which might act to shape lan- guage: cultural evolution, a necessary consequence of the fact that language is socially learned, is a second such mechanism; 2) selection for learnability will impact on language during its transmission. This model suggests that structure arises from cultural evolution when language is under pressure to be ex- pressive and learnable: pressure for expressivity arises from language use in communication, language learning by naive individuals introduces a pressure for simplicity arising from domain-general preferences for compressibility in learning. Crucially, both must be in play: pressure for expressivity or simplicity alone does not lead to structure. Structure in lan- guage is a linguistic adaptation, not a biological adaptation, in response to competing pressures for expressivity and learn- ability (Kirby, Cornish, & Smith, 2008; Steels, 2012). Cultural evolution of structure: previous work Models of the cultural evolution of linguistic structure typ- ically emphasise the role of learnability constraints in driv- ing the evolution of compositionality. Specifically, language is under pressure to be compressible: to allow the forma- tion of compressed mental representations, i.e. simple gram- mars. This pressure for compressibility is inherent in learn- ing (Chater & Vitanyi, 2003), and can be amplified by other constraints acting on language transmission (e.g. the mis- match between the infinite expressivity of languages and the finite set of data from which such languages must be learned). Learning and transmission therefore favour languages which admit to compressed representations, i.e. which permit gen- eralisations. Recursive compositionality is one such general- isation (e.g. Steels, 1998; Kirby, 2002; Brighton, Smith, & Kirby, 2005), and therefore represents an adaptation by lan-