Language learning, communication and the emergence of phonotactic constraints

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All spoken languages make words by combining speech sounds. For example, the English words "tap", "pat", "apt", "at", and "tat" all use the same 3 sounds in different combinations. This combinatoriality contributes to the extraordinary expressive power of human language, allowing us to create thousands of distinct words out of a few tens of speech sounds. But not all combinations of sounds in the language are equally permitted. For instance, no words in English can begin or end with the sequence "tp". Such constraints on permissible sound sequences are known as phonotactics. While phonotactic constraints can be idiosyncratic to each language, some occur across multiple languages. One constraint that is thought to be universal is called Similar Place Avoidance: Consonants separated by vowels are unlikely to have the same or similar place of articulation (i.e., part of the mouth where the consonant is produced). For example, there are fewer than expected words in English like "bop" and "mob" that contain two consonants produced by the lips. Why do languages have phonotactics at all, rather than just allowing maximally free recombination? And why do certain constraints such as Similar Place Avoidance crop up repeatedly? One prominent claim in the literature is that the explanation lies in learning bias, or the tendency for us to learn certain rules for sound combinations more readily than others. Such biases are thought to be part of every learner's underlying knowledge of phonology, enforcing a preference for patterns that are simpler or easier to produce/perceive. This explanation has been tested in many studies that examined how easily new phonotactic constraints can be learned. However, the results have not always been consistent with the prediction that patterns that are commonly found across languages should be easier to learn than those that are uncommon. It appears, then, that learning bias is not the only reason why some phonotactics recur in the world's languages.In this project we explore a new way to explain the ubiquity of some phonotactic constraints. Our approach is different from previous accounts in two important aspects. Firstly, we consider the impact of communication as a critical factor. As well as being learned, languages are also used to communicate, and sound sequences that work well in communication should proliferate. Successful communication requires us to minimise confusability by maximising contrasts between words. There is some evidence that the repetition of similar consonants within a word can lead to misidentification of the word, suggesting that this effect may be behind constraints like Similar Place Avoidance.Secondly, we consider phonotactic constraints to be generalisations about the shapes of words in a language, the composition of which can change in a particular direction as words are learned and used to communicate between speakers. We predict that the direction of such change reflects the interaction between learning (i.e., which types of words are easier to learn) and communication (i.e., which types of words are more accurately recognised).We will examine this theory through 3 sets of experiments, testing whether words violating Similar Place Avoidance are better (or worse) learned by adults and infants, whether they are more (or less) likely to be misidentified, and whether they decrease in number in a vocabulary when they are learned and used in communication across different groups of people.Our project will provide important insights into how language learning and communication account for the presence of phonotactic constraints in general, and Similar Place Avoidance in particular. Findings from the project will also contribute to our understanding of a larger question central to language and cognitive sciences: Where do fundamental characteristics of human language come from?