Is phoneme inventory size influenced by speaker population size and isolation?

Phoneme inventory size is related to speaker population size, but the cause of this relationship has remained a mystery. The correlation was first reported by Hay and Bauer (2007), who considered several possible causes of this relationship, including ascertainment bias, similarity between relatives, loss of phonemes in small, isolated populations, and maintenance of larger phoneme inventories in larger speaker populations. Here, we test these proposed causes of the relationship and show that the correlation between phoneme inventory size and speaker population size is largely driven by similarity between relatives and neighbours in both speaker population size and phoneme inventory size. However, we also show that phoneme inventory size is related to aspects of linguistic isolation—languages confined to islands or with few bordering languages tend to have lower phoneme inventory sizes than their relatives. We also explore the geographic bias of phoneme inventory databases and show that in some cases there is a positive bias towards regions with larger phoneme inventories. However, we find no evidence that less-studied languages tend to have lower reported phoneme inventory sizes. Our analyses demonstrate the importance of taking sources of non-independence and bias in typological data into account when conducting global analyses of linguistic diversity.