Investigating the universality of consonant and vowel co-occurrence restrictions

Certain phonotactic constraints on the co-occurrence of segments appear to be much more common across the world’s languages than others. In many languages, similar consonant co-occurrence is restricted through Obligatory Contour Principle (OCP) effects, while there are some exceptions for identical consonants. In vowels, the opposite pattern appears to hold: many languages have vowel harmony processes, where vowels within a domain are required to share some feature. Languages that encourage similar consonant co-occurrence or restrict similar vowel co-occurrence appear to be exceedingly uncommon. However, evidence of this pattern so far only comes from studies of individual languages or families, or of only consonants or vowels. We investigate patterns of co-occurrence in vowels and consonants in 107 Northern Eurasian languages across 21 families using Bayesian negative binomial regression to explicitly model the effects of aggregate similarity and segment identity on co-occurrence counts (the results of which can be interpreted similarly to observed/expected ratios). We find that the effect of similarity is remarkably consistent across languages: Similar consonant co-occurrence is disfavored, while aggregate similarity has no effect on vowel co-occurrence. Identical segment co-occurrence effects are much more variable across languages, with a tendency towards disfavoring identical consonants, and favoring identical vowels. We also find larger effects in consonants than in vowels, suggesting that consonant co-occurrence is more strongly constrained than vowel co-occurrence. We also find that there is no evidence for or against any correlations between vowel and consonant co-occurrence, suggesting that more data is needed to evaluate this possibility.