Chain shifts and transphonologizations are driven by homophony avoidance

Human languages balance pressures for effort reduction and communicative accuracy. This tension plays out in the structure of phoneme inventories and how they change over time. For example, merger of two phoneme categories (e.g., /p ∼ b/) results in a simpler inventory, but also eliminates a phoneme contrast which can distinguish words (e.g. pat ∼ bat). Previous work has shown that merger of a phoneme contrast is more likely when that contrast distinguishes few words. Within a diverse set of languages we extend this finding to two phoneme inventory changes which preserve word contrast. In chain shifts, two adjacent phonemes move in concert within the phonetic space. In transphonologizations, a primary phonetic cue to a phoneme contrast merges while another cue expands. A common feature of these two change types is that although the phonetics-phoneme category mapping changes, no new homophones are created. Here we show that the greater the contribution a phoneme contrast makes to word differentiation, the less likely that contrast is to merge, and conversely the more likely it is to undergo a chain shift/transphonologization. Traditional phonological theory assumes phonological systems are causally independent of actual words in the language. This work shows instead that change-type in inventories is strongly influenced by the particularities of the lexicon. These findings support a model in which the structure of phoneme inventories is shaped by usage-driven pressures to optimize effort reduction versus function in the transmission of meaning, paralleling broader cognitive work in efficient coding and in learning versus usage of category systems.