Talkers account for listener and channel characteristics to communicate efficiently

• We review the noisy channel theorem, distinguishing source and channel coding.
• Our framing clarifies efficiency accounts of speech reduction and their predictions.
• We examine predictability effects on word duration using spontaneous speech corpora.
• We find these effects are modulated by both listener and channel characteristics.
• Results support accounts that tie predictability effects to communicative efficiency.

A well-known effect in speech production is that more predictable linguistic constructions tend to be reduced. Recent work has interpreted this effect in an information-theoretic framework, proposing that such predictability effects reflect a tendency towards communicative efficiency. However, others have argued that these effects are, in the terminology of Gould and Lewontin (1979), spandrels: incidental by-products of other processes (such as a talker-oriented tendency for low production effort). This article develops the information-theoretic framing more fully, showing that information-theoretic efficiency involves different kinds of coding operations (predictability effects), not all of which are consistent with the spandrel account. Using mixed effects regressions, we analyze word durations in several spontaneous speech corpora, comparing predictability effects between infant-directed and adult-directed speech and between speech to visible and invisible listeners. We find that talkers adjust the extent to which production varies with predictability measures according to listener characteristics, and exploit an additional visual channel to eliminate phonetic redundancy. This pattern would demand multiple independent spandrel accounts, but is unified by an adaptive account. Our results broaden the scope of existing work on predictability effects and provide further evidence that these effects are tied to communicative efficiency.