Categorical perception and the convergence of social learning

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About 50 years ago, Alvin Liberman and his colleagues at Haskins Laboratories discovered ‘‘categorical perception’’ of certain phonetic dimensions, where discrimination is hardly better than identification. They proposed this as one piece of converging evidence for a special perceptual mode for speech, part of a species-specific evolutionary adaptation for articulate language. This special sensory-motor module, they argued, makes human speech so uniquely efficient at transmitting discrete symbol sequences in sound. Lively arguments about all aspects of this reasoning continue to this day. This paper broadens the discussion beyond perception in individuals, by suggesting a role for categorical perception in enabling communities of speakers to form and maintain consensus about the pronunciation of tens of thousands of morphemes. Through computer simulations, simple and plausible assumptions will be shown under which the ‘‘pronouncing dictionaries’’ of the members of a speech community will converge rapidly from random starting points. Among these assumptions, a form of categorical perception plays a key role. With it, simulations converge to a consensus from which deviations occur rarely and hardly ever spread. Without it, individual pronunciation beliefs wander chaotically across time in the phonetic space, and at a given time, differences in belief increase rapidly with social distance. The role of partly categorical perception, as in so-called ‘‘magnet effects,’’ will also be addressed.