Learning Allophones : What Input Is Necessary ?

The discovery of phonemes and allophonic relations, as surface alternations accumulate in children’s growing vocabulary, is a vital element of language acquisition. This paper quantitatively models allophone learning, when the child’s input triggers a merger of surface-distinct segments into a single underlying phoneme, illustrated here for the case of grouping US English alveolar flaps [R] together with voiceless alveolar stops [t] as allophones of an underlying /T/. The Tolerance Principle (Yang, 2016), grounded in considerations of processing efficiency, defines for the general case the tipping point at which any grammar becomes unsustainable for a child, given the linguistic items that they know a grammatical rule should apply to and how many of them evidently violate it. Integrating this cognitively motivated model of grammar evaluation with a principled model for the nature of the phonological system the child will be evaluating at some non-final acquisitional stage (Ringe & Eska, 2013), it is possible to trace a portion of the development of the phonological inventory, as it is continually reevaluated with growing vocabulary and potentially ultimately rejected in favour of another grammar. Therefore, the present model of allophone acquisition proposes firstly that the learner’s default initial grammar is roughly ‘what you hear is what you get’ in terms of the language’s surface contrasts, as specified by Invariant Transparency (an acquisitional hypothesis soundly implied by patterns of historical language change, §1.1; Ringe & Eska, 2013); and secondly, that this initial grammar is subject to the Tolerance Principle, such that if the child has too much violating input in which it is obvious that what is heard on the surface is not what is underlying then the initial grammar will become unsustainable. Input violates the initial grammar when e.g. a child hears eat [it] ∼ eating [iRIN], knows both words, and understands the morphology, so they are aware how the flap in eating and stop in eat relate (compared to [v] in drive∼ driving). Abandoning the default grammar is identical with learning an allophone; although the child of course may not immediately move to the adult grammar, if a