Variable Phonological Rules and 'Quantal' Perception as a Source of Probabilistic Sound Change: The Case of Intervocalic Voicing in Old Tuscan.

The origin and nature of 'irregular', 'sporadic' sound changes have been debated by different theories of phonological change since at least the Neogrammarians. They are often attributed to non-phonological factors, as analogy or borrowing, or to the non-(purely)-phonological mechanism of lexical diffusion. The aim of this paper is to show that an irregular sound change in the historical phonology of Tuscan - namely the voicing of some intervocalic voiceless stops - is not due to borrowing (as often argued), but to a variable and allophonic voicing rule, whose output was only partially subject to phonological recategorization. The most likely causes for this irregular outcome are the variable strength intervocalic voicing had in different environments, the interaction with another lenition process and the perceptual ambiguity of the lenited stops.