  This paper addresses a series of complex and unresolved issues in the
historical phonology of West Iranian languages. The West Iranian languages
(Persian, Kurdish, Balochi, and other languages) display a high degree of
non-Lautgesetzlich behavior. Most of this irregularity is undoubtedly due to
language contact; we argue, however, that an oversimplified view of the
processes at work has prevailed in the literature on West Iranian dialectology,
with specialists assuming that deviations from an expected outcome in a given
non-Persian language are due to lexical borrowing from some chronological stage
of Persian. It is demonstrated that this qualitative approach yields at times
problematic conclusions stemming from the lack of explicit probabilistic
inferences regarding the distribution of the data: Persian may not be the sole
donor language; additionally, borrowing at the lexical level is not always the
mechanism that introduces irregularity. In many cases, the possibility that
West Iranian languages show different reflexes in different conditioning
environments remains under-explored. We employ a novel Bayesian approach
designed to overcome these problems and tease apart the different determinants
of irregularity in patterns of West Iranian sound change. Our methodology
allows us to provisionally resolve a number of outstanding questions in the
literature on West Iranian dialectology concerning the dialectal affiliation of
certain sound changes. We outline future directions for work of this sort.
