Shift-Induced Phonological Restructuring: Complexity and Change in Shira Yugur and Butha Dagur

Abstract

Understanding the learning biases that shape language acquisition—whether universal or language-specific—remains a central goal in theoretical linguistics. A major challenge in this endeavour lies in the notions of complexity and markedness, which are highly theory-dependent and variably interpreted across empirical studies. This thesis proposes that shift-induced simplification—systematic grammatical restructuring observed in speakers of obsolescing languages under conditions of reduced intergenerational transmission—provides a naturalistic context for investigating learning biases and the robustness of phonological rules. Such simplification can be conceptualised as partial L1 acquisition that persists into adulthood (Montrul, 2008, 2023). Unlike Artificial Grammar Learning experiments with adults, which may reflect transfer from participants’ existing linguistic knowledge (Chabot, 2021; Moreton & Pater, 2012b), shift-induced simplification reflects outcomes of partial L1 acquisition under constrained input, offering a unique empirical window into which aspects of grammar are fragile, how structural and substantive biases shape phonological learning, and how morphologically complex rules are acquired. This study focuses on two interface phenomena in phonology. The first concerns the phonology–phonetics interface, examining the extent to which phonological processes are sensitive to phonetic substance. The second concerns the morphosyntax–phonology interface, investigating the emergence and acquisition of cyclic computation. Empirical data were collected from two endangered Mongolic languages: Shira Yugur and the Butha variety of Dagur, both undergoing significant language shift towards Mandarin. Shira Yugur was selected for its perceptually motivated yet structurally complex vowel harmony system. Analyses show that, under limited exposure, this process is simplified in the younger generation. Crucially, the outcome of simplification cannot be accounted for by substantive-bias accounts or similarity-based generalisations (Cristià et al., 2013; Finley, 2022; Kimper, 2011; McCollum, 2018). Instead, it aligns with predictions of rule-based generalisation constrained by structural biases (Moreton et al., 2017; Moreton & Pertsova, 2012), providing evidence for the role of articulatory representations in shaping learnable phonological patterns (Esling, 2005) and challenging theories that downplay the influence of phonetic substance in phonology. Butha Dagur was investigated because its phonological rules may apply cyclically. Analyses show that over- and underapplication of vowel harmony, V1-deletion, assimilation, and epenthesis naturally arise from the Phase Impenetrability Condition (PIC). Deviations from PIC occur through mechanisms such as pruning of null heads, the formation of prosodic constituents, and post-cyclic rule application. Comparisons with Shira Yugur reveal that similar patterns can emerge independently, demonstrating the tight coupling of phonological computation with morphosyntactic structure. Importantly, empirical evidence from younger Butha Dagur speakers indicates that cyclicity is an emergent property, subject to variation and reanalysis. While some speakers demonstrate fully cyclic rule application, others—particularly younger speakers—reinterpret cyclic rules as post-cyclic. These findings challenge assumptions that child-led innovations guarantee the unidirectionality of the life cycle of phonological rules. Instead, the data suggest that the emergence and loss of cyclic computation are shaped by both developmental constraints and sociolinguistic factors, in line with evidence from longitudinal studies. Overall, this thesis demonstrates that shift-induced simplification is neither random nor idiosyncratic. It reflects predictable patterns of partial L1 acquisition influenced by structural biases. Simplification in the adult grammars of obsolescing languages captures intermediate stages of acquisition during childhood that persist into adulthood, offering new insight into the cognitive mechanisms underpinning phonological learning and the interface between phonology, phonetics, and morphosyntax.