A recent surge of panel studies on language variation and change has shed light on the malleability of an individual’s grammar throughout their lifetime. These studies have provided valuable insights into how linguistic changes occur (e.g., Wagner & Buchstaller 2018). However, some of the findings have led to conflicting generalizations about the complexities of idiolects over time. It has been observed that while some individuals adapt their language use to reflect changes in the community (lifespan change), others may actually revert back to older usage patterns (retrograde change) (Sankoff & Blondeau 2007). The longitudinal stability of idiolects also varies depending on different levels of grammar: vocabulary items are more susceptible to change, while prosodic variables remain largely unaffected (Chambers 2009). In Japanese dialect standardization, on the other hand, lifespan changes are generally observed in lexical accents (Yokoyama & Sanada 2010), whereas retrograde changes are more common in segmental phonology (Yokoyama 2010). Additionally, the malleability of idiolects is influenced by lexical properties, the specific stages of change within a community, and the members’ loyalty to and pride in their local culture (Takano 2021).

Our study aims to uncover the factors that determine the malleability of idiolects over time. To achieve this, we utilize the extensive panelist information from NINJAL’s large-scale panel corpora, focusing on Japanese dialect standardization in Furano, Hokkaido. Data were collected in 1959 and 1986 but have remained largely unexplored until now. Rbrul analyses take into consideration both language internal (e.g., levels of grammar) and sociolinguistic factors (e.g., lexical familiarity, community-based stages of change), along with speaker demographics including age, gender, and education. We also consider the regionality index that measures the panelists’ integration into the local culture (Chambers & Heisler 1999). By examining these variables, our study ultimately contributes to the development of more accurate variationist sociolinguistic paradigms.