Parallel linguistic and sociocultural change: Modals and mores

Grammatical change triggered by sociocultural change has been invoked for English modals of obligation. In particular, the decline of must and increase of need to has been adduced as evidence of a shift from authoritative to interpersonal obligation. But how can changing functions be tested independently of their grammatical expressions? Here, we do this by assessing the relative frequency and conditioning not only of forms, but also of functions. Drawing on sociolinguistic corpora of Australian English, we establish a replicable three-way meaning classification based on source of
obligation. Within a well-defined variable context, must is most associated with Hierarchical authority, both have to and (have) got to with General circumstances, and should and need to with Personal choice. Need to arose rapidly between the 1970s and 2010s, likely as a communal change, favored in 1st and 2nd person contexts and by women and Middle Class men. Expression of Personal obligation also increased, not only with need to but across modal forms. Change in the nature of obligation is thus concomitant with, but independent of, changing modal forms. Parallel formal and functional change can be substantiated by applying the heuristic of the linguistic variable to the inherently variable data of everyday language use.