Form and function covariation: Obligation modals in Australian English

Shifts in the frequencies of English modals of obligation have been linked to shifts in modal function and changing interpersonal authority. Interpretation of over 2,000 tokens in spontaneous speech data recorded in Sydney, Australia, in the 1970s and 2010s establishes a replicable classification of obligation meanings, based on source of obligation, with a three-way distinction between Hierarchical authority, General circumstances, and Personal choice. Competing expressions for these obligation types, besides have to, have got to, and older must, include should and, recently, need to. Two sets of regression analyses provide evidence of covariation of form and function: first, the linguistic and social conditioning of forms, with meaning as one of the predictors; and second, the conditioning of function, with modal form as a predictor. Need to rises in real time and so does talk of personal obligation. However, the change in modal function is concomitant with, but independent of, shifting modal forms.