Grammaticalization studies have revealed many similar cross-linguistic patterns in the semantic development of grammatical categories.  In addition, the field has also been successful in discovering some of the psycholinguistic mechanisms responsible for these linguistic similarities. The present study attempts to further the discussion with respect to both points.  First, while linguists have identified a category of root possibility (e.g. I actually couldn't finish reading it because the chap whose shoulder I was reading the book over got out at Leicester Square [Coates 1983])), few researchers have mentioned an analogous modal category for obligation grams.  This paper presents evidence for a grammatical category termed root necessity as found in English (cf. Gamon 1993 for a similar proposal for German).  Therefore, just as root possibility is commonly defined as the modal category which develops out of ability grams and reports on general external enabling conditions (Bybee et al. 1994), so too, do obligation morphemes express the notion of root necessity, whereby no clearly identifiable obligator exists, but rather, general external circumstances logically necessitate some action or event.  Secondly, the existence of a semantic category such as root necessity suggests that the development of obligation morphemes proceeds along a gradual path, one very similar to the path proposed for ability grams in Bybee et al. (1994):  ability > root possibility > epistemicity. Specifically, it is predicted that bleaching and inference allow for the development of a root necessity category and that metaphor is not involved in the development of obligation grams as they progress towards epistemic markers, rendering the path: obligation > root necessity > epistemicity. The conversational data studied here reveal that the proposed development best accounts for the emerging American English modals have to and got to and their semantic distributions.

References

Bybee, Joan, Revere Perkins, William Pagliuca. 1994. The evolution of grammar. Chicago:   University of Chicago Press.
Coates, Jennifer. 1983. The semantics of modal auxiliaries. London: Croom Helm.
Gamon, David. 1993. On the development of epistemicity in the German modal verbs mögen  and müssen. Folia Linguistica Historica 14.125-176.

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