Why is grammaticalization irreversible?

Grammaticalization, the change by which lexical categories become functiona l categories, is overwhelmingly irreversible. Prototypical functional categ ories never become prototypical lexical categories, and less radical change s against the general directionality of grammaticalization are extremely ra re. Although the pervasiveness of grammaticalization has long been known, t he question of why this change is irreversible has not been asked until fai rly recently. However; no satisfactory explanation has been proposed so far . Irreversibility cannot be attributed to the lack of predictability, to th e interplay of the motivating factors of economy and clarity, or to a prefe rence for simple structures in language acquisition. I propose an explanation that follows the general structure of Keller'S (19 94) invisible-hand theory: language change is shown to result from the cumu lation of countless individual actions of speakers, which are nor intended to change language, but whose side effect is change in a particular directi on. Grammaticalization is a side effect of the maxim of extravagance, that is, speakers' use of unusually explicit formulations in order to attract at tention. As these are adopted more widely in the speech community, they bec ome more frequent and are I educed phonologically. I propose that degrammat icalization is by and large impossible because there is no counteracting ma xim of "anti-extravagance," and because speakers have no conscious access t o grammaticalized expressions and thus cannot use them in place of less gra mmaticalized ones. This is thus a usage-based explanation, in which the not ion of imperfect language acquisition as the locus of change plays no role.