Morpheme recognition and the learning of rules for derivational morphology

Derivational morphology is one of the most difficult and least studied of all the areas of linguistic description (cf. Lightner, 1968:71). There are two main problems which are largely responsible for this. The first is the question of morpheme recognition or lexical identity: how similar in meaning or in sound do two words have to be in order for the linguist or language learner to identify a common morphemic unit and thus to see a morphological relationship between the words? (This problem is discussed in detail in Derwing, 1973: 122-6.) Many of the morphological rules which are proposed by linguists, whether morphophonemic or phonotactic in presumed character, are posited primarily, if not solely, in order to capture certain kinds of supposed ‘lexical redundancies,’ i.e., systematic variations which appear in the phonological form of the same morpheme when the morpheme occurs in different syntactic constructions. The viability of all such rules is thus directly contingent upon the assumption that the words involved do, in fact, share a common morpheme. Consider, for example, the morphophonemic rule which Chomsky proposes for English which changes a /d/ to an /s/ before the suffix /lv/, and the phonotactic rule which changes a /d/ plus /i/ or /y/ into a /ž/ before a vowel (1964:90); both of these rules are motivated by the presumed fact that the English words decisive and decision, for example, contain in their ‘underlying’ or ‘lexical’ representations the common morpheme decide. But how does one decide whether this claim is justified for ordinary native speakers of the language, particularly in some of the more problematical cases discussed in Derwing (1973)?