The origins of inflectional classes
Keywords: inflection
inflection classes
morphomes
diachrony

Inflectional classes are classes of lexemes which share a content paradigm (they inflect for the same set of feature values) but differ in their form paradigm (the exponents for these feature values are at least partially different). Irregular lexemes will be viewed as classes with a very low number of members, which do not differ theoretically from classes with larger membership. Contrary to recent developments in morphological theory, inflectional classes are not viewed as classes of stems but classes of lexemes.
Inflectional classes have long been known to arise from sound change applied to inflectional paradigms, creating allomorphy. Other sources identified in the literature include grammaticalization and reanalysis, which have been identified in connection with Germanic languages.
This thesis investigates the origins of inflectional classes from a crosslinguistic perspective, including data on the Romance languages, Pama-Nyungan languages, Austronesian languages, various families of non-Austronesian languages from New Guinea, Niger-Congo languages, and a number of other families. It confirms that sound change, grammaticalization, and reanalysis are major sources for inflectional classes.
It shows that sound change is major source of inflectional classes, as well as the sound adaptation that occurs when affixes are created at affix boundary. For grammaticalization, drawing on data from Romance and Basque, it shows that the grammaticalization of auxiliaries can give rise to inflectional classes. Another major source is a cycle of two successive grammaticalizations of the same markers, which is shown to produce inflectional classes in Skou and Arapesh. Arapesh is peculiar in exemplifying a rare case of inflectional class through the linear ordering of affixes. Cases of sound change and grammaticalization produce systems of classes with very few classes. Sound change in particular partitions into two subclasses a preceding single class.
In contrast, when reanalysis gives rise to inflectional classes a larger number of classes are created at once, as is the case in Maori and Manam, as well as in Pama-Nyungan languages.
In addition to these three main sources, a number of new sources are identified and analysed, in particular the creation of inflectional classes through lexical strata in case of language contact, and morphological sources in the form of heteroclisis and deponency.
Finally, in two scenarios, it is shown that inflectional classes can arise from a previous system partitioning the lexicon into lexical classes. This is the case with systems of alienability distinctions in possessive paradigms, which often develop into systems of inflectional classes, although such systems have not been considered as inflectional classes previously. Alienability distinctions give rise to systems of inflectional classes presenting a large open class, opposed to a number of small, closed membership classes. Gender is another case where existing classes give rise to inflectional classes, if gender is marked directly on nouns together with another feature, most often number. This is shown to be the origins of systems of inflectional classes in Niger-Congo languages, as well as in Arapesh languages from New Guinea. In these cases, membership of the inflectional classes corresponds to the membership of each gender. Different sources for inflectional classes thus produce systems with different forms.