Explaining Typologically Unusual Structures: The Role of Probability

Among languages of the world, some morphological structures are common and expected, others are rare and unexpected. Still others may not be so infrequent as to be called rare, but they are at least uncommon. The rare or uncommon features include structures such as (i) multiple suppletion, (ii–iii) circumfixes and infixes (both found in Austronesian, for example), (iv) the coexistence of two or even three different patterns of case marking (as found, for example, in Georgian), (v) endoclitics (as in Udi), (vi) agreement morphemes repeated within a word (found in some Nakh-Daghestanian and Kiranti languages, for example), (vii) a case that marks non-focused nouns, but only in a subset of one gender (found in Dirayta), and (viii) tense marking on pronouns (found, for example, in Gurnu and certain other languages of Australia). These rare and unusual structures pose a challenge for linguists. On the one hand, linguists must explain why they are rare or unusual; on the other hand, they must explain, given the rarity, why they exist at all. Some explanations that might be given are mentioned below.