Rara and Theory Testing in Typology: The Natural Evolution of Non-canonical Agreement

Typological research seeks to make valid generalisations about the diversity of human language. It proceeds from observed linguistic traits, their distributions across geography and genealogy, and their relationships to other traits. Typological theories generate hypotheses, whose testing can prompt us to make more precise and systematic observations. Theories also propose explanations for observations. Corbett (2012), for example, generalises over extensive typological evidence to propose a theory of features, according to which, inflectional features are limited to only a handful of kinds. As in all sciences, theories in typology have apparent counter-examples. Rare linguistic traits, or rara (Plank 2000; Cysouw and