Generalisation of regular and irregular morphological patterns

Abstract Both regular inflectional patterns (walk-walked) and irregular ones (swing-swung) can be applied productively to novel words (e.g. wug-wugged; spling -splung). Theories of generative phonology attribute both generalisations to rules; connectionist theories attribute both to analogies in a pattern associator network; hybrid theories attribute regular (fully predictable default) generalisations to a rule and irregular generalisations to a rote memory with pattern-associator properties. In three experiments and three simulations, we observe the process of generalising morphological patterns in humans and two-layer connectionist networks. Replicating Bybee and Moder (1983), we find that people's willingness to generalise from existing irregular verbs to novel ones depends on the global similarity between them (e.g. spling is readily inflectable as splung, but nist is not inflectable as nust). In contrast, generalisability of the regular suffix does not appear to depend on similarity to existing regular verbs Regularly suffixed versions of both common-sounding plip and odd-sounding ploamph were reliably produced and highly rated, and the odd-sounding verbs were not rated as having worse past-tense forms, relative to the naturalness of their stems, than common-sounding ones. In contrast, Rumelhart and McClelland's connectionist past-tense model was found to vary strongly in its tendency to supply both irregular and regular inflections to these novel items as a function of their similarity to forms it was trained on, and for the dissimilar forms, successful regular inflection rarely occurred. We suggest that rule-only theories have trouble explaining patterns of irregular generalisations, whereas single-network theories have trouble explaining regular ones; the computational demands of the two kinds of verbs are different, so a modular system is optimal for handling both simultaneously. Evidence from linguistics and psycholinguistics independently calls for such a hybrid, where irregular pain are stored in a memory system that superimposes phonological forms, fostering generalisation by analogy, and regulars are generated by a default suffix concatenation process capable of operating on any verb, regardless of its sound.