Islands of Reliability for Regular Morphology: Evidence from Italian

The representation of regular morphological processes has been the subject of much controversy, particularly in the debate between single and dual route models of morphology. I present a model of morphological learning that posits rules and seeks to infer their productivity by comparing their reliability in different phonological environments. The result of this procedure is a grammar in which general rules exist alongside more specific, but more reliable, generalizations describing subregularities for the same process. I present results from a nonce-probe (wug) experiment in Italian, in which speakers rated the acceptability of novel infinitives in various conjugation classes. These results indicate that such subregularities are in fact internalized by speakers, even for a regular morphological process.^*

1. Overview of the issues

In the course of the debate between connectionist models (Rumelhart & McClelland 1986, MacWhinney & Leinbach 1991) and the dual-mechanism model of morphology (Pinker & Prince 1988, 1994), a substantial body of research has developed describing qualitative differences between regular and irregular inflectional processes.¹ This research program has grown to include an impressive number of domains, including language acquisition, linguistic productivity, lexical recognition tasks, brain imaging, and language pathology (see Clahsen 1999, Pinker 1999, and Ullman 2001 for overviews). Proponents of the dual-mechanism model argue that differences in how regular and irregular forms are produced and processed show that the two must be represented in fundamentally different ways; regular forms are generated by a relatively simple grammar of widely applicable rules, while irregular forms are stored in some type of associative network outside the grammar. Proponents of single-route models such as connectionist models, in contrast, argue that whatever differences there may be between regulars and irregulars emerge as a result of the way that speakers store and generalize over all of the words of their language—regular and irregular—using a single system.

To date, no model—single or dual route—has been implemented that can adequately capture the full range of data. But the large body of experimental evidence about regular/irregular differences poses interesting challenges for all models of morphological productivity. Those who believe in a single mechanism must take on the question of how a unified model can be used to derive seemingly dualistic behavior. Those who believe in a separate rule component, on the other hand, must address the question of how language learners use input data to posit rules, and how rules are evaluated for possible inclusion in the adult grammar.

In this article, I explore one approach to morphological rule learning, suggested by Pinker and Prince (1988:130–34) and further developed by Albright and Hayes (2002). This approach compares the morphological behavior of different words in order to hypothesize rules and evaluate their effectiveness in explaining existing words. The goal of learning under this model is to discover the best morphological rules and to provide a quantitative measure of their productivity. For reasons that are explained below, however, the very best rules in this system are not necessarily those that one would choose in a traditional linguistic analysis. In particular, in addition to the most general rules (such as ‘suffix -d to form the past tense’), the model also finds rules to describe more specific, but more reliable, subregularities for the very same change (such as ‘suffix -d after fricatives’). I use here the term islands of reliability to refer to such subgeneralizations about phonological environments in which a morphological process is especially robust. This article presents experimental evidence that such subgeneralizations are in fact internalized by speakers and may coexist in grammars alongside the more general rules.

The issue of subgeneralizations for regular and irregular processes has implications for various models of morphology. The dual-mechanism model and more traditional approaches to morphology agree that local subgeneralizations about regular processes are absent from the adult grammar. In other words, although irregular processes may be sensitive to subgeneralizations or neighborhood similarity effects, regular processes should be equally applicable to all new inputs, regardless of their similarity to existing words (Marcus et al. 1995:196). Prasada and Pinker (1993) present results from a nonce-probe (wug...