Learning Noun and Adjective Meanings:
A Connectionist Account
Computer Science and Linguistics Departments
Bloomington, IN 47405, USA
Linda B. Smith
Bloomington, IN 47405, USA
Why do children learn nouns such as cup faster than dimensional adjectives such as big? Most explanations of this well-known phenomenon rely on prior knowledge in the child of the nounadjective distinction or on the logical priority of nouns as the arguments of predicates. In this paper we examine an alternative account, one which seeks to explain the relative ease of nouns over adjectives in terms of the response of the learner to various properties of the semantic categories to be learned and of the word learning task itself. We isolate four such properties: the relative size and the relative compactness of the regions in representational space associated with the categories, the presence or absence of lexical dimensions in the linguistic context of a word (what color is it? vs. what is it?), and the number of words of a particular type to be learned. In a set of five experiments, we trained a simple connectionist categorization device to label input objects, in particular linguistic contexts, as nouns or adjectives. We show that, for the network, the first three of the above properties favor the more rapid learning of nouns, while the fourth favors the more rapid learning of adjectives. Our experiments demonstrate that the advantage for nouns over adjectives does not require prior knowledge of the distinction between nouns and adjectives and suggest that this distinction may instead emerge as the child learns to associate the different properties of noun and adjective categories with the different morphosyntactic contexts which elicit them.
1 The Phenomenon
Children learn to correctly label objects with dimensional adjectives such as red and big later than they learn to label objects with nouns such as block and dog (Blewitt, 1983; Carey, 1982; Ehri, 1976; Macnamara, 1982; Rescorla, 1980). From the point of view of category learning, this might be considered surprising for two reasons. First, children are faced with a much larger set of nouns than adjectives to learn. Second, the nouns would, at least from an adult perspective, seem to be organized in a much more complex fashion than the adjectives. Nouns are generally characterized by a such a wide range of features that many have argued that they almost defy definition (see, e.g., Murphy & Medin, 1985; Rosch, 1973; Wittgenstein, 1953). Dimensional adjectives, in contrast, refer to a restricted range of values on a single perceptual dimension.
Nonetheless, children learn the meanings of dimensional adjectives only slowly. The protracted course of learning dimensional adjectives has been well studied by students of child language. The evidence suggests that children have some ideas about dimensional word meanings before they understand the details of what it is that distinguishes the different words associated with a given dimension