Downloaded By: [University of Pittsburgh] At: 15:05 15 April 2008 Context Variation and Definitions in Learning the Meanings of Words: An Instance-Based Learning Approach Donald J. Bolger, Michal Balass, Eve Landen, and Charles A. Perfetti University of Pittsburgh This article proposes an instance-based theoretical framework to account for the in- fluence of both contexts and definitions on learning new word meanings and reports 2 studies that examine hypotheses about learning from context. One is that variation in contexts is important for allowing core meaning features of a word to emerge. The second is that definitions are effective because they can interact with contexts to com- municate core meanings. Both experiments tested the effects of context variation by presenting adult learners with context sentences that either varied or repeated with each training trial. Experiment 1 varied whether definitions were also provided, whereas Experiment 2 varied context variability without definitions and examined the role of reading comprehension skill and pre-training word familiarity. Results across several different measures were that exposure to variable contexts led to better learning of abstract meanings than did equivalent exposure to a single context. In ad- dition, definitions were more effective at conveying this knowledge than context alone. The instance-based framework accounts for the dual effects of contexts and definitions, suggesting how word learning results from abstraction across varied word encounters, both definitions and context sentences. Discourse context is considered to be a primary source for learning the meanings of unknown words (Jenkins, Stein, & Wysocki, 1984; Nagy & Anderson, 1984; Nagy & Herman, 1987). Accounts of word learning suggest that the acquisition of meaning is incremental with each incidental experience with a word in context (Fukkink, Blok, & de Glopper, 2001; Nagy, Anderson, & Herman, 1987; Nagy, Discourse Processes, 45:122–159, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 0163–853X print/1532–6950 online DOI: 10.1080/01638530701792826 Correspondence concerning this article should be addressed to Donald J. Bolger, Department of Psychology and the Learning Research and Development Center, University of Pittsburgh, 3939 O’Hara Street, Pittsburgh, PA 15260. E-mail: d-bolger@northwestern.edu