Copyright 1992 by the American Psychological Association, Inc. 0033-295X/92/J3.00
A Capacity Theory of Comprehension: Individual Differences in Working Memory Marcel Adam Just and Patricia A. Carpenter Carnegie Mellon University A theory of the way working memory capacity constrains comprehension is proposed. The theory proposes that both processing and storage are mediated by activation and that the total amount of activation available in working memory varies among individuals. Individual differences in working memory capacity for language can account for qualitative and quantitative differences among college-age adults in several aspects of language comprehension. One aspect is syntactic modularity: The larger capacity of some individuals permits interaction among syntactic and pragmatic information, so that their syntactic processes are not informationally encapsulated. Another aspect is syntactic ambiguity: The larger capacity of some individuals permits them to maintain multiple interpretations. The theory is instantiated as a production system model in which the amount of activation available to the model affects how it adapts to the transient computational and storage demands that occur in comprehension.
Working memory plays a central role in all forms of complex thinking, such as reasoning, problem solving, and language comprehension. However, its function in language comprehension is especially evident because comprehension entails processing a sequence of symbols that is produced and perceived over time. Working memory plays a critical role in storing the intermediate and final products of a reader's or listener's computations as she or he constructs and integrates ideas from the stream of successive words in a text or spoken discourse. In addition to its role in storage, working memory can also be viewed as the pool of operational resources that perform the symbolic computations and thereby generate the intermediate and final products. In this article, we examine how the human cognitive capacity accommodates or fails to accommodate the transient computational and storage demands that occur in language comprehension. We also explain the differences among individuals in their comprehension performance in terms of their working memory capacity. The major thesis is that cognitive capacity constrains comprehension, and it constrains comprehension more for some people than for others. This article begins with a general outline of a capacity theory of language comprehension. In the second section we use the capacity theory to account for several phenomena relating individual differences in language processing to working memory This work was supported in part by National Institute of Mental Health Grant MH 29617 and Research Scientist Development Awards MH-00661 and MH-00662 as well as a grant from the Andrew W Mellon Foundation. Sashank Varma played an important role in developing the capacity constrained interpreter and CC READER. We ace also grateful to Jay McClelland, Maryellen MacDonald, Chuck Clifton, Mike Masson, Marilyn Turner, and Jonathan King for their constructive comments on earlier drafts of the manuscript. Correspondence concerning this article should be addressed to Marcel Adam Just, Department of Psychology, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213.
capacity In the third section we describe a computer simulation model that instantiates the capacity theory. In the final section we discuss the implications of capacity theory for other aspects of cognition besides language comprehension. For the past 100 years, research on working memory (or short-term memory, as it used to be called) has focused on the storage of information for retrieval after a brief interval. A familiar example to illustrate the purpose of short-term memory is the storage of a telephone number between the time that the number is looked up in a directory and...