Measuring linguistic complexity independent of plausibility*

1. INTRODUCTION. Many factors affect the ease or difficulty of understanding a word in a sentence, including the lexical frequency of the word, the syntactic context, the discourse context, the intonation of the sentence, and the plausibility of the situation described by the sentence thus far (see Gibson & Pearlmutter 1998 and Tanenhaus & Trueswell 1995 for summaries of relevant factors and evidence). In order to investigate any one of these factors, it is therefore necessary to control for the others. One factor that has recently begun to be investigated in the psycholinguistics literature is the derivational complexity of different lexical structures. In order to investigate lexical derivational complexity, researchers have compared reading times and sentence-acceptability judgments for different kinds of noun-verb pairs. Unfortunately, these measures are necessarily contaminated by potential differences in plausibility between the real-world situations described by the particular sentences that are used to instantiate the structures. For example, consider McKoon and Macfarland's (2000; henceforth M&M) evidence that externally caused change-of-state verbs are more complex than internally caused change-of-state verbs (see Levin & Rappaport Hovav 1995 for a lexical-semantic distinction between the two types of verbs). M&M demonstrated that participants were slower in deciding that externally caused sentences like 1a were acceptable than they were in deciding that internally caused sentences like 1b were acceptable.