That vocabulary knowledge is strongly related to reading comprehension is among the most robust findings in the reading literature. From research in areas as diverse as readability (Dale & Chall, in press) and psychometrics (Thorndike, 1973–74), extremely strong correlations have been reported between measures of word knowledge and measures of comprehension. In a paper that had a strong influence on the resurgence of interest in vocabulary knowledge, Anderson and Freebody (1981) proposed three hypotheses to explain these correlations.