This study investigated incidental learning of word meanings from context during normal reading. A total of 352 students in third, fifth, and seventh grades read either expository or narrative passages selected from grade-level textbooks, and after six days were tested on their knowledge of difficult words from the passages. Small but reliable gains in knowledge of words from the passages read were found at all grade and ability levels. Effects of word and text properties on learning from context were examined in some detail. Word properties investigated included length, morphological complexity, and part of speech. Text properties included the strength of contextual support for each word, readability as measured by standard formulas, and several measures of density of difficult words. Among the word properties, only conceptual difficulty was significantly related to learning from context. Among the text properties, learning from context was most strongly influenced by the proportion of unfamiliar words that were conceptually difficult and by the average length of unfamiliar words.
Nagy, W. E., Anderson, R. C., & Herman, P. A. (1987). Learning Word Meanings From Context During Normal Reading. American Educational Research Journal, 24(2), 237-270. https://doi.org/10.3102/00028312024002237