Since E.L. Thorndike's landmark 1917 study of the complexity of reading comprehension, inferential research has generally focused on either inference as a developmental process or the nature of inferences made during reading. In his 1930 study, R. W. Tyler established that inference could be objectively measured. S. G. Paris conducted several studies in the 1970s showing that readers' age and ability level influenced their inferencing skills, and indicating that young children are capable of producing inferences but usually do not do so spontaneously. Examinations of the nature of inferences have not been as conclusive as developmental studies. E. T. Goetz's 1977 study of high school students found no main effects for the importance or salience of the reading material. A 1979 study by P. D. Pearson, J. Hansen, and C. Gordon revealed that prior knowledge has a simple effect for inferable questions but no effect for explicitly stated questions. In a major contribution on the nature of inferencing, R. Tierney, C. A. Bridge, and M. Cera discovered that good readers made more causal and conditional connections between propositions while poor readers more often overgeneralized. One point of unanimity in reading research is that inferential ability is vital to mature readers. (MM)