schoolbook simplification & the 50-point drop in SAT scores

I'm sure I've posted this before, but it bears repeating:

The 50+ point decline in mean SAT-verbal scores between 1963 and 1979 is widely attributed to changes in the composition of the test-takers. Several inconsistencies in that explanation are identified. That explanation also ignores the pervasive decline In the difficulty of schoolbooks found by analyzing the texts of 800 elementary middle, and high school books published between 1919-1991. When this text simplification series is linked to the SAT verbal series, there is a general fit for the three major periods: before, during, and after the decline. Long-term exposure to simpler texts may induce a cumulating deficit in the breadth and depth of domain-specific knowledge, lowering reading comprehension and verbal achievement. The two time series are so sufficiently linked that the cumulating knowledge deficit hypothesis may be a major explanation for the changes in verbal achievement. Only an experiment can establish whether this is causal, so we describe a simple, low-cost experiment schools can use to test how schoolbook difficulty affects their students’ verbal achievement levels.

Schoolbook Simplification and Its Relation to the Decline in SAT-Verbal Scores
Donald P. Hayes, Loreen T. Wolfer, Michael F. Wolfe
American Educational Research Journal
Summer 1996, Vol. 33, No. 2, pp. 489-508

cumulating knowledge deficit...