From City Journal, "A Wealth of Words: The key to increasing upward mobility is expanding vocabulary" by E D Hirsch, JR:
Early in the twentieth century, a well-meant but inadequate conception of education became dominant in the United States. It included optimism about children’s natural development, a belief in the unimportance of factual knowledge and book learning, and a corresponding belief in the importance of training the mind through hands-on practical experience. In the 1920s and 1930s, these ideas began spreading to teacher-training institutions. It took two or three decades for the new teachers and administrators to take over from the old and for the new ideas to revolutionize schoolbooks and classroom practices. The first students to undergo this new schooling therefore began kindergarten in the 1950s and arrived in 12th grade in the 1960s.The entire article is worth reading.
Their test scores showed the impact of the new ideas. From 1945 to 1967, 12th-graders’ verbal scores on the SAT and other tests had risen. But then those scores plummeted. Cornell economist John Bishop wrote in the 1980s of “the historically unprecedented nature of the test score decline that began around 1967. Prior to that year test scores had been rising steadily for 50 years.” The scores reached their nadir around 1980 and have remained low ever since.
Some scholars thought that the precipitous fall of verbal SAT scores simply reflected the admirable increase in the percentage of low-income students taking the SAT. But Bishop observed that the same downhill pattern had occurred in verbal scores on the Iowa Test of Educational Development—a test given to all Iowa high school students, who were 98 percent white and mostly middle-class in attitude. He argued that the declining effectiveness of American schools was a leading indicator for the shrinking income of the American middle class. The evidence today suggests that he was right. The decline in the educational productivity of our schools tracks our decline in income equality. For 30 years after 1945, Stiglitz observes, economic equality advanced in the United States; after about 1975, it declined.
Later, another Cornell scholar, the sociologist Donald Hayes, showed that the decline of the verbal SAT scores was indeed correlated with a dumbing-down of American schoolbooks. Following the lead of the great literacy scholar Jeanne Chall, Hayes found that publishers, under the influence of progressive educational theories, had begun to use simplified language and smaller vocabularies. Hayes demonstrated that the dilution of knowledge and vocabulary, rather than poverty, explained most of the test-score drop.