So there’s a positive correlation between a student’s vocabulary size in grade 12, the likelihood that she will graduate from college, and her future level of income. The reason is clear: vocabulary size is a convenient proxy for a whole range of educational attainments and abilities—not just skill in reading, writing, listening, and speaking but also general knowledge of science, history, and the arts. If we want to reduce economic inequality in America, a good place to start is the language-arts classroom.

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.

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.