Vocabulary test score trends from the General Social Survey contradict the widespread conclusion that scores on standardized intelligence tests have systematically increased over the past century. We use a vocabulary test included in 20 nationally representative surveys administered since 1974 to test three hypotheses proposed to account for these trends, including changes in the formal measurement properties of the test, over-time changes in the meaning of education, and intercohort differences in exposure to words on the test. We find no support for the idea that test scores have declined because of changes in the structure of the test. Instead, our results show that education selectivity accounts for some cohort differences among prewar cohorts and that cohort-specific differences in exposure to words on the test account for nearly all variation in vocabulary scores of respondents born after 1945, suggesting different causal processes have influenced cohort verbal ability during distinct historical eras.