Describing and explaining age, period, and cohort trends in Americans' vocabulary knowledge.

For a quarter of a century researchers have been documenting and trying to explain trends in Americans' vocabulary knowledge using data from the General Social Survey (GSS) and its WORDSUM test. Trends in Americans' vocabulary knowledge have important practical implications-for example, for educational policy and practice-and speak to the American workforce's competitiveness in the global knowledge economy. We contribute to this debate by analyzing 1978-2018 GSS data using an improved analytical approach that is consistent with theoretical notions of cohort effects and that permits simultaneously estimating inter-cohort average differences and intra-cohort life-course changes. We find that WORDSUM scores peak around age 35 and gradually decline in older ages; the scores were significantly lower in the 1980s and higher in the late 2000s and 2010s; and the 1940-1954 birth cohorts and the 1965 and later birth cohorts had notably higher and lower scores, respectively, than the expectation based on age and period main effects. We provide new evidence that such cohort differences tend to persist over the life course. Interestingly, the effects of increasing educational attainment and decreasing reading behaviors seemed to cancel out, leading to a relatively flat overall period trend. Trends in television viewing and word obsolescence did not appear to affect age, period, or cohort trends in WORDSUM scores. Supplementary Information: The online version contains supplementary material available at 10.1007/s11113-023-09771-5.