The widely adopted Common Core State Standards (CCSS) call for raising the level of text complexity in textbooks and reading materials used by students across all grade levels in the United States; the authors of the English Language Arts component of the CCSS build their case for higher complexity in part upon a research base they say shows a steady decline in the difficulty of student reading textbooks over the past half century. In this interdisciplinary study, we offer our own independent analysis of third- and sixth-grade reading textbooks used throughout the past century. Our data set consists of books from 117 textbook series issued by 30 publishers between 1905 and 2004, resulting in a linguistic corpus of roughly 10 million words. Contrary to previous reports, we find that text complexity has either risen or stabilized over the past half century; these findings have significant implications for the justification of the CCSS as well as for our understanding of a "decline" within American schooling more generally.