The “primary-secondary learning gap” has long been discussedin Japan. Many students suddenly have difficulties inunderstanding subjects when they enter junior high school (7thgrade in Japan). Despite the fact that textbooks are one of themost important learning instruments, the qualitative andquantitative change in the content of textbooks has not beenexamined in light of the primary-secondary learning gap. Inthis paper, we show that students are overloaded with the steepincrease in the definitions of scientific concepts in textbooks.While the number of definition expressions in textbooksincreases rapidly toward junior high school, students’ skills inunderstanding definitions develop only gradually. Wedemonstrated this through a quantitative linguistic analysis oftextbooks and psychometric results of students’ reading skills
