Learning from textbooks is challenging because students must understand novel concepts while also comprehending the language used to convey those concepts. In the domain of science, one posited reason for the perceived difficulty in the reading comprehension of science texts is the low frequency of logical connectives (words that signal relationships between sentences and ideas). To test this claim and discuss its potential effects on the reading comprehension of texts used at the middle school level, this study measured whether the usage of logical connectives (e.g., therefore, so) differed between science and social studies textbooks. Our findings from a large corpus of 12 science and 12 social studies textbooks showed that science texts contained a higher rate of logical connectives than social studies texts. This main effect of subject area also interacted with grade level: The rate of logical connectives usage increased over grade levels in science but not in social studies. Our results showed further differences in the types of logical connectives used across subject areas, with science texts favoring inferential connectives (e.g., furthermore) and social studies texts favoring contrastive connectives (e.g., however). The implications of these findings for the development of science-specific literacy practices are discussed here.