Our research aims to identify domain-specific similarities and differences of externalized cognitive structures. Cognitive structure, also known as knowledge structure or structural knowledge, is conceived as the manner in which an individual organizes the relationships of concepts in memory. By diagnosing these structures precisely, even partially, the educator comes closer to influencing them through instructional settings and materials. Our assessment and analysis of cognitive structures is realized within the HIMATT tool, which automatically generates four quantitative indicators for the structural entities of written text or causal maps. In our study, participants worked on the subject domains biology, history, and mathematics. Results clearly indicate different structural and semantic features across the three subject domains. Additionally, we found that written texts and causal maps seem to represent different structure and content across the three subject domains when compared to an expert's representation. We conclude with a general discussion, instructional implications and suggestions for future research. (Contains 2 figures and 5 tables.)