The purpose of this study was to compare: 1) the connective expressions (CEs) used to signal causal coherence relations in primary school textbooks from four different school subjects (variety); 2) patterns of use of these CEs in terms of the types and number of causal relations they signal across subjects (functionality); 3) patterns of use of these CEs regarding the causal relations they specialize in across subjects (specificity). 2216 causal coherence relations extracted from school textbooks of Language, Mathematics, Science, and History were analysed. To describe variety and functionality, a descriptive statistical approach was adopted, while to determine specificity, a logistic model was built, which allowed us to predict the co-occurrence of CEs and a specific type of causal relation. 76 linguistic devices were identified in the overall corpus, being Conjunctions the predominant category. Regarding functionality, in each subject the most frequent CEs signal from two to five different types of relations. Four CEs showed a profile of specificity in the overall corpus and only two were determined as specific in the across subject analysis. Results suggest that polyfunctionality is the main pattern of use of Spanish causal CEs.