During language acquisition children must learn to use causal connectives, such as because. Acquiring these linguistics markers of causal coherence relations between utterances can be regarded as learning one of the most important ‘building blocks’ of language at a discourse level. This study investigates how children’s use of causal connectives develops. It takes a cross-linguistic approach by investigating the English causal connective because as well as its German counterpart weil and the Dutch equivalents want and omdat. Growth curve analysis is used to track young children’s production of causal connectives in longitudinal corpora. This method is used to investigate two factors that may influence connective acquisition. First, the parental input: does parental connective use and parental scaffolding through why-questions influence the development of causal connectives? Second, the cognitive complexity of the causal relation: does the relative complexity of objective and subjective causal relations influence the subsequent development of these relations? As an additional measure, this study also includes an innovative eye-tracking experiment – based on the preferential looking paradigm – that test young children’s comprehension of causal relations. Overall, results show that connective acquisition is an intricate system in which cognitive complexity and parent-child interaction play an important role.