Requirements taxonomies help to classify and channel the requirements in a project. A very simple taxonomy is the distinction between functional and non-functional requirements. Furthermore, a taxonomy helps to decide if a statement is a requirement at all or just something else (e.g., 'information'). The quality of a taxonomy is important as we do not want to put a statement in the wrong category.In this paper, we argue that we need to take cognitive psychology into account in this task of requirements classification. Cognitive psychology focuses on the abilities and limitations of the human mind. We present a controlled experiment and a replication in which we compare three requirements taxonomies.The participants had to evaluate a set of requirements based on the given taxonomies. The results of these experiments show that there are differences between the taxonomies: Interestingly, the question whether a statement is identified as a requirement or not depends on the taxonomy. These experiments present initial results, we assume that these results are related to phenomena of cognitive psychology.We conclude that the wording should be carefully taken into account in the definition of the categories of a high quality requirements taxonomy.