Usually, the formulation of research questions and hypotheses draws upon previous research and existing theories and concepts. Theories provide a framework for explaining and predicting phenomena researched. Concepts relate to how data are described and interpreted, including how particular phenomena are categorized. For that it is crucial to be aware of gendered contents in (some) scientific theories and to avoid theories that are biased. Theories and concepts must be underpinned by the best available information on sex and gender