Causal selection refers to the common practice of singling out one cause among all known causes of some event, and calling it "the" cause. Suppose that a fire breaks out in a house, and we say that this fire is caused by a short circuit. In this case we know that other factors (such as the presence of combustible material and oxygen) are causally relevant, but we still consider the short circuit to be the most important or decisive cause. This is an instance of causal selection. Causal selection not only occurs in everyday causal discourse, but also in biomedical research. Epidemiologists tend to single out genetic factors for inclusion in disease explanations, and to relegate other factors to the background. For example, when epidemiologists put forward the BRCA 1 and BRCA 2 genes as causes of breast cancer, they are well aware that these genes are only two of the many factors involved in an incidence of breast cancer. Nevertheless, the genetic causes are emphasised. My project investigates these causal selection practices in epidemiology and provides instruments for their critical assessment. I study four diseases intensively: phenylketonuria, tuberculosis, breast cancer and obesity.