To date, a large number of researchers have documented associations between lower birth weight and a range of health problems later in life. However, for a substantial amount of existing studies, it is unclear whether these associations reflect causality. Aiming at providing causal evidence in this field of research, Jonas Helgertz (University of Minnesota and Lund University) and Anton Nilsson (Lund University) analysed full-population Swedish register data on siblings and twins born between 1973 and 1994, and followed until 2011.