period, the causes of death were grouped based on the same coding procedure. We particularly focused on young age groups, to avoid dilution of the findings by large mortality effects among older people. Data on cohort members were collected by linking 4 Danish population registers (see the eAppendix in the Supplement). All residents of Denmark, including immigrants, get a unique personal identification number that is used in all national registers, which enables us to link data across these registers. Our study was approved by the Danish Data Protection Agency. The investigators were blind to the identity of study members, and because the study did not result in any contact with the participants, no written informed consent was required according to Danish law.
