Individuals diagnosed with schizophrenia, mood disorders, bipolar affective disorder, autism and attention-deficit/hyperactivity disorder were identified through linkage between Danish population-based registers along with a random sample of the same population that supplied the cases.15 Dried blood spots for virtually all individuals were retrieved from the Danish Neonatal Screening Biobank and processed for genotyping. The design includes the ability to efficiently analyse prospectively collected cohort data within the iPSYCH case–cohort sample.15 This particular design provides several advantages: As the cohort is randomly selected from the entire population, we are able to generate unbiased absolute risks and incidence rates and to estimate the effect sizes of genetic markers on risk of mental disorders, which is representative of the entire Danish population. To date, most genetic and epidemiological studies are based on convenient case-control samples, which are prone to biases.15, 16 The iPSYCH2012 sample was preceded by four smaller Danish samples,17, 18, 19, 20, 21, 22, 23, 24 all aiming to investigate the potential interplay between genes and the environment. Collectively, these forerunners informed on the best possible study design to use in
