The longitudinal prospective study of populations at risk is considered a powerful strategy towards disentangling hereditary and environmental factors. Data from Mednick and Schulsinger's 1962 study in Copenhagen, on children at high-risk for schizophrenia are used as an illustration. Pregnancy and birth complications, as well as institutional rearing in early childhood contributes towards schizophrenia in the risk children, but not in the low-risk controls. Risk children with an outcome of schizotypal personality disorder were hardly exposed to perinatal complications. To some extent, schizophrenia may be considered as a complicated form of schizotypal personality disorder, which again may be a genetically transmitted condition.