Studies on the offspring of dual mating psychiatric inpatients present a special high-risk strategy, analogous to the diallele cross methods of plant and animal genetics for Mendelian traits, except that the parents in dual matings have complex phenotypes, and all combinations of disorders may be observed. Previous studies have mainly been on the effects of homotypic parent combinations of schizophrenia or manic-depressive disorders, showing a 2-5-fold increase in the risk for similar disorders in the offspring compared to the risk for offspring of one affected parent. A study on heterotypic parent combinations allowing an exploration of genetic relationships among various disorders has been made possible through the use of the nation-wide Danish Psychiatric Register. All psychiatric inpatients who had offspring with other psychiatric inpatients were identified, yielding a total of 139 couples with non-organic mental disorders. Their 378 offspring were followed well into the risk periods for the major mental disorders. As an example to illustrate the complexities involved in analysis of data from a dual mating study, results for parent combinations involving reactive psychoses are presented.