Psychiatric population research in Denmark has been facilitated by the existence of various national registers. especially the National Psychiatric Register and the Adoption and Twin Registers. A number of important American-Danish collaborative studies have been performed in the areas of the schizophrenic spectrum disorders, alcoholism, antisocial personality, and suicide using high-risk and adoption samples. The National Psychiatric Register has also been used in prevalence studies of inpatient populations. Other than notable case studies of criminal psychopaths, no well-designed clinical studies on personality disorders have been carried out in Denmark. Epidemiological studies on phenotypical differences and similarities in early-separated twin and psychological studies on men with sex chromosome aberrations have contributed to the understanding of normal personality and genetics. Overall the Danish contributions to the research in the behavioral domain suggest strong genetic and biological factors in normal and in abnormal personality. Further, the studies suggest that if genetic factors are not taken into account, then studies on environmental predisposing factors are confounded with hereditary factors.