Psychiatric population research in Denmark has been facilitated by the
existence of various national registers, especially the National Psyc
hiatric Register and the Adoption and Twin Registers. A number of impo
rtant 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 Na
tional Psychiatric Register has also been used in prevalence studies o
f inpatient populations. Other than notable case studies of criminal p
sychopaths, no well-designed clinical studies on personality disorders
have been carried out in Denmark. Epidemiological studies on phenotyp
ical differences and similarities in early-separated twin and psycholo
gical studies on men with sex cromosome aberrations have contributed t
o the understanding of normal personality and genetics. Overall the Da
nish contributions to the research in the behavioral domain suggest st
rong genetic and biological factors in normal and in abnormal personal
ity. Further, the studies suggest that if genetic factors are not take
n into account, then studies on environmental predisposing factors are
confounded with hereditary factors.