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SummaryA description is given of the Danish Twin Register which is based on the total twin population born in Denmark in the years 1870-1910 with a more recent addition of all same-sexed pairs from the period 1911-1920. Twins belonging to pairs where one or both partners are found to have died before attaining the age of six years are disregarded in the more intensive studies whereas the remaining part of the twin population is being followed by periodical inquiries.The methods used in the collection of the material, in the establishment of the zygosity diagnosis and in specific investigations of selected diseases and abnormalities are described and discussed.A survey is given of the more important results of the research based on the Register. In the group of somatic diseases, greatest attention has so far been paid to malignant growths, diabetes mellitus and peptic ulcer. A number of psychiatric disorders and deviations are at present being studied with schizophrenia and criminality as the first subjects for intensive research. Some of the main results obtained are given as illustrations of the potentialities and scope of this approach to the application of twin studies in a broad variety of research areas.

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Suicide is a complex problem and a phenomenon of mankind which is obviously influenced by a multitude of ethnological, socio-cultural, psychological and various other factors.That a man takes his own life does not necessarily mean that he was mentally abnormal. On the other hand, it is a well established fact that the frequency of mental illness — affective disorders in particular — is high among suicides, and that one in six patients suffering from manic-depressive psychosis will commit suicide (Sainsbury, 1968). A family history of suicide is not quite uncommon, and beforehand one cannot exclude the possibility that somehow genetic factors may play a part in suicide; but evidence in favour of such an assumption seems to be scanty.A survey of the literature reveals that there have been very few twin studies of suicide, and so far concordance in MZ twins seems to have been reported in three cases only.The first case was observed, already in 1812, by a Dr. Stephens Williams from Massachusetts. He described twin brothers, presumably MZ, who both were captains and had distinguished themselves in the War of Independence. One of the twins settled down in the beautiful state of Vermont, whereas the other remained in Springfield. For several years, they continued to live apart, until, within short interval, they both committed suicide by cutting their throat. Williams points out that the mother and two siblings of the twins had presented symptoms of mental illness, and that, prior to their death, both had been in a state of melancholy, probably an endogenous depression.

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