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The island Bornholm has been known since the ninth century and has been under Danish government since the tenth century. The population consists of forty-six thousand inhabitants who in every respect give a fair representation of the Danish population in Denmark proper. In their psychology the people are very similar to their Danish brothers; an important character trait is their pride
at being a member of the island population. Further, they strive always not to show their emotions; they are 'true democrats' of conservative coloring, suspicious of every change. The advantage of a thorough psychiatric examination of such a limited population is the possibility of getting a nearly complete picture of the psychiatric morbidity. The methods used to obtain this complete picture are described in the main part of the book and include an investigation of all inmates of the hospitals, the study of twenty thousand case records, and the geneological investigation of family trees with tens of thousands of members. The results of the author's investigation confirm some findings that were known before the investigation: for instance, that the ratio of the frequency of manic depressive psychoses to the frequency of schizophrenia is not one to two but one to four; that epilepsy may be expected in an average population twice as often as cases of manic depressive psychoses. If the findings of the island Bornholm are applied to the total population of Denmark the expectation for schizophrenia is 0.66±0.10, for manic depressive psychoses, 0.20±0.07; for epilepsy, 0.35±0.07; for general paresis, 0.33±0.10. Among the total population twelve percent are 'psychiatrically suspicious'.
So far as the specific hereditary factors of the main psychoses are concerned it seems that nothing contradicts the opinion that both the schizophrenics and the manic depressives show a monomeric hereditary basis, recessive in the case of the former, dominant in the latter.