A 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.

A twin study was undertaken with the twofold aim (a) of studying the hereditary behaviour of digital dermatoglyphic traits both at the qualitative and quantitative level, and (b) of working out a method for discriminating MZ and DZ twins by means of fingerprints.

Fingerprints of 50 MZ (25 ♂ and 25 ♀) and 50 DZ (25 ♂ and 25 ♀) twin pairs were thus examined and analyzed by means of a special methodology and of a 7044/K32 IBM computer.

The qualitative analysis has shown a significantly higher concordance in MZ than in DZ twin pairs, with a certain variability of single finger concordance values. The quantitative analysis has shown significantly higher correlation values in MZ than in DZ twin pairs, with very limited confidence intervals in the former. Single ridge counts apparently behave as cumulative counts on the five or ten fingers, although with an obviously higher random variability.

Digital dermatoglyphics thus appear to show practically complete genetic conditioning, which, rather than at a cumulative level for the ten fingers, as is largely believed, appears to act on single finger quali-quantitative traits. The total finger ridge count, rather than a trait, only appears to be a useful, but artificial cumulative value. Actually, applied to the diagnosis of zygosity, it provides, by itself, a fairly high, general probability (0.86) of a correct diagnosis.

Data obtained in a study of mentally defective twins permitted the examination of interdependence and efficiency of blood groups and fingerprints in zygosity diagnosis. After exclusion of Down's syndrome, the series consisted of 49 MZ pairs, 52 DZ pairs and 18 pairs of uncertain type.

Same-sex pairs discordant for Down's syndrome seemed to include some pairs of uniovular or irregular origin, but this could not be investigated cytologically. Concordance appeared to occur independently in different blood group systems. However, DZ twins concordant in the ABO blood groups were significantly more similar in their fingerprints than were other pairs, and the within-pair differences were more uniform.

Linear discriminant functions designed for zygosity diagnosis make much better use of dermatoglyphic information than do simpler measures, and the latter could not be much improved by comparing left and right hands separately.

1. The Author has investigated on a twin material of 34 twin pairs the papillary line patterns on the fingers, the interdigital areas of the palms, and the hallucal area of the soles according to a systematics accounted for by the Author in earlier works.

2. The diagnosis of zygosity has been based on a comparison for the morphological particularities and on blood group analyses, in the cases in which the relative chance for dizygosity for the concordant pairs has been calculated.

3. The frequency distribution for the papillary line patterns of the twin pairs showed no significant divergence with respect to the large Swedish population, earlier investigated.

4. The 19 MZ twin pairs showed a concordance of 84.2-89.5% for the total qualitative value, total ridge-count and for the hallucal patterns.

Dosage of the hematic level of ATP has been afforded in 20 (7 MZ and 13 DZ) twin pairs aged 4-6 years. The intrapair correlation coefficient has been calculated and resulted to be significantly higher in MZ than DZ twin pairs. The hereditary “quantum” in the determination of the hematic level of ATP was also estimated and resulted to be of about two thirds.

A multiple birth frequency of 5.59% has been found in 25 Klinefelter sibships, which is statistically significantly higher than expected (P < 0.001).

A family investigation of a patient with Klinefelter's syndrome, who is a triplet himself, revealed 5 twin births and 2 triplet births in three generations.

The mortality of the 19 children from the 9 multiple births born in the 25 Klinefelter sibships was 53% up to the age of 3. Such a high mortality indicates that some of the twins and triplet births might have suffered from chromosome disorders contributing to the high mortality.

If the comparatively high frequency of twins in the sibships with Klinefelter's syndrome as well as Turner's syndrome is confirmed by further studies, this will indicate a relation between the cause of sex chromosome non-disjunction and multiple births or between the predisposition to non-disjunction and multiple births.

A follow-up EEG study in 28 cases with closed head injuries, using MZ co-twins as controls, is described. A new concept in EEG analysis is introduced. A combined EEG code and point scale are more effective in discriminating subtle differences between two EEG records. A high concordance was found whether the EEG showed normal or abnormal patterns. Not only the head-injured twins but also the controls showed more abnormalities than in the normal population. Without this check by co-twins the EEG abnormalities could have been taken for persisting signs of the proband injury. It was found, however, in late follow-up that proband twins showed more EEG aberrations and abnormalities than co-twins, but the probands differed already in pre-traumatic mental make-up from their genetically identical controls. Neither differences in mental make-up nor in EEG characteristics showed any statistically significant correlation with the severity of the proband injury. These observations limit the significance of EEG abnormalities in late concussional states.

This study also indicates that there are individually specific EEG patterns, which in health retain their characteristics throughout life. The EEG pattern is therefore concordant in identical twins. Thus, when a twin is influenced by a brain disease or lesion, the co-twin may serve as a biological control in both acute and follow-up studies.

The fact that controls as are maintained in most biological experiments, could even be studied ebulliently in human population, by observing a MZ pair, a member of which is exposed to a certain anomaly and the other acts as control, becomes more vivid and recommended from the present studies. For, they can be directly related to human health and welfare.

Permanent changes due to typhoid incidence in physical, mental and anthropometric traits of a member of MZ twin pair may be suggestive of some “beneficial interaction of typhoid bacillus with certain intricate mechanisms like RNA and protein synthesis, as has been envisaged earlier.