Seventy-nine pairs of same-sex twins were examined at the age of three years at a municipal clinic and their mothers were interviewed to assess the twins' current and past behaviours. The zygosity was determined after the interview by fingerprints and/or bloodtyping in the majority of cases. The following significant differences in concordance between monozygotic and dizygotic twins were found: 1) fear of strangers during observation at the clinic; 2) marked fear of strangers in the first year of life; 3) whether or not the child was startled by sudden noises during infancy; 4) whether or not the child was able to sleep alone at three years without a parent sitting nearby; 5) susceptibility to motion sickness; 6) nocturnal enuresis; 7) short attention span or restlessness during the test.

Nineteen variants of the iron-binding protein, transferrin, have been described in human serum. The world literature on the distribution of these variants in human populations is surveyed in comprehensive tables and attention is drawn to some of the outstanding deficiencies in our present knowledge of this distribution. It is pointed out that transferrin variants are important markers in anthropological studies.

Fetal risk in twin pregnancy is recognized as being about 2 to 4 times higher than in singletons. The main factors of this increased mortality are reviewed. Pathological effects associated with MZ origin are mainly due to the “third circulation” occurring in monochorial twins. Three situations can be distinguished: (a) hemodynamic equilibrium resulting in fetal growth impairment, (b) strong hemodynamic imbalance inducing fetal death, (c) moderate imbalance as the cause of “intertwin transfusion syndrome”. The acardiac and to some extent the papyraceus fetus can be considered as particular cases of the same problem. Other risks associated with MZ twins are the higher rate of malformations and umbilical cord entanglement in monoamniotic twins. The conjoined effects of prematurity and intrauterine growth retardation are undoubtedly the main cause of fetal death in multiple births. Therefore, some consideration is given to the underlying physiopathogenic factors. Prevention of twin weight defects demands early diagnosis of multiple pregnancy. As little can be expected from classical clinical methods, the obstetrician has to rely on more sophisticated techniques such as fetal ultrasonography. Even with an early diagnosis, at the present time there is no satisfactory treatment capable of preventing prematurity and retarded fetal growth. In this specific context, bed rest would appear as a fallacy.

It has been recently shown that the human requirement for exogenous ascorbic acid and the disease, scurvy, are the result of a typical genetic disease syndrome caused by a defect on the gene controlling the synthesis of the enzyme protein, L-gulonolactone oxidase. The lack of this active enzyme in the human liver prevents Man from producing his own ascorbic acid; a synthesis which is regularly carried out by nearly all other mammals. This genetic disease has been named, Hypoascorbemia. This new concept of the genetic etiology of scurvy gives a much broader outlook and opens perspectives which were lacking in the previous fifty year old nutritional or trace “vitamin C” hypothesis. “Correction” of this genetic defect in Man is now possible since the availability of ascorbic acid in large quantities. By “correction” is meant the long-term administration of ascorbic acid in the large amounts the human liver would be synthesizing had this genetic defect not occurred. The mammals have long used the increased liver biosynthesis of ascorbic acid, under stress, to maintain homeostasis. The genetic defect prevents Humans from utilizing this important mammalian biochemical protective mechanism. Supplying exogenous ascorbic acid at the proper high dosage for full “correction” is merely duplicating a normal mammalian reaction. The medical implications of the full “correction” of this genetic disease are discussed and speculations on the effects of “correction” in the rheumatoid diseases, cardiovascular conditions, strokes, cancer and the aging process are extrapolated from the meager data already in the medical literature. This paper is mainly a plea for more thought along the medical possibilities opened by this new concept and for more clinical tests based on the rationales derived from the genetic disease viewpoint.

Linear regression models are used to explain the variations in the twinning rates. Data sets from different countries are analysed and maternal age, parity and marital status are the main regressors. The model building technique is also used in order to study the secular decline in the twinning rate. Linear regression technique makes it possible to compare the effect of different factors but the method requires sufficiently disaggregated data.

Stillbirth rates of MZ and DZ twins in Japan gradually decreased during the period 1960–1967 and in 1974. The stillbirth rates of MZ and DZ twins were 0.270 and 0.224 in 1960, respectively, whereas the corresponding figures in 1974 were 0.135 and 0.099. The stillbirth rate was higher in MZ than DZ twins, in males than females, and in the second- than in the first-born. In both zygosities, the rates were higher in the first birth order than in the second, and then increased with birth order except in the sixth birth or more for MZ twins and for DZ twins born to mothers aged 35–39 years. The shorter the gestational age, the higher the stillbirth rate in both MZ and DZ twins, as expected. Mean gestational age was slightly shorter in MZ than in DZ twins. The decrease of the stillbirth rates can be explained partly by the reduction of twins with higher birth order, where high stillbirth rates of MZ and DZ twins are seen, and the reduction of older mothers.

Provided the assumptions underlying Weinberg's method of differentiation of types of twin pairs are correct, this analysis of the statistics of twin births in Italy in the two years 1949 and 1950 shows that there are variations in the frequency of monovular twin births greater than can be attributed to chance deviations from a constant frequency of monovular twinning. There is a tendency for the frequency to increase with maternal age, and for young mothers, a tendency for it to increase also with parity. In general, the occurrence of monovular twinning is probably more closely related to maternal age than to parity.

The frequency of binovular twinning appears to be independently related to both maternal age and parity. Amongst births of each rank the frequency of twin births increases to a maximum at maternal ages 35-39 years; and, at each age level, the frequency does not change between birth ranks 1 and 2 but subsequently increases fairly continuously. There is a suggestion that there may be a modal birth rank as well as a modal maternal age, and that binovular twinning may be no more closely related to maternal age than to parity.

In a sample of 105 concordant sex MZ and DZ twin pairs, the following characteristics were measured: red cell count, haemoglobin concentration, package cell volume, mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration, reticulocytes, platelets, white cell count and the six types of leucocytes, lymphocytes, monocytes, band and segmented neutrophils, eosinophils and basophils. The statistical model employed in the univariate twin analysis allows for three sources of variation: genetic (h2), shared environmental (c2) and specific environmental influences (e2). A genetic component was significant for red cell count, haemoglobin and mean cell haemoglobin (0.64, 0.60 and 0.46 respectively), with heritable variation suggested for package cell volume, mean cell volume, mean cell haemoglobin, lymphocytes and monocytes. Shared environmental variation was only present for neutrophils.

A survey of twin births in Denmark in relation to the total number of births from 1911 to 1974 shows a significant decrease in the dizygotic twin birth rate for all maternal age groups. The possible environmental causes of such a decrease are discussed.

The present study has confirmed previous evidence that young twins show a marked delay in language development. This delay averaged six months at the age of 48 months, compared to a control group of singletons; it was evenly reflected in all nine subtests of the Illinois Test of Psycholinguistic Abilities, and was shown equally in MZ and DZ twins. Biological variables such as birth weight, gestational age and reproductive complications were only weakly associated with language scores at four years. The twins were not significantly retarded on non-verbal tests of general intelligence, compared with population norms or with the singleton controls. Environmental factors, particularly social class and family size were strongly correlated with language scores, though the association was stronger in singletons than in twins. Middle class twins were relatively much more retarded in language development compared with middle class singletons than working class twins compared with working class singletons. Comparison of intra-class correlations between 28 MZ and 64 DZ pairs yielded a heritability index of 44% for the test as a whole, with considerable variations in h2over the nine subtests. Heritabilities were highest for tests sampling the visual motor channel, and lowest for tests on the auditory-vocal channel. In confirmation of this finding, an examination of the test profiles of the singleton controls suggested that social class differences were most marked on tests of the auditory-vocal channel. It is suggested that the latter are more susceptible to environmental variables.

Self-reported cold data have been analyzed for 95 pairs of identical twins aged 14–64 who took part in a double-blind trial of vitamin C tablets. One member of each twin pair took a tablet containing 1 g vitamin C and the other took a well-matched placebo each day for 100 days. In the total sample there was no effect of vitamin C in preventing colds. However, subdivision of the data showed a significant preventive effect of the placebo in the 51 pairs living together and an equal and opposite preventive effect of the vitamin C in the 44 pairs living apart. The placebo effect in the pairs living together may be attributed to the large proportion who wrongly perceived which treatment they were taking. The reduction of colds in the vitamin C group of the pairs living apart was about 20%. There were significant correlations between cold symptoms reported and the personality trait of neuroticism. No side effects or substantive changes in serum biochemistry could be attributed to the vitamin C dose.

Detailed twin birth rates for the United States are unavailable since 1964. In 1983 the crude twinning rate for women of white race was higher than in 1964, but there had been great changes in maternal age and parity. Indirect standardization for maternal age and birth order provides estimated total twinning rates that can be compared over the entire period. The adjusted rates for whites show a nearly continuous increase except after a 2-year reporting hiatus, 1969-70, when rates dropped back 10%. In blacks the adjusted rate increased between 1966 and 1978, except for the 1968-71 shift. The distributions of rate increases by maternal age and by race argue against effects of medical ovulation stimulants, but a disproportionate increase of triplets argues for such effects. Study is needed of rates specific for maternal age and parity, rather than of total rates.

The aim of this study on all twin births in Sweden between 1973 and 1985 was to evaluate the effect of maternal age, parity, and sex on twin perinatal mortality (PNM), by using national data held at the Medical Birth Registry, National Board of Health and Welfare, Stockholm. Zygosity in this study was estimated by the Hardy-Weinberg formula. In all, 22,374 twins were born; 972 (4.3%) of these died perinatally. The relative risk for perinatal death (after standardization for maternal age, parity, and like sex) decreased significantly during the study period, and reached its lowest value (0.3) in 1983. Maternal age was not associated with PNM, while women having their first delivery had a significantly higher PNM than the rest of the parturients. The MZ twinning rate increased fom 0.36% during 1973-76, to 0.40% during 1977-81 and 0.44% during 1982-85 (trend analysis: χ2 = 40.1, p < 0.001). After stratification for year of delivery and birthweight, like-sexed twins had a significantly higher PNM than unlike-sexed twins (odds ratio, OR = 1.25, 95% confidence interval, CI = 1.02-1.54). Males had a significantly higher PNM than females in like-sexed pairs (OR = 1.39, 95% CI = 1.16-1.66), and in unlike-sexed pairs (OR = 1.46, 95% CI = 1.01-2.10). MZ twins had a significantly higher estimated PNM than DZ ones (OR = 1.31, 95% CI = 1.02-1.67).

Monozygotic twins with typical trisomy 13 are reported. Despite an identical karyotype, the twins were dimorphic for the presence of an omphalocele. Reasons for the rarity of MZ twins with trisomy 13 are presented. It is suggested that the presence of a chromosomal abnormality in MZ twins may predispose to dimorphism.

The dizygotic twinning rate in Italy, adjusted for maternal age and parity, declined from 8.9/1000 in 1949-1953 to 6.0/1000 in 1978-81, but may have stabilized or recovered slightly in the early 1980s. The long decline was greatest in the oldest mothers, and the recent increase, if any, has been principally in first births and in women under 30. The monozygotic rate rose from 3.8/1000 in 1969-73 to 4.3/1000 in 1982-85, having been relatively stable prior to that period. No good explanation for these trends is apparent.

Old-age twinning rates were evaluated over a period of 250 years (1611-1860), from 80,256 parish records of baptism in a church office in Görlitz, East Germany. Two periods with high and two periods with low twinning rates were observed. Twinning rates were relatively stable in spring and fall, but varied greatly in winter and summer.

By cytological diagnosis of chromosomal sex it has been shown, that about 80 per cent of all cases of gonadal dysgenesis are genetically male. In about 8 per cent of male individuals various defects of red-green sensitivity may be expected. Deuteranopia was observed in a hetero-gametic pseudo-female with testicular dysgenesis. Tests for colour vision should be made in all patients with the syndrome of testicular feminization. From such tests a decision may be expected between sex- linked recessive and autosomal dominant transmission with sex limitation.

A Cox proportional hazard regression analysis was carried out that evaluated age-specific death risk among 21,890 twins born in Sweden during 1886 through 1925 and followed during 1962 through 1980. Cotwin's survival was used as the primary covariable, and auxiliary covariables were smoking, marital status and, among men, police registration for alcohol abuse. In each age, sex and zygosity group, except the oldest DZ males, cotwin's mortality had a significant, independent, positive relationship to the mortality risk of the individual. The auxiliary covariables, except marital status among females, had significant, independent, positive relationships to mortality among the youngest twins of both zygosity groups and in the middle age group of MZ twins. In the oldest age group, the death of MZ cot wins was the only variable significantly related to the individual's mortality. Heritability estimates for the age-specific probability or death risk, developed by different methods for different analysis groups, range between 0.4 and 0.6. They have reasonable internal consistency, are not much affected by the covariates, and are in agreement with other studies that did not control covariates.

A group of 34 twin pairs was studied at four years of age. Characteristics of the group were a homogeneous social background, good parental interrelationship and conscientious care during pregnancy, delivery and the neonatal period. All the children were tested with the Griffiths' Mental Development Scales. The girls generally showed a higher development quotient than the boys. Birth weight seemed to be the most important factor; at 4 years of age, the twins with a birth weight of less than 2500 grams had a significantly lower locomotor quotient. The langage quotient showed a similar trend. This difference was, however, not significant. No significant differences in either locomotor or language development could be observed between MZ and DZ twins.

A family investigation has been carried out on 301 consanguineous and the same number of nonconsanguineous couples, who married in Florence in the years 1939 through 1958. The sample turned out to be homogeneous as regards age of the couple, year of marriage, period of cohabitation, social level, and methods of survey.

The average rate of consanguinity in Florence in the above years was of 0.458 × 10−3, with a decrease from 0.595 to 0.327 × 10−3 between the first and the second decade.

While the number of pregnancies is not significantly different in the two groups, abortions are more frequent among blood relatives. As a result, the number of stillbirths being not significantly different, the birth rate is higher among the controls. Infant mortality is more than double in blood relatives. As regards morbidity, hereditary defects or anomalies show an average increase of 2.6 times in the offspring of blood relatives: the increase being higher for serious hereditary defects (3.44) than for lighter ones (1.98). The finding of slight delays in mental development was 4.8 times more frequent in the offspring of blood relatives.

By applying Morton-Crow-Muller's formula, it was possible to calculate the A and B values, and to find out, in the observed population, the presence of 2.2-2.4 pathological gene equivalents per gamete as to early mortality (abortions + stillbirths + infant mortality); 1.6-1.7 gene equivalents as to hereditary defects in general; and 0.82-0.85 gene equivalents as to slight delays in mental development.