In each species, natural selection has resulted in an optimal litter size, to ensure the largest average number of surviving offspring and the lowest maternal mortality. The terrestrial insectivores from which the primates evolved had large litters. It has been argued that in primates adaptation to an arboreal mode of life led to a reduction of litter size to a single offspring because of the difficulty of producting and caring for a large litter in a tree [6]. A recent critical survey of the literature indicates that twinning frequency in most nonhuman primates is lower than in man [20].

There are clear ethnogeographic differences in the incidence of human twinning. Among peoples of Eastern Asia, multiple maternities are rare; in Japan for instance, the twinning rate is only 3-7 per mill, according both to the official statistics and hospital records. These low rates seem not to be caused by the facts that it is customary in Japan to have children very early in life or that twin births were at one time viewed with displeasure and concealed, or sometimes twins were even killed in some areas [21].

In hospital series among some Negro tribes almost 10 times higher twinning frequencies than among Japanese have been reported e.g. the Yorubas in western Nigeria with values above 60 per mill [24]. However, where national birth statistics of Blacks are available, twinning is lower, e.g. in U.S.A. 13-15 per mill [1].

The course and outcome of 23 monoamniotic (MA) twin pregnancies, delivered in Tampere University Central Hospital during the years 1964-1984, were studied retrospectively and compared to 1056 diamniotic (DA) twin pregnancies. The frequency of MA twins was 2.1% of twin pregnancies. Polyhydramnion complicated the pregnancy in 26% of MA vs 6% of DA pregnancies. Two cases were defined as acute polyhydramnion. Preterm labour was stated in 70% of MA pregnancies and deliveries before the 34th week were 4 times more common in MA than DA pregnancies. The cesarean section rate was more than double in MA pregnancies (39%). Entanglement of the umbilical cords was noted four times, and prolapse of the cord in three vaginally delivered cases. Perinatal mortality was 28% in MA vs 5% in DA twins. The most common causes of death were respiratory distress syndrome, congenital malformation and feto-fetal transfusion.

The multiple birth family is more likely to have a dispute with the education system than with any other service. So many potential areas of conflict exist over the abilities and behaviour of multiples and over such issues as separation or keeping back one twin. One reason for disputes is the lack of good data to adequately reflect the different perspectives of parents and teachers and the differing needs of families: the same solution does not apply to all. To provide the first large-scale data base and building upon an initial survey of 85% of all primary school teachers in South Australia, the LaTrobe Twin Study and AMBA worked with Education Departments to set-up in each state Education Research Teams (ERTs) of parents of multiples who were also teachers. The ERTs were crucial in three phases. 1) Developing and circulating questionnaires and publicising the nationwide survey. 784 families and 1264 teachers of their children completed these questionnaires, many reporting that simply having to address the issues raised in the questionnaire was a valuable learning experience. 2) Exploring the data base. Issues arising included the very different bases on which parents and teachers judged separation desirable, with teachers emphasising the unsubstantiated claim that separation is essential to individual development. Separation became more common over the first three years of schooling but 20-25% of twins separated one year were back together the next. 3) Running regional meetings of parents, teachers and administrators to discuss the results and to pool experiences and plan policies at the local level. A need clearly exists to improve the level of consultation between families and school personnel and to ensure the widespread availability of information which identifies key issues in making decisions for that multiple birth family.

Monozygotic twin (MZ), dizygotic twin (DZ), and sibling (SIB) pairs were taste-tested for hydrochloric acid, 1-quinine sulfate, and 6-n-propylthiouracil (PROP). The numbers of pairs involved were 75 MZ, 70 DZ, and 78 SIB, for the latter two compounds; 26, 45, and 45, respectively, for the acid.

There was a significant difference in intrapair variance, between the MZ and the same-sex DZ pairs, in thresholds for bitter-tasting 6-n-propylthiouracil (p < .001). The difference in intrapair threshold variance was not significant for bitter-tasting quinine (p > .05) or for sour-tasting hydrochloric acid (p > .10).

The male MZ pairs had a significantly lower intrapair threshold variance than the male DZ or male SIB pairs for hydrochloric acid (p < .01), but the female pairs manifested no such difference. The intrapair variance in hydrochloric acid threshold was significantly less for the nine male MZ pairs than for the 17 female MZ pairs (p < .02).

Repeated taste tests on the same subjects reproduced results similar within a single threshold range in a high proportion for each compound: for hydrochloric acid, 72.9% (N = 44); for quinine, 76.9% (N = 221); and for PROP, 76% (N = 225).

Correlations between thresholds for the different substances were positive and significant (N = 308): between PROP and quinine, r = + 0.44 ± .05 (p < .01); between quinine and hydrochloric acid, r = + 0.35 ± .05 (p < .01); between PROP and hydrochloric acid, r = + 0.17 ± .06 (p < .05).

The first case of MZ twins concordant for oculo-auriculo-vertebral dysplasia is reported. In the light of the data from the literature concerning the etiology of the syndrome, this observation does not allow an absolute conclusion but points towards the possible existence of a genetic form of this syndrome.

87 affected twins (in 81 pairs) were identified among 3,982 children with pyloric stenosis. The proportion of twins among affected (1 in 46) is about the same as in the general population of births. The incidence of pyloric stenosis in partners of affected twins is 8.6 per cent, considerably higher than in the general population of births (0.3 per cent) but only slightly higher than in subsequent sibs (5.8 per cent).

The frequency of concordance is about the same in MM (4 of 31) and MF (2 of 23) pairs. Of 39 like sexed pairs 4 were concordant; the expected number of monozygotic pairs is 18 (assuming the usual proportion of monozygotic pairs), which suggests that the frequency of concordance among them is not greater than 4 in 18. It is shown that the distribution of MM, FF and MF pairs in the series corresponds closely to the distribution expected in a twin universe which contains the usual proportions of monozygotic and dizygotic twin pairs.

The Finnish Twin Cohort material was used to estimate genetic and environmental effects in the etiology of hyperopia (farsightedness). All twin pairs in the cohort born before year 1927 (age 60 years and over at the time of the study), with both members alive, were sent a questionnaire. The questionnaire included questions of past and present eye diseases, visits to ophthalmologists, use of glasses and other vision-related questions. The hyperopia was assessed by asking the patients to send their last prescription for glasses to the authors. Twins with any eye disease affecting refraction (cataract, corneal damage), operation or trauma to their eyes were discarded from the present study. In 191 pairs (80 monozygotic and 111 dizygotic pairs) one or both members of the pair had a hyperopic refractive error. The correlations of refraction between right and left eyes of both MZ and DZ pairs were high (Spearman Rank Correlations of 0.86-0.89). The intrapair correlations among MZ pairs were higher (0.44 for right and 0.45 for left eyes) than intrapair correlations among DZ pairs (0.24 for right and 0.15 for left eyes). The variances were not significantly different among MZ and DZ pairs. The classical analysis of heritability gave an estimate of 0.75 for hyperopia. The result suggests that genetic factors are important in hyperopia and especially in hyperopia of higher degree.

All of the recorded twin live births in Washington State birth certificates between 1984 and 1988 were used a retrospective cohort study to determine the risk of zygosity on pregnancy complications and birth outcomes (n = 3458). Relative risks comparing different sex (DS) twins to same sex (SS) twins were corrected to relative risks relating dizygotic (DZ) to monozygotic (MZ) twins, using the Weinberg rule. A higher proportion of DS twin pregnancies (3.5%) than SS pregnancies (1.6%) were complicated by gestational diabetes, resulting in an estimated risk for DZ twin pregnancies relative to MZ pregnancies of 8.6 (95% CI = 3.5-21.0). DZ twin pregnancies were at a lower risk for complications of polyhydraminios (RRDZ∣MZ = 0.2, 95% CI = 0.1-0.4) and of pyelonephritis, (RRDZ∣MZ = 0.3, 95% 0 = 0.1-0.8). MZ twins were more likely to have low birthweight and to have shorter gestations. The proportion of first-born babies of MZ twin pairs who died during their first year was similar to that of first twins of DZ pairs; however, the second-born of MZ twins were more likely to die in infancy than were second-born DZ pairs. First twins of DZ pairs were more likely to die of SIDS (sudden infant death syndrome) than the first of MZ twins (RRDZ∣MZ = 1.5, 95% CI = 0.4-5.1). In contrast, DZ second-born were less likely to die of SIDS than were MZ second-born twins (RRDZ∣MZ = 0.1, 95%CI = 0.1-0.7). DZ twins were less likely to have adverse newborn conditions or malformations. The high risk for gestational diabetes for DZ twin mothers is possibly due to the presence of two placentas which may support the development of greater insulin antagonism than the single placenta in the mother of MZ twins. The reduced risk of DZ relative to MZ twins for selected adverse birth outcomes may result from the increased tendency of MZ twins to be premature.

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.

A survey was carried out among fisher-women in Pallepalam, in Nellore District of Andhra Pradesh, to determine the prevalence and pattern of consanguinity and its effects on fertility, mortality, and morbidity. Of the 106 marriages investigated, 54.72% were consanguineous with predominant proportions of first cousin (36.79%) and uncle-niece (13.21%) marriages. The average inbreeding coefficients for autosomal and sex-linked genes were found to be 0.0405 and 0.0448 respectively. The differences between consanguineous and nonconsanguineous marriages were studied with respect to fertility, mortality and morbidity.

Twenty-one couples with complete sets of triplets aged between four and six years were interviewed about their experiences of being “triplet parents”. The diagnosis of triplets had been a shock for most. All triplets were born prematurely, the mean birth weight being 2,000 g. The first time at home was chaotic for most of the parents. They spent more time organizing and arranging their day and less time on emotional care than did parents of single infants. Growing up as a triplet, with constant competition for attention, stimulation and love from the mother (parents), differed a great deal from the situation for singletons and twins.

The early relationship between mothers and triplets must be managed differently from that between mothers of singletons and twins. The study shows how important it is for obstetricians, paediatricians and other professionals to understand the specific needs of these families.

For epidemiological purposes, it is customary to assume that same-sex (SS) dizygotic (DZ) twin pairs are approximately equal in number to unlike-sex (OS)-DZs, the remainder of the SS pairs being monozygotic (MZ). It is also customary to consider OS-DZs to be epidemiologically representative of all DZs, which can only mean that difference in frequency of any trait between OS and SS twins is due to the MZ fraction of the SS twins. Since this is assumed as a premise, there is little value in its usual appearance as the result. The basic tenet of twin biology, that most twin excess anomalies are due to MZs, is a myth self-perpetuated by a methodological tautology, and is false, at least for mortality. In a consecutively ascertained and prospectively studied sample of 616 twin pairs, over 80% diagnosed for zygosity, it can be shown that the standard assumption mentioned above have given impossible answers. The most probable possible answer is that mortality does not differ greatly with zygosity overall, but that SSDZ mortality is much higher than that of OS twins, and probably even higher than that of MZs. Race differences in the probable answers further suggest that standard assumptions of the Weinberg method may have consistently provided false explanations for race differences in the OS fraction of twin pairs.

Seventeen head and face measurements of 205 twin pairs, aged 18 to 25 years, are analyzed. In both sexes a significant genetic variance component is found for head length, head breadth, and frontal breadth, for seven breadth measurements of the face, for physio-face height, and nose height. A significant genetic variance component is found for nasion-gnathion, nasion-stomion, and lips height in males and for the two ear measurements in females. We suggest that the sex difference for heritability may be due to random factors and to continued growth from 18 to 25 years in males.

A survey questionnaire of 200 families with 2-month-old twins assesses the economic, social and psychological impact of the arrival of twins on family life. The study shows the extent of material difficulties mothers of twins are confronted with, and the ways they cope with them — recourse to mother's helpers, assistance from the father and other members of the family (analyzed in terms of parity and socioeconomic/cultural status), twincare organization strategies, and impact on the decision to stop working. The findings provide an overall picture of the real situation, a necessary prerequisite to an understanding of psychological problems.

The Klippel-Trénaunay-Parkes Weber-syndrome (K.T.W.) is a congenital malformation with partial gigantism (osteohypertrophy), naevi and varicose veins in the fully developed trias of symptoms. According to arterio- and phlebographic findings varicose veins are usually the result of mal-and deformations of the deep venous system or arteriovenous anastomosis, rarely of the arterial system alone.

In a critical consideration of all hypotheses about aetiology and pathogenesis of the K.T.W.-syndrome is discussed that there is no satisfactory explanation for the assumption that the syndrome is a exogenous malformation.

It is pointed out that so far there have been no family investigation of a greater extent. The statistic survey of the existing family investigations showes that the K.T.W.-syndrome is most variable in its phaenotypical manifestation and also has a strong intra-and interfamiliar variability. Heredopathologically is suggested that apart from a weak (?) principal gene secondary genes are essential for the manifestation. The mode of inheritance may be either dominant or recessive. The existence of various biotypes is considered probable. It is proposed to count the K.T.W.-syndrome into the group of the ectome-sodermal malformations (Oehler), the hamartomata (Wohlwill), the phacomatoses (van der Hoewe) or the neuro-cutaneous syndromes. 2 own cases are described.

Although familial aggregation of blood pressure is well documented, few studies have considered the changing contribution of genetic and environmental influences during adulthood. Applying maximum likelihood model fitting to blood pressure covariation in balanced pedigrees including both parents and their young adult twin offsprings (25 MZ, 32 DZ, aged between 16 and 24 years), it is shown that the increased variation in parents is explained by such developmental changes. For DBP, an apparent reduction in heritability from 68% to 38% from young adulthood to middle age results from the increasing impact of individual environmental experience (E1), with little or no influence from shared family environmental (E2). For SBP, shared environmental effects may play a part. Given the relatively small size of the present sample, the conclusions are to be seen as tentative. An augmented family study, incorporating middle aged twins and their young adult offspring, will clarify the causation of these developmental changes.

Hospitalization rates of monozygotic (MZ) and dizygotic (DZ) twin pairs in Finland were compared for schizophrenia, neuroses, and alcoholism. Record-linkage of hospital records and death certificates for the years 1972-1979 was carried out for persons in the Finnish Twin Cohort (16,649 like-sexed twin pairs). The ratio of the number of observed vs that of expected concordant pairs and the ratio of concordance rates between MZ and DZ pairs were greater among males than females, and greater among young (40 years old or less) than among older pairs. The highest difference was found in schizophrenia and the lowest in neuroses. Pairwise concordance rates for schizophrenia (11.0% for MZ and 1.8% for DZ) seem to indicate great environmental influence (high proportion of discordant pairs) with apparent genetic liability (6.1-fold ratio in concordance between MZ and DZ pairs). In neurotic disorders, the difference of pairwise concordance rates between MZ and DZ pairs (6.8% vs 4.0%) was quite low, not strongly supporting a genetic hypothesis. Of the MZ pairs concordant for psychiatric hospitalization, 47% had lived together for their whole life time; of those discordant, 16% lived together. The corresponding figures for DZ pairs were 18% and 15%. The effect of intrapair relationships in disease-concordant pairs should be taken into account when evaluating the effect of genetic and environmental factors in psychiatric disorders.

In this preliminary report the question of the inheritance of amyotrophic lateral sclerosis is discussed. Through systematic search within the province of Westphalia and especially in the District of Münster, we found 251 index cases out of a population of about 250.000 neuro-psychiatric patients observed during the last 30 years. This material is being analysed from a clinical as well as from a genetic standpoint. The family study will be a basis for determination of the mode of inheritance. As for the sporadic cases of Charcot's disease, the effect of mutations must be taken into consideration.

A criticism of twin studies has been that the difference between the behavioral similarities of identical and fraternal twins is largely created by parental influences based on their perception of the twins' zygosity. This issue is examined for differences in the IQ scores found within pairs classified by parents and bloodtyping. The systematic differences in IQ scores could be attributed to zygosity classified by bloodtyping rather than by parental belief. The available evidence indicates that the twin method is still appropriate for human behavior genetics.

Certain congenital malformations occur in excess among twins and also among first-degree relatives of twins. In the general population, these anomalies are familially associated with each other, and, like twinning, familially associated with unusual brain function asymmetry. They affect structures built by fusion of bilateral embryonic halves and remodeled under major influence of neural crest mesenchyme. This conjunction of associations suggests that twinning, symmetry development, and this group of malformations might share causai elements at least some of which are heritable. The problem here is the absence of zygosity differences in these relationships, because of which they cannot be explained within the biology of twinning as it has been understood. A potential resolution is offered by way of a mechanism common to MZ and DZ twinning, involving a relationship between oocyte organization and the determination of body symmetries.