Birthweight was measured on 188 monochorionic monozygotic, 54 dichorionic monozygotic, 102 like-sexed dizygotic, and 94 unlike-sexed dizygotic liveborn twin pairs. Overall, males were found to be significantly heavier than females. These differences were not significant, however, when birthweights were compared within zygosity/chorion-type categories. Males were also characterized by a slightly greater overall total variance. Comparisons of intrapair variation of monochorionic and dichorionic monozygotic twins revealed significant differences between monochorionic pairs and dichorionic separate pairs and no significant differences between monochorionic pairs and dichorionic fused pairs. The results of this study suggest that placental proximity may have as important an influence on variation in birthweight as does the presence or absence of vascular anastomoses.

A preliminary analysis of twins or triplets with heart defects, ascertained in five centres, confirms earlier suggestions that monozygotic (MZ) twins are over represented among twins with heart defects, even after excluding persistent ductus arteriosus and conjoined twins. An MZ twin individual has a risk of cardiovascular malformation approximately twice that of DZ twins and singletons. It is suggested that the twinning process itself affects one of the pair. Disturbance of laterality (‘mirror imaging’) is probably a more important mechanism than twin-twin transfusion. Inappropriate use of the twin method in the past has caused the importance of genetic factors in the etiology of congenital heart defects to be underestimated. Neverthless, twins do provide a useful illustration of the likely importance of epigenetic factors in heart development.

The malformative syndrome with cryptophthalmos is characterized by cryptophthalmos or congenital symblepharon of the upper lid, dyscephalia with malformations of the ears, nose and mouth, syndactylia, malformations of the urogenital organs and other less constant anomalies.

The population-based twin registries of Denmark, Finland, Norway and Sweden represent an extraordinary resource for scientific research. Although each register has its own history and composition, they share certain common qualities. All the Nordic countries have a long tradition of population registration, a high standard of living and health-related registers of high quality. The large size of the registers means that they are uniquely placed for representative studies of rare occurrences. Examples of studies that these registers make possible are illustrated with data from the Finnish Twin Cohort, which in its first phase consisted of over 17,000 like-sexed twin pairs born before 1958. It has been recently expanded to include multiple births between 1958 and 1986 (nearly 23,000 sets) and their first-degree relatives. During this period the DZ/MZ ratio decreased, while an increase in MZ twinning rates was observed between 1974 and 1986.

The classical twin study method was used to assess the relative contributions of genetic and environmental components to individual variation in several aspects of cognitive functioning. Tests of logico-mathematical concept formation, as well as vocabulary, nonverbal reasoning, and visual memory, were administered to 137 MZ and 72 DZ, same-sex white twin pairs. These children were individually tested on the Piagetian Mathematical Concepts Battery (PMCB), Peabody Picture Vocabulary Test (PPVT), Raven Coloured Progressive Matrices (PM), and a Visual Memory (VM) test. The Attitudes Toward Education (ATE) questionnaire and the Moos [63] Family Environment Scale (FES) were used to collect additional data from the parents. Twins were 4 to 8 years old, with a mean age of 71 months, and most were from middle- and upper-middle-class families. Zygosity was determined from dermatoglyphic information and responses to a questionnaire asking mothers about twin similarities and confusion between the twins by others. These data were analyzed by a simple pair concordance procedure and by a discriminant function analysis. In addition, blood typing was done on 32 pairs for whom zygosity was not possible to determine by these methods.

Previously reported patterns of intercorrelations among the 10 subscales of the FES, as well as the subscale structure, were verified by factor analysis. A factor analysis of the ATE yielded three factors: Basic Academic Education, Parental Participation, and General Utility of Education. These factors correlated significantly (P < 0.01) with various environmental indices (including father's occupation and education, Achievement Orientation, Expressiveness, etc). A factor analysis of the PMCB tasks gave some support for the existence of Piaget's underlying concepts of conservation of number, seriation, and classification.

No sex differences were found for any of the specific cognitive abilities or any of the environmental variables. Correlations with age were substantial: 0.75 for PMCB, 0.70 for PPVT, 0.59 for PM, and 0.43 for VM. Because of the high correlations with age, the effect of age on these variables was partialed out in all further analyses. PMCB correlated most highly with PM (r = 0.41), and with PPVT (r = 0.36). Nonverbal reasoning and vocabulary were relatively independent of each other (r = 0.23). Correlations between visual memory and all other tests were low.

MZ and DZ intraclass correlations for height and weight were similar to values reported in other studies. After correcting for test reliability, significant genetic variance (P < 0.01) was found for both PMCB and PPVT, and was suggested for VM. Genetic variance for PM was not significant (P > 0.05). Correction for reliability could not be employed in this case because an accurate estimate of the PM test–retest reliability is not available. There was no significant effect of age on the magnitude of the MZ or DZ intraclass correlations.

A stepwise multiple regression on the environmental variables was performed for each cognitive test. The environmental variables considered were number of siblings, parental education and occupation, the 10 FES subscales, and the three ATE factors. Age was entered first in the regression equation for each test, and it accounted for 18% to 57% of the total variance in cognitive performance. Parental education accounted for 3% of the total variance in both PMCB and PPVT performance. This was considered as an environmental influence, but the possible confounding with a genetic element in parental IQ was discussed. Achievement Orientation exhibited a significant negative relationship (R2 = 0.02) with PM performance. Cohesion in the Family was positively related to PPVT performance (R2 = 0.02). In addition, Intellectual–Cultural Orientation predicted VM performance (R2 = 0.02). Overall, those environmental variables found to have a small effect suggest the value of a warm, stimulating, supportive (but not “pushy”) family environment for normal cognitive development in young children.

Examination of the genetic and environmental results indicated that 49% of the variance in age-corrected PMCB performance was accounted for by the genetic variance (estimated from twin comparisons) and parental education. Similarly, variables identified in this investigation accounted for 60% of the variance in age-corrected PPVT performance, 29% of the age-corrected PM performance, and 32% of age-corrected VM performance.

In conclusion, this was the first large twin study to find both genetic and environmental influences on the development of Piagetian logico-mathematical concepts and other specific cognitive abilities. The results illustrate the feasibility of investigating cognitive development in a theoretical framework such as Piaget's.

An examination of ascertainment bias in identification of twin pairs in the Vietnam Era Twin Registry has been conducted. A complete listing of all male-male Vietnam era veteran twin pairs born in Connecticut between 1939 and 1955 was obtained (N = 150). An attempt was made to match these pairs with a listing of Vietnam era veteran twin pairs derived from the United States Department of Defense's Defense Manpower Data Center (DMDC) computer files. The results indicate that the DMDC files identified only 46.7% of the 150 Connecticut born Vietnam era veteran pairs. Statistically significant differences (P < 0.05) between pairs found on the DMDC files and Connecticut veteran pairs missing from the DMDC files are observed for the following variables: a) year of discharge from military service, b) total length of active military service, c) branch of service, and d) foreign service. No consistent pattern of bias is observed for factors related to the physical and psychosocial healt of veteran pairs. The implications of the ascertainment biases in the Vietnam Era Twin Registry are discussed.

Fifteen cases were reviewed over a five-year period at a perinatal centre with intrauterine demise of one member of a multiple gestation. Nine cases were monozygotic twin pairs, two were dizygotic, and two were triples. Gestational age ranged from 27 to 39 weeks. The management protocol consisted of delivery in all cases after confirmation of the diagnosis. In 4 cases delivery was immediate because of spontaneous labor. In the other cases elective delivery was performed if the gestational age was 37 weeks or greater or there was evidence of preeclampsia or if amniocentesis revealed a mature lecithinsphingomyelin (L/S) ratio. Steroids were given if the L/S was immature or the attempt at amniocentesis was unsuccessful and delivery was performed 48 hours after initiation of steroid therapy. Cesarean section was the mode of delivery in 14 of the 15 cases. All of the cotwins and cotriplets survived. One survivor of a monozygous twin pair has multicystic encephalomalacia possibly implicating perinatal arterial occlusion or in utero disseminated intravascular coagulation (DIC). The intrauterine deaths are categorized into possibly avoidable deaths (2/15), unavoidable due to congenital anomalies (3/15), and unknown or unavoidable deaths (8/15).

We examined the placentas of 182 like-sexed live-born twins: 73 placentas (40.1%) were monochorionic and 109 (59.9%) were dichorionic. All twin pairs with monochorionic placentas were monozygotic (MZ), but 28.9% of pairs with dichorionic placentas were MZ. Analysis of birth weights demonstrated that dichorionic and dizygotic (DZ) twins were heaviest, and suggested that the chorion status is a more important determinant of birth weight than zygosity. Vascular anastomoses were identified only in monochorionic placentas and occurred in 79.5% of cases. All placentas with deep anastomoses had superficial anastomoses. A higher proportion of velamentous and marginai insertions of the umbilical cord in monochorionic placentas (27.4%) compared to dichorionic placentas (13.8%) supports the belief that lateral placental growth is greatest in twin gestations in which the embryos are initially most closely apposed — The theory of trophotropism.

During 1967-79 the population-based Medical Birth Registry of Norway registered 7,660 twin pairs (1% of births) born to 7,596 mothers, who gave birth to 6,608 additional infants (twin siblings). The total rate of malformations among twins (278.1/10,000) was not significantly different than among singletons (302.1/10,000), nor among twin siblings (314.8/10,000). By specific type of defect, twins had significantly higher rates than singletons of central nervous system (CNS) defects (Rate Ratio = 1.8) and cardiovascular defects (RR = 1.5). The twins also had a significantly low rate of congenital hip dislocation (RR = 0.4), which may explain the relatively low incidence of malformations in twins. Like-sex (LS) twins had a slightly higher rate of malformations than unlike-sex (US) twins (RR = 1.1), as well as a higher rate of CNS defects (RR = 3.0). The siblings also had a significantly increased rate of CNS defects compared to singletons (RR = 1.9), but not of cardiovascular defects (RR = 0.9). The results indicate that twins have elevated rates of at least some congenital malformations. The observations about CNS defects suggest common factors that can lead to either like-sex twinning, CNS defects, or both. The increased frequency of cardiovascular defects in twins appears to be associated with the biologic conditions of twinning.

In order to study the influence of heredity on resting metabolic rate (RMR), 20 monozygotic and 19 dizygotic male twin pairs aged 20.6 (SD 2.9) and 21.4 (SD 3.1) years, gave their consent to participate in the experiment. Fat free weight (FFW) was estimated from underwater weighing. RMR was measured by indirect calorimetry using an open circuit system. RMR was expressed as kJ · min−1, kJ/m2 · h−1, kJ/kg · h−1 and kJ/kgFFW · h−1. Significant intraclass coefficients were observed in MZ twins for the different expressions of RMR. The values ranged from r = 0.45 (P < 0.05) to r = 0.81 (P < 0.01). However, DZ twins demonstrated lower intraclass coefficients for RMR, with a range from r = 0.21 to r = 0.44. Significant (P < 0.05) DZ resemblance was revealed only when RMR was expressed as kJ · min−1 and kJ/kg · h−1. Results of the present study suggest that variations in RMR may have a genetic component. Implications for human energy balance and body fat are discussed.

The monozygotic (MZ) twinning rate in Japan had remained nearly constant from 1975 (3.74 per 1,000 births) to 1994 (4.23), whereas the dizygotic (DZ) twinning rate had remained nearly constant from 1975 (1.86) to 1986 (2.27), and had gradually increased up to 1994 (3.89). The higher DZ twinning rate since 1987 has been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to in-vitro fertilisation in Japan. As for maternal age, MZ twinning rates have remained nearly constant for maternal age groups except the youngest and the oldest age groups. On the other hand, DZ twinning rates increased up to the 35-39 years of age group and decrease thereafter. In 1994, the twinning rate was higher in DZ twins than in MZ twins for maternal age groups of 30-34 years and of 35-39 years. As for geographical variations in twinning rates, DZ rates statistically significantly increased with the year during the period from 1986 to 1994 in 31 out of 47 prefectures. In 1994, twinning rates in 21 out of 47 prefectures were higher in the DZ than the MZ rate, and the DZ rate is equal to the MZ rate in two prefectures. Geographical variations in twinning rates by zygosity in 1994 drastically changed from those during the period from 1955-1959 and in 1974.

Mean twin, triplet, and quadruplet birth rates in Japan from 1951 to 1968 were 6.41, 0.056, and 0.00094 per 1,000, respectively. In 1974 the corresponding figures were 5.83, 0.059, and 0.00329. No quintuplets were born in the former period, but a set was born in the latter year, the rate being 0.47 per million. From 1955 to 1966 the MZ twinning rate increased slightly, but decreased thereafter. This increase was limited to live-born MZ twins, particularly in the higher maternal age groups. The DZ twinning rate declined in the entire period, particularly in higher maternal age groups. This decline appeared to be essentially limited to fetal deaths. Among live births the MZ twinning rate underwent a nearly linear increase with maternal age, whereas the DZ twinning rate attained a mode in the maternal age group 35–39 years. The MZ and the DZ twinning rates among fetal deaths by maternal age had unimodal distributions with modes in maternal age groups 25–29 and 30–34 years, respectively. As to the DZ twinning rate, a geographical cline was noted, with a high rate in the northeast of Japan; the rate was positively correlated with latitude, which also positively correlated with the presence of multiple births among relatives. A negative but nonsignificant correlation was seen between the DZ twinning rate and the proportion of mothers treated with ovulation-inducing hormone. The proportion was higher in mothers of unlike-sexed twins than in those of like-sexed twins and in mothers of triplets than in those of twins. An association between DZ twinning rate and age-specific fertility per married woman is suggested to exist among higher maternal age groups in the northeast part of Japan in earlier years.

A preliminary series of quantitative genetic models was applied to a subset of longitudinal height data, spanning birth to maturity, gathered from twin families in the Louisville Twin Study. Descriptive Cholesky factor parameterization was found to give more satisfactory results than did a system of constraints based on a model of developmental transmission of a time-constant and time-specific factors. The results from application of two autosomal sex-limitation models are contrasted with those from a model specifying both autosomal and sex-chromosomal patterns of inheritance. The latter model was more conducive to parameter reduction. Although these models do not constitute conclusive tests of autosomal sex-limitation versus sex-linkage, the more parsimonious model is consistent with previous research suggesting a stature locus on the long arm of the Y chromosome. Heritability of height is estimated at about 90% or greater from 6 years of age on. Substantial and fairly constant longitudinal genetic correlations are found from 3 years of age on. Shared environmental effects unrelated to parental height were seen for birth length, corrected for gestational age, to height at 3 years of age, but these are not satisfactorily differentiated from possible twin effects in the present sample. The genetic consequences of assortative mating are emphasized since failure to take assortment into account can lead to overestimation of shared environmental effects and under-estimation of genetic effects. The results indicate that about 20% of within-gender variability for mature height can be attributed to the genetic consequences of assortment, even though the phenotypic marital correlation of 0.22 is quite modest. The importance of testing the assumption of multivariate normality underlying the application of the method of maximum-likelihood is also highlighted.

It is proposed that transgenerational modulation of gene expression might be possible, if the metabolic response of the parent to some physiological or social stress modified imprint setting. Transcription regulators could theoretically mediate this process. The nature of imprinted genes poised, as it were, between a transcriptionally active and silent state, makes them good candidates for incorporation into the evolution of transgenerational adaption systems where coordinated changes in gene expression over the generations is a selective advantage. The coordination of human fetal (head) growth with the existing size of the mother's pelvis is suggested as just such a circumstance. The reduced birth weight of Dutch babies where their grandmothers suffered acute starvation in mid pregnancy, supports the notion of transgenerational adaption to nutrition, as does the secular change (increase) in child growth over the last century. The recent indication that there may be functional polymorphism in the imprinting of the human IGF2 and IGF2R genes suggests these ideas could be explored using association studies at the population and individual level.

The mothers of a consecutive series of 125 same sexed pairs of twins aged 6 months to 6 1/2 years completed a mailed questionnaire concerning the physical similarity of the twin partners. Zygosity diagnoses were first determined blindly on the basis of the answers given by the mothers, who did not know the result of the blood group test when fullfilling the questionnaire. Next, the results were compared with the zygosity determination based on examination of erythrocyteserum- and enzyme-groups. It turned out that a few simple questions distinguished well enough between MZ and DZ pairs, with a frequency of misclassification of 4%, leaving 5% as unclassifiable. It is concluded that twin zygosity can be estimated by a simple questionnaire with sufficient accuracy even in very young twin pairs. This has relevant implications for more extensive twin studies where the use of a wide spectrum of serological characters would imply too high expenses.

Secular changes in twinning and triplet rates were analyzed using vital statistics in Austria, the Czech Republic, the Slovak Republic, England and Wales, Germany, the Netherlands, Switzerland, Denmark, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. Among those 17 countries, the twinning and triplet rates in the Czech Republic and in the Slovak Republic remained constant from 1972 to 1994, whereas these rates increased significantly year by year in the other 15 countries during the examined period in each country. Twinning rates increased from 1.2-fold in Austria to 2-fold in Denmark from 1972 to 1996. As for triplets, the rate increased from 3-fold in Denmark to 9-fold in Norway during that period. With one exception, that being the Slovak Republic, the triplet rate was highest in the Scandinavian countries, followed by the other European and Asian countries. The rising twinning and triplet rates have been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to IVF.

Trends of twinning rates were analyzed using vital statistics in Austria, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. The twinning rates increased significantly year by year in each country. During these periods, the twinning rate increased by twenty percent in Austria and Canada, and by sixty percent in Norway and Sweden. The twinning rate was 1.6 times higher in Sweden than in Hong Kong in 1972 and by eighty percent in Israel in 1995. Twinning rates were higher in European countries, Canada, and Australia than in Asia. The variations of the twinning rates among countries were not only due to biological factors, but also to assisted reproductive techniques. In Australia, the overall twinning rate was 1.3 times higher in the nuptial births (14.1 per 1000 births) than in the ex-nuptial births (10.7) during the period 1994-1996. As for maternal age, twinning rates in Sweden increased year by year for maternal age groups except the youngest and the oldest age groups. In Sweden, the rising twinning rate has been attributed to the higher proportion of mothers (for the 25-39 year old age groups) treated with ovulation-inducing hormones and attributed to in-vitro fertilization.

This study investigated the degree of risk of handicap in twins, triplets, quadruplets and quintuplets and associated factors, and examined the clustering tendency of handicaps. The sample was recruited from the Kinki University Twin and Higher Order Multiple Birth Registry. This panel consisted of 705 pairs of twins (1410 twins), 96 sets of triplets (287 triplets excluding 1 infant death), 7 sets of quadruplets (27 quadruplets excluding 1 infant death), and 2 sets of quintuplets (10 quintuplets), all of whom were born after 1977. The incidence of handicap was 3.7% in twins, 8.7% in triplets, 11.1% in quadruplets, and 10.0% in quintuplets. The risk of prodicing at least 1 handicapped child was approximately 1 in 13 pairs of twins (7.4%), 1 in 4 or 5 sets of triplets (21.6%), and 1 in 2 sets of quadruplet and quintuplets (50%). There was a significantly higher clustering tendency of handicaps in twins and triplets compared with the expected frequency calculated from the incidence rate of handicap. Four significant risk factors for subsequent handicap were found by logistic regression: gestation number, shortening of gestational age, premature rupture of the membrane, and toxemia during pregnancy.

Respiratory sinus arrhythmia (RSA) has been shown to be a sensitive index of vagal cardiac control. We studied the genetic and nongenetic influences on individual differences in RSA in a sample of 160 adolescent twins. RSA was measured during rest and across two different tasks. Results show that heritability is task dependent. The amount of genetic variance is the same, however, during rest and task conditions. Because nonshared environmental variance decreases during tasks, heritability is larger for RSA measured under more stressful conditions than for RSA as measured during rest. Multivariate models assessed the continuity of the genetic and environmental influences and show genetic influences to be the same across different conditions, while environmental influences are different. More specifically, a one-factor model is found for genetic influences and a second-order autoregressive model for the environmental factors.

A study has been made of 35 patients belonging to 20 families, all diagnosed as Usher's syndrome (retinitis pigmentosa and deafness). The results indicate that there are four clinical types, which have been called Types I to IV. Genetically, they represent at least two, and possibly three or four, separate entities.