Although it is currently believed that the Palestinian population has relatively high twinning rates, to our knowledge no specific studies on this have yet been conducted. Therefore, and as a first step in our institutional program to develop medical and social care, as well as research programs specifically directed to twins in this area of the world, a number of observations have been conducted in order to gather estimates of twinning rates in the Palestinian population of East Jerusalem and the West Bank of the Jordan. This corresponds to the regions of Judea and Samaria, largely composed of small villages where most of the population lives, as well as a number of more important centers, such as Jenin, Nablus, Ramallah, Bethlehem, and Hebron.

Although we have, unfortunately, been unable to obtain vital statistics on the entire Palestinian population of the West Bank from the administrative offices of Judea and Samaria, information on total maternities and twin maternities by sex, for the period 1984-1988, was kindly provided by all Arab hospitals and maternity clinics in Jerusalem, and also by the United Nations Refugees World Agencies (UNRWA) with respect to the refugee camps.

The population we have studied, therefore, is limited to the Palestinians of East Jerusalem and the West Bank refugee camps, and corresponds to about one-third of the entire Palestinian population of the area. However, it does include both an urban setting (East Jerusalem) as well as a rural one (the refugee camps), and both Moslems and Christians, so it can be safely considered as representative of the entire Palestinian population. As for the period considered, we elected to limit our survey to the five-year period 1984-1988 because the records for then are considered to be more reliable than they had been previously, when little, if anything, was known on statistics, particularly on twin statistics relating to the West Bank population.

The rate of multiple births and the incidence of congenital anomalies in Tokyo Metropolitan Hospitals were studied during the period 1979-1990. The number of twins was 968 pairs (8.23 per 1,000 deliveries) and of triplets 18 sets (15.3 per 100,000 deliveries) among 117,672 deliveries including 1,587 stillbirths after 16 weeks gestation. Multiple birth rates increased yearly. Stillbirth rates in twins and triplets were 5.5% and 16.7% respectively, which were both significantly higher than that in singletons (1.3%). The number of congenital anomalies was 42 in 1,936 twins (2.17%), 2 in 54 triplets (3.7%) and 1721 in 116,686 singletons (1.47%). The most common defects in twins were those of the cardiovascular system (0.72% in twins vs 0.52% in singletons) and of the musculoskeletal system (0.72% in twins vs 0.50% in singletons), followed by upper respiratory tract and/or mouth conditions (0.67% in twins vs 0.35% in singletons), all of which had no significant difference in frequency between twins and singletons. Though some anomalies had a significantly higher frequency in twins than in singletons, the concordance rate in the like-sexed twins was very low.

Intrauterine death of one fetus in a multiple gestation is associated with significant morbidity and mortality in the surviving infant. This study is a retrospective review of 38 twin and 3 triplet gestations involving the intrauterine death of at least one fetus. The obstetrical history, placental pathology, autopsy findings, and neonatal history of the surviving infant are reviewed. Three cases involved the recent stillbirth of both twins, the remaining cases involved a surviving infant. In one case, neonatal death of a surviving twin occurred on day 19. In two sets of triplets, two stillbirths occurred, in the third case two infants were liveborn. The incidence of preterm delivery was 34%, which decreased to 18% if fetal cotwin death had occurred before 20 weeks gestation. Cesarean section was the method of delivery in 16 cases. There was an excess of velamentous cord insertions, which was most pronounced in the stillborn twin. Monochorionic placentation was found in 72%, also an excess.

Neurological damage was known to have occurred in 19 of the 39 surviving infants. Fifteen of these 19 (79%) were associated with monochorionic placentation. The neurologically damaged twin infants, when compared to the normal infants, had the cotwin die later in gestation (31 vs 16.5 weeks), had a shorter duration between the death of the cotwin and delivery (2.5 vs 21 weeks), and delivered earlier in gestation (36.5 vs 39.5 weeks). The probable cause of neurological damage, in our opinion, was either exsanguination into the dead twin fetus, or disseminated intravascular coagulation which occurred in at least 13 cases. The incidence of antepartum death in a multiple gestation, and the potential for neurological damage is probably higher than previously thought. A review of the literature is presented and the clinical implications of this phenomenon are discussed.

Familial aggregation for disease is important; strong familial risk factors must exist even if the increased risk to a relative of an affected individual is modest. It is in practice difficult, however, to conduct studies in genetic epidemiology which conform to strict epidemiological principles. For twin studies there are two major questions: Are twins ‘no different’ from the population on which inference is to be made? Are study twins ‘no different’ to twins in the population? The importance of each question of bias depends on the scientific question, the trait(s) studied, and sampling issues. The strength of the twin design is its ability to refute the null hypothesis that genetic factors do not explain variation in a trait. Following the Popperian paradigm, alternate hypotheses should be considered in depth (both theoretically and empirically), with a design and sample size sufficient to exclude not just naive explanations. More sophisticated statistical techniques are now being applied, so the philosophy, assumptions, and limitations of statistical modelling must be appreciated. The concept of ‘heritability’ has, in the past, been misunderstood and misused. New advances in DNA technology promise to revolutionise epidemiological thinking, and so case-control-pedigree designs may well become standard tools. The strengths and limitations of studies based on related individuals as the sampling unit are discussed.

In a previous Swedish twin project (the SLU-project), approximately 300 MZ and DZ twin pairs and controls were followed through the Swedish compulsory school from grade 3 to grade 9. Results from this study indicated an increase of genetic influences on school achievement over time for children from a permissive home environment and a decrease for children from a restrictive home environment. These types of data have generated a more general model for studying heredity-environment interaction in educational settings. To test this model, a cross-cultural comparison over time of twins and controls in the Israeli kibbutz school and in the Swedish compulsory school has been made. Restrictions on the child were originally assumed to be more apparent in the kibbutz environment.

The International Society for Twin Studies is a unique organization. It is unique in the status of members composing the Society. In addition to specialists of science and medicine we have non-specialist members including twins and their families, all enjoying excellent collaboration in undertaking research and related activities of common interest. This kind of collaboration is very important for academic societies, particularly for those devoting themselves to the studies of man. We aim at a better understanding of man; our purpose is to learn more about ourselves. This activity is not a monopoly of specialists. Our achievements should also be shared by lay people. ISTS can serve as a model for academic societies aiming at the studies of man. Science has the solid ground of logos. At the same time, we realize that the accomplishments of highly specialized science should be reevaluated in the light of human values. I am inclined to believe that our task is to integrate the logos of scientists and ethos of the public into one.

The purpose of twin studies is two-fold; studying multiple birth phenomena per se, and making inquiry into the genetic and environmental sources of human variation. The latter task was initiated by a number of pioneers in the early twentieth century. At that time, however, genetic studies of man including twin studies remained largely descriptive. We could not, and still cannot, perform genetic experiments on human individuals and populations, and the methodology available was limited to observation of a character presented in pedigree members and twins. However, despite the limitation of methodology, classical pedigree analysis has permitted us to accumulate a large amount of data on normal hereditary traits and genetic diseases, or more precisely, simple Mendelian characters.

We describe a rare case of acardius in a triplet pregnancy terminated by Caesarean Section at 32 weeks gestation. Morphological and chromosomal abnormalities of the fetus as well as structural abnormalities of the placenta are presented. Cytogenetic analysis and examination of the single disc triplet placenta provide evidence for the two major theories of pathogenesis of acardius, the twin reversed arterial perfusion (TRAP) sequence and the genetic theory, which we believe are not necessarily mutually exclusive.

A reanalysis of the data presented in a recent large twin study suggests that opposite-sexed (OS) pairs may be not as exhaustively tested as same-sexed (SS) pairs on genetic markers. This is contrary to an assumption I made in estimating that there are about 8 SS dizygotic pairs to every 7 OS pairs (thus impugning Weinberg's differential rule). If this assumption is false also in regard to the samples I discussed, then that estimate is unsound and Weinberg's rule is unscathed by empirical data. However, regardless of such considerations, there are strong theoretical reasons for questioning the status of Weinberg's rule. It is based on two assumptions, namely that p (the probability that a dizygotic twin zygote is male) is equal and independent for all dizygotic twin zygotes. Data are adduced here to suggest that both assumptions are false. The upshot is that, at present, without testing, we cannot know, of any given population, whether the rule holds or not. Otherwise, though Weinberg's rule may be useful (like Hellin's law) as a rule-of-thumb, it cannot be assumed as a basis for serious scientific argument.

Reading and mathematics performance data from a sample of 264 reading-disabled twin pairs and 182 matched control twin pairs were subjected to multivariate behavior genetic analysis. The factor structure of reading and math performance measures was found to be highly similar for both groups. Consistent with previous findings obtained using alternative methods, a significant heritable component to individual differences in reading performance was found both within the reading-disabled (h2 = 0.78) and control (h2 = 0.74) twin samples. In addition, a substantial genetic influence on mathematics performance was found (h2 = 0.51 and 0.60 in the reading-disabled and control samples, respectively), although shared environmental influences common to both members of a twin pair also contribute significantly to the variance in math scores of both groups (c2 = 0.44 and 0.37). Moreover, genetic influences accounted for 98% of the observed correlation between reading and math performance within the sample of reading-disabled twin pairs, and for 55% of the observed correlation in the control sample. Thus, individual differences in both reading and mathematics perfomance are highly heritable and appear to be caused by many of the same genetic influences.

This follow-up study of twins includes 35 families from the Stockholm area. One section concerns the physical and mental development of the twins. The other sections compare the development of the children with their parents' expectations of them and the parents' feelings about the delivery. Seven twin pairs (20%) in the total group were born before 37 complete gestational weeks. Thirty-seven of the twins weighed less than 2,500g at birth; of these, 23 (9 boys and 14 girls) weighed less than 2,500g, although they were born after 37 gestational weeks. At four years of age, 22 of the 68 twins still showed some impairment of locomotor, language/speech and emotional development, as assessed by the Griffiths Mental Development Scales. All the prematurely born and small-for-date infants were in this group. There also seems to be a relationship between developmental problems and the mother's negative or ambivalent expectations concerning the twin pregnancy.

During the years 1964 to 1971 a group of twins and a control group of singletons in the same classes, were followed from grade 3 at 10 years of age to grade 9 at 16 years of age in the Swedish compulsory school. The study was called the SLU-project (Skolöverstyrelsens och Lärarhögskolans Utvecklingsprojekt) and the main purpose was to study physical and mental growth during puberty as well as heredity and environment influences on these growth processes. Originally 323 twin pairs, MZ and DZ, and 1193 controls were included in the sample. Among the DZ twin pairs there were both same-sex and opposite-sex pairs. They were a nationally representative sample. Several kinds of information were collected, such as, a) Physical measurements (height and weight measurements, menarche, ratings of secondary sex characteristics); b) Ability and achievement measures (intelligence tests, standardized achievement tests); c) Self ratings; d) Ratings by teachers and classmates; e) Socioeconomic background data (fathers occupation and income). For the boys, supplementary data on physical growth and mental ability was collected on enrollment to military service at the age of 18. In this pilotstudy, we have made a follow-up of the original SLU-sample after 20 years. They are now in their mid-thirties. The main purpose has been to investigate if they are willing to participate in a new study on health and well-being at mid-life.

A newly discovered case of heteropaternal superfecundation (HS) is reported. Three HS cases were found in a parentage test database of 39,000 records. The frequency of HS among dizygotic twins whose parents were involved in paternity suits is 2.4%. Although the study population appears similar to the general population with respect to twinning data, inferences about the frequency of HS in other populations should be drawn with caution.

A nine-year-old girl with short stature was referred to the department of pediatrics at Kyushu University. The clinical diagnosis was Turner syndrome; karyotypic analysis performed on peripheral blood, using GTG techniques, demonstrated a 45,X/47,XYY (17:83) mosaicism. Her twin brother, a phenotypically normal male, had the same karyotype; 45,X/47,XYY (3:97) on peripheral blood. Their skin fibroblast karyotypes showed the same mosaicism, ie. 45,X/47,XYY (41:59 and 31:69 respectively). On eleven biochemical genetic markers the twin pair were concordant, thus the likelihood of monozygosity was 0.99527034. In addition, the analysis of variable number of tandem repeat (VNTR) markers revealed the likelihood of monozygosity to be 0.99944386. The most plausible explanation of the X/XYY mosaicism was nondisjunction of the Y in the first cleavage division of the 46,XY zygote. A disproportionate rate of cell populations with 45,X and 47.XYY in the twinning process of the X/XYY embryo, especially in the germ lines, would result in discordant sex in twin pairs.

Interindividual variation in odor identification and olfactory sensitivity has been explained primarily with reference to age, sex and/or experiential factors. A twin study of olfaction can, therefore, substantially contribute to current research in this area. Thirty-nine monozygotic and twenty dizygotic twin pairs have completed the University of Pennsylvania Smell Identification Test (UPSIT), an olfactory preference questionnaire, and two odor detection threshold tests (phenyl ethyl alcohol and butanol). A genetic influence on odor identification, as assessed by the UPSIT, has been demonstrated. Future plans and directions for this research program are discussed.

A case report is presented with a prolonged interval between delivery of 25 days. A quintuplet pregnancy resulted from hormonal stimulation of ovulation. Two fetuses remained after an embryo reduction was performed at 11 weeks gestation. At 22.5 weeks gestation the first twin (310 g) was delivered after spontaneous rupture of membranes. Using tocolytic agents, the second twin (710 g) was born at 26 weeks of gestation. This case is discussed and a review of the literature is given.

In 1988, questionnaires were received from 1,400 twin pairs (17% MZM, 23% MZF, 17% DZM, 19% DZF, 24% DZO) aged 11 to 18, registered with the Australian NHMRC Twin Registry. Twins reported independently on themselves and on the perceived behaviour of their parents, siblings and friends. For smoking and for drinking in the previous month, the prevalence was modelled as a logistic function of age, sex, perceived smoking or drinking behaviour of family and friends, and the Junior Eysenck Personality Questionnaire (JEPQ) scales. Strenghts of association were: family behaviour, odds ratio (OR) ≤2; Extraversion and Psychoticism, interquartile OR ≈ 1.6; behaviour of friend, OR ≈ 3 to 6. Twin associations were represented by odds ratios. For smoking they were 16 in MZ and 7 in DZ same-sex pairs, and 3 in DZO pairs. Although the former is consistent with genetic factors determining adolescent smoking behaviour, the reduced association in DZO pairs and strong association with smoking by friends argue to the contrary. For drinking, twin odds ratios were 11 in MZM, MZF and DZF pairs, and 4 in DZM and DZO pairs, consistent with genetic factors determining alcohol use in male but not female, adolescents. Twin odds ratios were not influenced by adjustment for the JEPQ scales; this does not support the hypothesis that genetic factors which determine personality also determine smoking or drinking behaviour during adolescence.

The laterality effects in 10 symmetrical EEG derivations in twins (20 MZ and 20 DZ pairs with a mean age of 20.5 years) were examined. The quantitative and qualitative analyses gave the following results: (1) cotwins in the MZ and DZ pairs differed particularly in the intensity of asymmetry for EEG parameters — one was more asymmetrical than the other; (2) among the MZ twins there were no “mirror” pairs (opposite asymmetry of the EEG), even where opposite-handedness existed. For example, a right-handed twin had an asymmetrical EEG, while the other, a left-hander, had a symmetrical one; (3) the most asymmetrical EEG was in the temporal derivations showing a more active left hemisphere; and, (4) there was no evidence of genetic influence in the intensity of EEG asymmetry.

This study compares standardized measures of childhood behavior problems in a community-based twin sample with those for normative samples from the general population. Maternal parent ratings on the Child Behavior Checklist (CBCL) for 1824 twins were compared with the CBCL normative sample. The results indicated that twins showed small but consistently higher levels of problem behaviors. These elevations were significant for older children on both internalizing and externalizing behaviors; for younger children the elevations were significant for externalizing but not internalizing behaviors.