We describe subject recruitment from the University of Chile School-Age Children Twin Registry (REMEUCHI). The research aim of REMEUCHI is to quantify the impact of genetic and environmental factors on scholastic achievement in a multicausal approach. The Ministry of Education of Chile, in collaboration with the Registry Office, provided the list of possible twin pairs graduated from high school in 2004 in Chile's metropolitan region. From a population of 70,065 school-age children who had graduated from high school, 434 possible twin pairs were analyzed. Of these, 327 were twins reared together (75.3% of the 434 possible twins pairs) and born between 1986 and 1987 in Chile (mean age 18 years), and approximately 8% were not twins despite matches on full name and birth data. The rest of the possible twin pairs were probably twins reared apart, since one member of the pair had moved to study in another region of Chile. Zygosity was determined through questionnaires, maternal reports of twin similarities, and by the hospital records of the twins at the time of birth. Three hundred and twenty-seven pairs were identified, where monozygotic (MZ) and dizygotic (DZ) twins represented 46.8% and 53.2% of pairs, respectively, with a DZ/MZ ratio of 1.14. Considering same-sex MZ pairs, the percentage of female pairs was greater (55.6%) than male pairs (44.4%). When DZ pairs were analyzed, 47.7% were of opposite sex, 20.1% were male pairs and 32.2% female pairs. In Chile, these findings represent a baseline study to contribute to the establishment of a national twin registry in the future.

This study explores the use of an individual's genetic (IGFS) and environmental factor score (IEFS), constructed using genetic model fitting of a multivariate strength phenotype. Maximal isometric and dynamic strength measures, one maximal repetition load (1RM) and muscle cross-sectional area (MCSA) were measured in 25 monozygotic and 16 dizygotic twin pairs. The use of IGFS and IEFS in predicting the sensitivity to environmental stress was evaluated by the association of the scores with strength training gains after a 10-week high resistance strength training programme. Results show a high contribution of genetic factors to the covariation between maximal strength and muscle cross-sectional area (84–97%) at pre-training evaluation. Individual factor scores explained the largest part of the variation in 1RM and other strength measures at pre-training and post-training evaluation respectively. Genes that are switched on due to training stress (gene–environment interaction) could explain the decrease in explained variation over time. A negative correlation was found between IGFS and strength training gains (−0.24 to −0.51, P < 0.05); individuals with a high IGFS tend to gain less strength than individuals with low IGFS. Individual environmental factor scores have lower differential power. The predictive value of the IGFS has potential utility in identifying an individual's susceptibility to environmental stress in a variety of multifactorial characteristics, eg diseases and impairments, and for selection of sib pairs for QTL analyses. Twin Research (2000) 3, 99–108.

The National Academy of Sciences-National Research Council (NAS-NRC) Twin Registry is one of the oldest, national population-based twin registries in the USA. It comprises 15,924 White male twin pairs born in the years 1917–1927 (N = 31.848), both of whom served in the armed forces, chiefly during World War II. This article updates activities in this registry since the most recent report in Twin Research and Human Genetics (Page, 2006). Records-based data include information from enlistment charts and Veterans Administration data linkages. There have been three major epidemiologic questionnaires and an education and earnings survey. Separate data collection efforts with the NAS-NRC registry include the National Heart, Lung, and Blood Institute (NHLBI) subsample, the Duke Twins Study of Memory in Aging and a clinically based study of Parkinson’s disease. Progress has been made on consolidating the various data holdings of the NAS-NRC Twin Registry. Data that had been available through the National Academy of Sciences are now freely available through National Archive of Computerized Data on Aging (NACDA).

In order to investigate a possible association between maternal smoking during pregnancy and twinning, information on 1 096 330 single births and 12 342 twin births in 1983–95 was obtained from the Swedish Medical Birth Registry (MBR). All odds ratios (OR) were estimated after stratification for year of birth and maternal age, parity, and educational level. Smoking women, compared with non-smoking women, were at increased risk of having dizygotic (DZ) twins, but the risk increase was only evident among multiparas. A strong association between previous involuntary childlessness and dizygotic (DZ) twinning (especially in primiparas) was found. The strongest association between maternal smoking and DZ twinning was found among multiparas without any history of involuntary childlessness (OR: 1.35, 95%CI:1.22–1.49), whereas among women who had experienced involuntary childlessness, the opposite was seen (OR: 0.82, 95%CI:0.66–1.00, no difference between parity strata). Weinberg's differential method was used to estimate the number of monozygotic (MZ) twins, and a method of estimating stratified ORs among mothers of MZ twins was presented. No association was found between MZ twinning and maternal smoking (OR: 0.96, 95%CI:0.86–1.07), and no confounding by parity or previous involuntary childlessness was indicated. Several non-causal explanations to the positive association between DZ twinning and maternal smoking among multiparas were discussed, but homogeneity over strata indicated that maternal smoking may be a true risk factor for double ovulation.

Little is known about the extent to which aging trajectories of different body systems share common sources of variance. We here present a large twin study investigating the trajectories of change in five systems: cardiovascular, respiratory, skeletal, morphometric, and metabolic. Longitudinal clinical data were collected on 3,508 female twins in the TwinsUK registry (complete pairs:740 monozygotic (MZ), 986 dizygotic (DZ), mean age at entry 48.9 ± 10.4, range 18–75 years; mean follow-up 10.2 ± 2.8 years, range 4–17.8 years). Panel data on multiple age-related variables were used to estimate biological ages for each individual at each time point, in linear mixed effects models. A weighted average approach was used to combine variables within predefined body system groups. Aging trajectories for each system in each individual were then constructed using linear modeling. Multivariate structural equation modeling of these aging trajectories showed low genetic effects (heritability), ranging from 2% in metabolic aging to 22% in cardiovascular aging. However, we found a significant effect of shared environmental factors on the variations in aging trajectories in cardiovascular (54%), skeletal (34%), morphometric (53%), and metabolic systems (53%). The remainder was due to environmental factors unique to each individual plus error. Multivariate Cholesky decomposition showed that among aging trajectories for various body systems there were significant and substantial correlations between the unique environmental latent factors as well as shared environmental factors. However, there was no evidence for a single common factor for aging. This study, the first of its kind in aging, suggests that diverse organ systems share non-genetic sources of variance for aging trajectories. Confirmatory studies are needed using population-based twin cohorts and alternative methods of handling missing data.

The objective was to investigate the genetic epidemiology of figural stimuli. Standard figural stimuli were available from 5,325 complete twin pairs: 1,751 (32.9%) were monozygotic females, 1,068 (20.1%) were dizygotic females, 752 (14.1%) were monozygotic males, 495 (9.3%) were dizygotic males, and 1,259 (23.6%) were dizygotic male-female pairs. Univariate twin analyses were used to examine the influences on the individual variation in current body size and ideal body size. These data were analysed separately for men and women in each of five age groups. A factorial analysis of variance, with polychoric correlations between twin pairs as the dependent variable, and age, sex, zygosity, and the three interaction terms (age x sex, age x zygosity, sex x zygosity) as independent variables, was used to examine trends across the whole data set. Results showed genetic influences had the largest impact on the individual variation in current body size measures, whereas non-shared environmental influences were associated with the majority of individual variation in ideal body size. There was a significant main effect of zygosity (heritability) in predicting polychoric correlations for current body size and body dissatisfaction. There was a significant main effect of gender and zygosity in predicting ideal body size, with a gender x zygosity interaction. In common with BMI, heritability is important in influencing the estimation of current body size. Selection of desired body size for both men and women is more strongly influenced by environmental factors.

New legislation giving parents a voice in deciding their twins' school placement situation (same or separate classrooms) was passed in Minnesota in spring 2005. Since then, parents in several other states have started petitions and lobbying efforts to persuade their senators and representatives to follow suit. A review and update of these exciting developments is presented, followed by a report from the recently established Asian Society for Twin Studies. Reviews of two new twin studies — bases of political attitudes and age effects on dizygotic twinning — are presented. The uniquely informative lives of several twin pairs are also documented.

The presence of situs inversus totalis (full reversal of internal organs) in twins is briefly reviewed. Information gathered from 35-year-old monozygotic (MZ) female twin pair discordant for this condition is presented. This is followed by summaries of research on the frequency of trisomy 21 (Down syndrome) in twins, the first case of MZ twin concordance for bilateral coronoid hyperplasia, prenatal hormonal effects in mixed-sex non-human primate litters, and links between insurance mandates and twinning following in vitro fertilization. The final section of this article describes twin-related events reported in the news, namely, the first recorded birth of identical twin puppies; the 100th birthday celebration of a pair of fraternal female twins, the passing of an award-winning identical twin production designer, and the first running of the New York City Marathon by a set of quadruplets.

The Boston University Twin Project (BUTP) uses a multimethod, longitudinal approach to study the role of genetic and environmental factors on the development of child temperament and related behaviors in early childhood. There are two phases in this project. The first, described in the previous Twin Research and Human Genetics special issue on twin registries, focused on activity level and comprised over 300 twin pairs assessed in the home and laboratory at ages 2 and 3. In this article, we describe subject recruitment, sample characteristics, and study procedures and measures of the second phase of the BUTP. This recent study focuses more broadly on the development of multiple temperament dimensions and explores associations between temperament trajectories, parenting and child adjustment in a new cohort of approximately 300 twin pairs assessed at 3, 4 and 5 years of age.

We report the process of adaptation into Sinhala of a questionnaire given to mothers of twins to determine zygosity. Adaptation and validation was carried out in three stages. Firstly, we used a nominal group to translate the English version and to assess the extent of agreement (consensus measurement) on the appropriateness of the translation and resolve disagreement (consensus development). Secondly we used a qualitative interview with 25 mothers of twins. The three main stems of the translated questionnaire were used as a semi-structured interview, and the responses noted verbatim. These were categorised and analysed, and the translated full questionnaire was then presented as closed questions with fixed choice responses. The categorised responses generated during the qualitative interview were compared with the responses to the fixed choices in the full questionnaire. The third stage was the appraisal of the questionnaire by 17 bilingual parents of twins. The source and translated version of the questionnaire were given to them at least 3 days apart. The responses were rated and the total scores were computed to determine the zygosity. This step was carried out to measure the validity and reliability of the Sinhala version. A perfect correlation between the original and adapted version was obtained, with a kappa of 1. The results suggest that the Sinhala version of the questionnaire is conceptually equivalent to the original questionnaire. Comparison of the zygosity determination by using this adapted questionnaire with results from analysis of genetic markers on Sri Lankan twins is needed for final validation of the translated questionnaire. Twin Research (2000) 3, 205–212.

It has been assumed that sexual activity during pregnancy would lead to an increased risk for miscarriage and other complications of pregnancy. Various studies showed no association of sexual behavior and pregnancy complications in single pregnancies. The aim of our study was to evaluate changes in sexual activity in women with twin pregnancies and whether a higher frequency of sexual intercourse was associated with an increased risk for pregnancy complications. We report on 50 women with twin pregnancies who answered a questionnaire on sexual behavior during pregnancy. At the time of delivery, patients were aged 31.0 ± 4.8 years. All women reported to have had sexual intercourse during pregnancy. In contrast to the first trimester in the vast majority of patients (41/50, 82.0%) the coital frequency decreased in the last month of pregnancy. Patients who had become pregnant after in-vitro fertilization were significantly less likely to have regular sexual intercourse than patients after spontaneous conception during early pregnancy (p = .002). No statistical significances were found when the rates of preterm delivery before the 37th gestational week were compared to the frequencies of sexual intercourse during early pregnancy and during late pregnancy (p >.05). In conclusion, our study demonstrates a decrease in frequency of sexual intercourse from early to late pregnancy in the specific collective of women with twin pregnancies, especially in women after in-vitro fertilization. There was no association between sexual activity and preterm delivery.

The French mathematician Bertillon reasoned that the number of dizygotic (DZ) pairs would equal twice the number of twin pairs of unlike sexes. The remaining twin pairs in a sample would presumably be monozygotic (MZ). Weinberg restated this idea and the calculation has come to be known as Weinberg's differential rule (WDR). The keystone of WDR is that DZ twin pairs should be equally likely to be of the same or the opposite sex. Although the probability of a male birth is greater than .5, the reliability of WDR's assumptions has never been conclusively verified or rejected. Let the probability for an opposite-sex (OS) twin maternity be pO, for a same-sex (SS) twin maternity pS and, consequently, the probability for other maternities 1 − pS − pO. The parameter estimates $\hat p_O$ and $\hat p_S$ are relative frequencies. Applying WDR, the MZ rate is m = pS − pO and the DZ rate is d = 2pO, but the estimates $\hat m$ and $\hat d$ are not relative frequencies. The maximum likelihood estimators $\hat p_S$ and $\hat p_O$ are unbiased, efficient, and asymptotically normal. The linear transformations $\hat m = \hat p_S - \hat p_O$ and ${\skew6\hat d} = 2\hat p_O$ are efficient and asymptotically normal. If WDR holds they are also unbiased. For the tests of a set of m and d rates, contingency tables cannot be used. Alternative tests are presented and the models are applied on published data.

The aim of this study was to compare maternal and neonatal outcome of twin births according to mode of delivery and to isolate the factors predicting a successful vaginal delivery and those predicting a failed trial of labor (TOL) leading to an emergent cesarean section. We reviewed all twin deliveries during the years 1995 to 2004. Parameters studied include maternal age, parity, gestational age, maternal antepartum complications and postpartum complications, fetal presentations, birthweight, mode of delivery of each twin, Apgar scores and cord pH. During the study period there were 40,710 deliveries of which 804 (1.9%) were twin deliveries. Of the 804 twins, 398 (49.5%) had planned cesarean sections (PCS) and 406 (50.5%) entered a TOL. Maternal age and parity were similar among the groups. Neonatal outcomes and postpartum complications did not differ between the groups. Of 406 women who had a TOL, 84.9% eventually delivered both twins vaginally. A significantly higher percentage of antepartum complications were noted among those who failed the TOL compared to those with successful TOL (8.2% vs. 1.7%, p =.01). The number of neonates with pH of less than 7.0 did not differ between the groups although more neonates (2.5% vs. 0.4%, p =.05) among the failed TOL had an Apgar score of less than 7.0 at 5 minutes compared to successful TOL. Vaginal delivery of both twins after TOL occurred in 91% of vertex/vertex compared with 71.8% of vertex/nonvertex presenting twins ( p < .01). Neonatal outcomes did not differ between both groups. Our results indicate that both vaginal and PCS are comparable options for vertex presenting first twin regardless of second twin presentation.

We study the situation in which a cheap measure (X) is observed in a large, representative twin sample, and a more expensive measure (Y) is observed in a selected subsample. The aim of this study is to investigate the optimal selection design in terms of the statistical power to detect genetic and environmental influences on the variance of Y and on the covariance of X and Y. Data were simulated for 4000 dizygotic and 2000 monozygotic twins. Missingness (87% vs. 97%) was then introduced in accordance with 7 selection designs: (i) concordant low + individual high design; (ii) extreme concordant design; (iii) extreme concordant and discordant design (EDAC); (iv) extreme discordant design; (v) individual score selection design; (vi) selection of an optimal number of MZ and DZ twins; and (vii) missing completely at random. The statistical power to detect the influence of additive and dominant genetic and shared environmental effects on the variance of Y and on the covariance between X and Y was investigated. The best selection design is the individual score selection design. The power to detect additive genetic effects is high irrespective of the percentage of missingness or selection design. The power to detect shared environmental effects is acceptable when the percentage of missingness is 87%, but is low when the percentage of missingness is 97%, except for the individual score selection design, in which the power remains acceptable. The power to detect D is low, irrespective of selection design or percentage of missingness. The individual score selection design is therefore the best design for detecting genetic and environmental influences on the variance of Y and on the covariance of X and Y. However, the EDAC design may be preferred when an additional purpose of a study is to detect quantitative trait loci effects.

Twin pregnancies discordant for neural tube defects (NTD) is a management dilemma. Risks of preterm delivery from polyhydramnios must be balanced with the risks of selective termination (ST) of the anomalous fetus. We investigated the prevalence of twin pregnancies discordant for NTD and the rate of pregnancy complications in our institution over a 10-year period. Cases were obtained by searching the hospital ultrasound database and findings were confirmed by expert review of ultrasound images. Outcomes of ST and expectant management were assessed. Each unaffected co-twin was assigned to three consecutive twin pregnancy controls matched by chorionicity and maternal age. Primary outcome was birth before 34 weeks’ gestation. Secondary outcomes were small for gestational age, mode of delivery, neonatal unit admission, and neonatal death. In total, 13 pregnancies were identified as potential cases. Of these, 11 were included in the analysis: 9 dichorionic diamniotic and 2 monochorionic diamniotic twins. Seven cases had ST and four were managed expectantly. We found 100% (4/4) of expectantly managed pregnancies delivered <34 weeks compared with 14% (1/7) of the ST group (p = .015). Polyhydramnios complicated three expectantly managed pregnancies and one pregnancy in the ST group. The birthweight SD score of all unaffected co-twins was ≥-2. The case–control analysis showed a higher rate of polyhydramnios in twin pregnancies discordant for NTD compared with controls, but little evidence for differences between groups in delivery rates <34 weeks, birthweight, neonatal unit admission, or neonatal death. ST warrants serious consideration to avoid potential complications to the unaffected co-twin.

With the development of large twin-family registers worldwide (see the December 2006 issue of Twin Research and Human Genetics for an overview) new options for research projects become feasible, including longitudinal studies. In this special issue on Longitudinal Twin and Family Studies, several aspects of longitudinal research are highlighted and an overview of results is given from European, Australian and United States twin studies. These studies deal with a time span from birth to adulthood and a wide range of phenotypes is covered. Data collection in some studies is age/cohort driven; in others, subjects are followed from different ages at regular time intervals. Most studies are based on data collection in twins, but adoption and parent–offspring data designs are described as well. The overview of results of current longitudinal projects makes this special issue a reference resource for longitudinal research in developmental behavior genetics.

According to the Sasang theory, humans can be categorized into one of the four Sasang constitution (SC) types. The four SC types are Tae-Yang (TY), Tae-Eum (TE), So-Yang (SY), and So-Eum (SE), which are determined mainly on the basis of anthropometric characteristics, personality, and the balance of the physiological functions of the major organ systems. There is a growing recognition in the complementary and alternative medicine area that SC types have the potential to be a useful scientific tool for the prevention, diagnosis, and treatment of diseases (Cooper, Evidence Based Complementary and Alternative Medicine, Vol. 6 (Suppl. 1), 2009, pp. 1–3). The main purposes of the present study are to estimate genetic and environmental influences on SC types, and to explore genetic and environmental correlations that affect phenotypic associations among the SC types. In total, 1,742 (365 monozygotic male, 173 dizygotic male, 675 monozygotic female, 271 dizygotic female, and 258 opposite-sex dizygotic) twins (mean age = 19.1 ± 3.1 year) completed a Sasang questionnaire. Univariate and multivariate model-fitting analyses were performed. Total (additive and non-additive) genetic influences were 71% for males and 81% for females in TE, 70% for males and 71% for females in SE, and 47% for both sexes in SY. Non-additive genetic effects were substantial, and shared environmental influences were negligible in most SC types. Multivariate model-fitting analysis revealed that non-additive genetic and individual-specific environmental correlations between TE and SE were -0.92 (95% CI [-0.89, -0.93]) and -0.62 (95% CI [-0.57, -0.68]), respectively. The corresponding estimates were -0.55 (95% CI [-0.48, -0.61]) and -0.44 (95% CI [-0.37, -0.51]) between TE and SY and 0.19 (95% CI [0.09, 0.29]) and -0.40 (95% CI [-0.32, -0.47]) between SE and SY. These results suggest that the phenotypic associations among SC types may be mediated by pleiotropic mechanism of genes.

It has been argued that culture significantly influences the developmental basis of self-concept. The goal of the present study is to compare the relative importance of genetic and environmental factors to explain individual differences in various dimensions of self-concept in female preadoles- cents of Minnesota in the United States and Seoul in South Korea. Two hundred and eighteen monozygotic (MZ) and 137 dizygotic (DZ) twin pairs from the Minnesota Twin Family Study (MTFS) and 74 MZ and 42 DZ twin pairs from the Seoul Twin Family Study (STFS) completed the 6 cluster scales of the Piers–Harris Children's Self-Concept Scale (P–H). The 6 cluster scales of the P-H include Popularity, Physical Appearance and Attributes, Behavior, Intellectual Competence and School Status, Anxiety, and Happiness and Satisfaction. Univariate model- fitting analyses were performed. In both samples, a model incorporating shared and nonshared environmental influences fitted the data best for Popularity, Anxiety, and Intellectual Competence and School Status, whereas a model including additive genetic and nonshared environmental factors provided the best fit for Physical Appearance and Attributes, and Behavior. The univariate model did not yield an adequate fit for Happiness and Satisfaction. For Physical Appearance and Attributes, and Intellectual Competence and School Status, estimates of additive genetic and environmental factors were significantly different between the MTFS and the STFS samples. For Popularity, Anxiety, and Behavior, however, the genetic and environmental estimates were comparable between the two samples.

We aimed to investigate whether the alterations of time intervals and velocity–time integrals (VTIs) of ductus venosus (DV) Doppler flow could predict evolving twin-to-twin transfusion syndrome (TTTS) in monochorionic diamniotic (MD) twin pregnancies in the second trimester. DV Doppler waveforms of MD twin pregnancies were retrospectively analyzed. Time intervals and VTIs of the accelerating phase of S-wave (S1 and VTI-1), decelerating phase of S-wave (S2 and VTI-2) and phase of D-wave (D and VTI-diast) were measured in the recipient twins of TTTS and the larger twins of uncomplicated MD twins. Each parameter was normalized to total time intervals and VTIs. DV parameters of prerecipient twins in pre-TTTS conditions were analyzed. The pre-TTTS condition was defined as the condition within 2 weeks before when TTTS cases fulfilled the criteria. Both recipient twins in TTTS cases and prerecipient twins in pre-TTTS condition showed significantly shorter S1, longer S2, shorter D, smaller VTI-1, larger VTI-2 and smaller VTI-diast than did the larger twins in uncomplicated MD twins (p < .05). Receiver-operating characteristic curve analysis showed that VTI-2 had better performance, and VTI-2 >45.5% had 87.0% sensitivity, 90.9% specificity, 81.0% positive predictive value and 93.0% negative predictive value for detecting prerecipient twins. Recipient twins complicated with TTTS have characteristic alterations in time intervals and VTIs of DV Doppler flow, which is present even before fulfilling the TTTS criteria. Furthermore, VTI-2 >45.5% is a possible predictive parameter for TTTS in the second trimester.

All psychological and psychophysiological traits vary, often widely, about their stable set-point values, due to transitory environmental influences. Because it is this stable set-point that embodies the genetically determined component of the trait, twin and family data based on one-time trait measurements must underestimate true trait heritability. The means of multiple measurements, taken months or years apart, then correlated within pairs of monozygotic twins, would yield an accurate estimate of the broad heritability of the set-point value, but such data are rare and expensive. Given just two measurements sufficiently far apart, the cross-twin cross-time correlation (RCT), divided by the retest or within-twin cross-time correlation (RWT), provides a valid estimate of set-point heritability. This article examines data from young and middle-aged twins who were tested twice, 3 or more years apart, on heart rate and blood pressure, personality traits, self-rating items, occupational and recreational interests, as well as on Wechsler Intelligence Scales. In every case, the disattenuated RCT revealed substantially higher heritability than indicated by correlations based on single measurements.