The University of British Columbia (UBC) Twin Project is a registry of approximately 1500 pairs of reared-together twins recruited from Vancouver, British Columbia and surrounding municipalities. The focus of the project is to examine personality and its disorders from a behavioral genetic perspective. The primary measures include self-report measures of variables from the major models of personality and personality disorders. Subsamples of the study have also been surveyed on a wide range of psychiatric conditions and symptoms, including, for example, substance use, mood, anxiety, coping, posttraumatic stress disorder, schizotypy, and several measures of the environment and experience. Also surveyed are general health and basic psychological processes including cognitive ability. This broad assessment has enabled us to examine not only the structure of personality, but also its potential role in psychopathology and other psychological processes. A feature of the project is that the measures selected reflect current thinking in the field as opposed to traditional psychiatric diagnostic criteria. The UBC Twin Project has been used in a number of collaborative projects on personality and psychopathology with other worldwide twin registries. At the present time, no DNA has been collected; however the facility to collect these data is available. Collaborative projects on this and future questionnaire studies are welcome.

Attention-deficit hyperactivity disorder (ADHD) is currently recognized as a neurobiological, genetically based disorder in both children and adults. In this article we examine whether, by using a sample of middle-aged male twin veterans, the phenotypic characterization, prevalence, heritability and the longitudinal course of the illness is comparable to results observed in samples of children and adolescents. We evaluated the utility of adult reports of lifetime ADHD symptoms by examining the heritability of retrospectively reported childhood symptoms, using both symptom-based and discrete classification- based approaches, as well as examining the persistence of ADHD symptoms into adulthood for that subsample of individuals who were judged to possibly have ADHD as children. Our results showed prevalence rates that were approximately similar to those observed in other studies, demonstrable familiality, similar item endorsement patterns, a strong genetic association between hyperactive and inattentive subtypes, and a longitudinal decline in symptom severity. We concluded that while assessing ADHD in adult probands may be less accurate than with children or adolescents, since it demonstrates several characteristics in common with other assessment techniques it remains a viable diagnostic and research strategy, even with population samples.

This article is concerned with the power to detect the presence of genotype by environment interaction (G × E) in the case that both genes and environment feature as latent (i.e., unmeasured) variables. The power of the test proposed by Jinks and Fulker (1970), which is based on regressing the absolute difference between the scores of monozygotic twins on the sums of these scores, is compared to the power of an alternative test, which is based on Marginal Maximum Likelihood (MML). Simulation studies showed that generally the power of the MML-based test was greater than the power of the Jinks and Fulker test in detecting linear and curvilinear G × E interaction, regardless of whether the distribution of the data deviated significantly from normality. However, after a normalizing transformation, the Jinks and Fulker test performed slightly better. Some possible future extensions of the MML-based test are briefly discussed.

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.

The twin method assumes that the trait under study is similar in twins and singletons so that the results from twin samples can be generalized to the singleton population. The purpose of the present study was to compare self-concept in twins and singletons matched in parental level of education. The 6 cluster scales of the Piers-Harris Children's Self-Concept Scale (P-H) were administered to 630 singleton children and 635 twins aged 7 to 12 years. Model-fitting analyses yielded two conclusions. First, self-concept was generally similar between first- and second-born twins and between monozygotic and dizygotic twins across the 6 cluster scales of the P-H. Second, twins consistently exceeded singletons for all 6 cluster scales of the P-H. However, the effect sizes of differences between twins and singletons were small except for one scale, Physical Appearance and Attributes.

It is often debated whether migraine with aura (MA) and migraine without aura (MO) are etiologically distinct disorders. A previous study using latent class analysis (LCA) in Australian twins showed no evidence for separate subtypes of MO and MA. The aim of the present study was to replicate these results in a population of Dutch twins and their parents, siblings and partners (N = 10,144). Latent class analysis of International Headache Society (IHS)-based migraine symptoms resulted in the identification of 4 classes: a class of unaffected subjects (class 0), a mild form of nonmigrainous headache (class 1), a moderately severe type of migraine (class 2), typically without neurological symptoms or aura (8% reporting aura symptoms), and a severe type of migraine (class 3), typically with neurological symptoms, and aura symptoms in approximately half of the cases. Given the overlap of neurological symptoms and nonmutual exclusivity of aura symptoms, these results do not support the MO and MA subtypes as being etiologically distinct. The heritability in female twins of migraine based on LCA classification was estimated at .50 (95% confidence intervals [CI] .27 – .59), similar to IHS-based migraine diagnosis (h2 = .49, 95% CI .19–.57). However, using a dichotomous classification (affected–unaffected) decreased heritability for the IHS-based classification (h2 = .33, 95% CI .00–.60), but not the LCA-based classification (h2 = .51, 95% CI .23–.61). Importantly, use of the LCA-based classification increased the number of subjects classified as affected. The heritability of the screening question was similar to more detailed LCA and IHS classifications, suggesting that the screening procedure is an important determining factor in genetic studies of migraine.

Twin designs, comparing correlations in monozygotic (MZ) versus dizygotic (DZ) twins, have an extensive history. One major confounder in such studies is that MZ twins may share postnatal environmental influences more so than do DZ twins. To avoid such confounding, twins separated at or soon after birth have been studied, but their scarcity often makes this approach impractical. Another method has been to measure the degree of contact twins have maintained over time, and adjust the observed correlations. Here, we remove confounding by utilizing the discrepancy between biological and self-perceived zygosity to separate environmental from genetic sources of twin similarity. We analyzed dietary patterns and physiologic traits in 350 female twin pairs of the 1988 Kaiser Permanente Twin Registry. Among twin pairs, 175 were MZ by self-report and genetic testing (MZC), 136 were DZ by self-report and genetic testing (DZC), 30 were MZ by genetic testing but not by self-report (MZW), and 9 were DZ by genetic testing but not by self-report (DZW) but were excluded due to small sample size. For healthy food patterns, MZC and MZW intraclass correlations were similar and greater than for DZC, yielding positive and significant heritability estimates. For unhealthy food patterns, the MZC, MZW and DZC correlations were similar with no significant heritability. For physiologic traits, MZC and MZW correlations were similar and higher than those for DZC, indicating significant heritability, except for insulin for which MZW and DZC were similar and which showed modest heritability. Twins of mistaken zygosity (TOMZ) provides a useful approach to robust determination of heritability.

The purpose of this study is to present curves of estimated placental growth in twins and to evaluate the relative contribution of gestational age, zygosity, chorionicity, fusion of the placentas, sex of the individual and of the twin pair, site of the umbilical cord insertion, birth order, maternal age, and parity. Perinatal data and placental data were obtained from 6315 live-born twin pairs from the East Flanders Prospective Twin Survey. Of 4318 twin pairs, with no missing values, the placental weights of different gestational ages were analyzed using a nonlinear multivariate Gaussian regression. Two groups were distinguished: (1) twins with two separate placentas, and (2) twins with only one placental mass (one placenta in case of monochorionic twins or two fused placentas in case of dichorionic placentas). Overall, placental weight was influenced by gestational age, fusion of the placentas, and parity. In the case of one placental mass, monozygotic dichorionic twins had the lowest weights. If two separate placentas were present, birth order played a role in favor of the first-born twin. For parity and zygosity, the differences were most pronounced between 27 and 29 weeks, whereas the difference for birth order was most pronounced between 33 and 37 weeks. In conclusion, basic physiological characteristics, routinely examined at birth, influence placental weight. Taking these covariates into account allows a better evaluation of the placental weight given a gestational age, as an indicator of growth.

The objective of this study was to evaluate the impact of variations of the angiotensinogen (AGT) and angiotensin II type I receptor (AGTR1) genes on progression of blood pressure (BP) and left ventricular mass (LVM) in multiethnic youth. The study was longitudinal involving 581 European American (EA) and African American (AA) youth with 12 assessments over a 15-year period. AGT M235T and three AGTR1 polymorphisms (C-521T, L191L and A1166C) were genotyped and individual growth curve modeling analyses were conducted. Single nucleotide polymorphism (SNP) analyses found a significant 3-way interaction between M235T, ethnicity and gender on BP levels. Systolic BP (SBP) levels were 5.8 mmHg (p = .00003) and diastolic BP (DBP) levels were 2.6 mmHg (p = .005) lower in carriers versus noncarriers of the M235 allele in AA males only. Furthermore, the AGTR1 L191 allele showed a SBP lowering effect in subjects with a high socioeconomic status (SES; p = .048) and a DBP lowering effect in AAs (p = .038). Haplotype analyses identified a protective haplotype (C-521, 191L and A1166) for LVM levels (p = .03). LVM in individuals homozygous for this haplotype was 12.9 g lower than those homozygous for the most common haplotype (–521T, 191L and A1166). No significant interactions were found between the AGT M235T polymorphism and any of the single SNPs or haplotypes of the AGTR1 gene. Our results in multiethnic youth uncover an ethnicity and gender-specific effect of the AGT M235T polymorphism and a SES or ethnicity-specific effect of the AGTR1 L191L polymorphism on the progression of hypertension risk. A protective AGTR1 haplotype for LVM was also identified.

We sought to estimate levels of risk for stillbirth subtypes associated with twin gestations among pediatric mothers (10–14 years). Analysis was on twin pregnancies covering the period 1989 to 2000 in the United States. We classified stillbirth as term, preterm, small-for-gestational-age (SGA) or preterm-SGA. We then assessed the risks of these stillbirth subtypes in pediatric mothers using two comparison groups consisting of women aged 15 to 19 years old (adolescent mothers) and 20 to 24 years old (mature mothers). Adjusted risk estimates were by means of hazard ratios generated from a Cox proportional hazards regression model. We adjusted for dependence of observations within twin clusters using the robust sandwich estimator. The rate of stillbirth was highest among pediatric mothers (56/1000), followed by adolescent gravidas (29/1000) and lowest in mature mothers (20/1000; p for trend < .01). Overall, preterm stillbirth was the most frequent stillbirth phenotype while term stillbirth was the least frequent. Not a single case of term stillbirth was recorded in pediatric mothers. Among pediatric gravidas, the risk for preterm stillbirth was more than tripled (adjusted hazard ratio [AHR] = 3.4; 95% confidence interval [CI] = 2.5–4.6), and that of preterm-SGA stillbirth more than doubled (AHR = 2.6; 95% CI = 1.8–3.7) that of mature mothers respectively. The 30% risk elevation for SGA stillbirth among pediatric mothers was not found to be statistically significant (AHR = 1.1; 95% CI = 0.3–4.3). Pediatric motherhood is a risk factor for stillbirth in twin gestation, especially, preterm and preterm-SGA stillbirth phenotypes. Prevention of stillbirth among this category of mothers should target the period preceding full term.

Both genetic and environmental factors affect smoking initiation and maintenance, but less is known about the genetic architecture of various other smoking-related behaviors. The aim of this study is to examine the genetic architecture of smoking behavior in a large twin cohort. Questionnaires with an extensive smoking history section were mailed to same-sex adult twins of the Finnish twin cohort. The final study population included 2923 monozygotic and 6018 dizygotic twin pairs aged 24 to 88 years. Two-stage bivariate genetic modeling of age at initiation with amount smoked (less than 20 cigarettes per day vs. 20 or more) and age at initiation with smoking cessation was done by using the Mx statistical package. For men the heritability estimate for age at initiation was .59 (95% confidence interval [CI] .49–.69), for amount smoked .54 (95% CI .45–.62) and for smoking cessation .58 (95% CI .50–.65). For women the heritability estimates were .36 (95% CI .28–.43), .61 (95% CI .46–.70) and .50 (95% CI .39–.60), respectively. The genetic correlations between age at initiation and amount smoked or smoking cessation were at most .22 in magnitude, indicating that genetic influences in age at initiation accounted for at most about 4% of the genetic factors in amount smoked or in cessation. Genetic factors are important in amount smoked and smoking cessation and they are largely independent of genetic influences on age at initiation. This has implications for defining phenotypes in searches for genes underlying smoking behaviors.

We analyzed depressive and psychosomatic symptoms in relation to parental preference in 419 twins at the age of 22 to 30 years. Depressiveness was elicited with Children's Depression Inventory and reported as a total score and three subscales (low self-confidence, anhedonia and sadness) based on factor analysis as reported in a previous epidemiological study conducted in Finland. Items assessing nervous complaints and somatic symptoms were adapted from Finnish studies of juvenile health habits. Twins reported the preference in two directions: experienced parental preference towards either twin, and twin's own preference towards either parent. About half of the twins were from pairs where both twins experienced having been equally close to both parents, while about 30% were from ‘equal and mother's’ pair, where one twin evaluated having been preferred by the mother and the co-twin evaluated having been equally close to both parents. According to the twins' own preference, about one third of the twin pairs were ‘both equal’, one third ‘both mother's’ and one third ‘equal and mother's’. Those male twins who were equally close to both parents (experienced parental preference) had least total depressiveness, while females in the intermediate situation had the highest self-confidence and least anhedonia and nervousness. According to twins' own preference, twins who felt equally close to both parents had the least depressiveness and anhedonia. The intermediate position seems to be the best alternative, as these twins had the least symptoms.

The consensus from published studies is that plasma lipids are each influenced by genetic factors, and that this contributes to genetic variation in risk of cardiovascular disease. Heritability estimates for lipids and lipoproteins are in the range .48 to .87, when measured once per study participant. However, this ignores the confounding effects of biological variation measurement error and ageing, and a truer assessment of genetic effects on cardiovascular risk may be obtained from analysis of longitudinal twin or family data. We have analyzed information on plasma high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, and triglycerides, from 415 adult twins who provided blood on two to five occasions over 10 to 17 years. Multivariate modeling of genetic and environmental contributions to variation within and across occasions was used to assess the extent to which genetic and environmental factors have long-term effects on plasma lipids. Results indicated that more than one genetic factor influenced HDL and LDL components of cholesterol, and triglycerides over time in all studies. Nonshared environmental factors did not have significant long-term effects except for HDL. We conclude that when heritability of lipid risk factors is estimated on only one occasion, the existence of biological variation and measurement errors leads to underestimation of the importance of genetic factors as a cause of variation in long-term risk within the population. In addition our data suggest that different genes may affect the risk profile at different ages.

The Chinese National Twin Registry is the first and largest population-based twin registry in China. It was established in 2001. The primary goal of this program is the establishment of a population-based twin registry of 45,000 twin pairs from several regions representing north, south, urban, and rural areas in China. A secondary goal is to study genetic contributions to complex diseases, and to test associations of candidate genes with related phenotypes. Seven thousand, four hundred and twenty-three twin pairs have been enrolled in the registry, in which 1613 pairs have undergone detailed questionnaire assessments and physical examination. Based on the baseline registry, a twin cohort was established. Continued research includes studies on intermediate phenotypes of cardiovascular and cerebrovascular diseases and psychological studies in adult twins, studies on growth and development in adolescent twins, and so forth. The current state and future plans for the Chinese National Twin Registry will be discussed in this article.

The Guangzhou Twin Registry, initiated in 2005, is a population-based registry of twins born between 1987 and 2000. To date, over 9700 pairs of twins, regardless of their health and medical history, were enrolled in the database using the Official Household Registry of Guangzhou City. The twins were subsequently verified by door-to-door visits based on the registry address. The primary goal of this registry is to develop a resource for genetic epidemiological studies on common diseases in the southern Chinese population. The initial focus is to distinguish the genetic and environmental determinants of eye diseases, in particular myopia and glaucoma. About 1000 pairs of twins living close to the Zhongshan Ophthalmic Center were invited for the first phenotyping examination, questionnaire administration and DNA collection in July and August 2006. An annual eye examination and other phenotype data collection have been scheduled for up to 5 years in order to investigate changes in phenotypes including the myopia progression, physical development and the changes of other eye-related phenotypes. Recruitment of adult twins aged 50 years and over is underway in the same city with the assistance of the government.

The stillbirth rate in twins is a more sensitive indicator of environmental hazards than the stillbirth rate in singletons. Medical care or other socioeconomic factors may be more influential for perinatal survival in twin than in single deliveries. Studies have indicated that stillbirths among children in a set of multiple maternities are not independent. Models were considered assuming independent outcomes within a set of multiple maternities. Analyses of the stillbirth rates confirm that the risk of stillbirth among males is almost constantly higher than among females. Any model introduced should assume different stillbirth rates for males and females. The models were tested with both maximum likelihood and minimum χ2 methods. Data was analyzed from Sweden, the Åland Islands, Saxony, England and Wales, and significant discrepancies obtained from the independence models. The same-sexed twin data contain both monozygotic and dizygotic twin sets with apparently different stillbirth rates. Consequently, for same-sexed twins the proposed model could be considered too simple. After improvement by splitting the same-sexed data into monozygotic and dizygotic twin sets, the dependence still remains. The proportion of both same-sexed and opposite-sexed twin pairs that contain two stillborn is greater than what the stillbirth rates and the independence should indicate. Consequently, stillbirth rate estimates based on the relative frequency of twin sets with two stillborn children have a positive bias. When the stillbirth rate decreases, the number of sets with two stillborn children decreases more slowly than would be indicated by independence.

Twinning rates in developed countries have recently registered an increase. At the end of the 1970s, the change in mother's age structure has partially contributed to the growth in the proportion of multiple births. In fact, the evolution of twinning rates is related to the calendar of maternity since, comparatively to younger mothers, older women more frequently have twins. Moreover, the growing frequency of multiple births also depends on fertility treatments, which are largely used in the developed countries. National data from the civil birth registration systems are taken into account in order to describe, in a comparative study, the main trends of twinning rates in the 20th century.

In the classic twin design, estimation of genetic and environmental effects is based on the assumption that environmental influences are shared to the same extent by monozygotic and dizygotic twins (equal environment assumption, EEA). We explore the conditions in which the EEA can be tested based on multivariate phenotypic data. We focus on the test whether the correlation between shared environmental factors in dizygotic twins (rC) is less than 1. First, model identification was investigated analytically in Maple and Mx. Second, statistical power was examined in Mx. Third, the amount of bias caused by violation of the EEA was evaluated. Finally, applications to empirical data concern spatial ability in adolescents and aggression in children. Bivariate and trivariate models include several instances in which the EEA can be tested. The number of twin pairs that is needed to detect violation of the EEA with a statistical power of .80 (α = .05) varied between 508 and 3576 pairs for the situations considered. The bias in parameter estimates, given misspecification, ranged from 5% to 34% for additive genetic effects, and from 4% to 34% for shared environmental effects. Estimates of the nonshared environmental effects were not biased. The EEA was not violated for spatial ability or aggression. Multivariate data provide sufficient information to test the validity of the EEA. The number of twin pairs that is needed is no greater than the number typically available in most twin registries. The analysis of spatial ability and aggression indicated no detectable violation of the EEA.

Previous studies have shown that Glutathione, a tripeptide found in blood, is involved in protecting against toxins. Glutathione levels are known to drop in response to cadmium. Using 15 twin pairs, we modeled the effect of cadmium on glutathione levels. The heritability of glutathione content was 91%. The application of cadmium significantly reduced the mean level of GSH. However, this reduction in GSH was not due to additive genetic influences in our sample.

The aim of this study was to estimate the heritability of variation in testosterone levels in 12-year-old children, and to explore the overlap in genetic and environmental influences on circulating testosterone levels and androgen-dependent pubertal development. Midday salivary testosterone samples were collected on 2 consecutive days in a sample of 183 unselected twin pairs. Androgen-induced pubertal development was assessed using self-report Tanner scales of pubic hair development (boys and girls) and genital development (boys). A significant contribution of genetic effects to the variance in testosterone levels was found. Heritability was approximately 50% in both boys and girls. The remaining proportion of the variance in testosterone levels could be explained by nonshared environmental influences. The relatively high correlation between testosterone levels of opposite-sex dizygotic twins suggests that sex differences in genes influencing variation in testosterone levels have not yet developed in preand early puberty. Variance in pubertal development was explained by a large genetic component, moderate shared environmental influences, and a small nonshared environmental effect. Testosterone levels correlated moderately (r = .31) with pubertal development; the covariance between testosterone levels and pubertal development was entirely accounted for by genetic influences.