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AbstractThe Colorado Twin Registry (CTR) is a population-based registry formed from birth and school records including twins born between 1968 and the present. Two previous reports on the CTR [Rhea et al., (2006). Twin Research and Human Genetics, 9, 941–949; Rhea et al., (2013).Twin Research and Human Genetics, 16, 351–357] covered developments in the CTR through 2012. This report briefly summarizes previously presented material on ascertainment and recruitment and the relationships between samples and studies, discusses developments since 2012 for four previously described twin samples, describes two new samples and their complementary studies and expands on two subjects briefly mentioned in the last report: a history of genotyping efforts involving CTR samples, and a survey of collaborations and consortia in which CTR twins have been included. The CTR remains an active resource for both ongoing, longitudinal research and the recruitment of new twin samples for newly identified research opportunities.

In this article we introduce the first twin study in Turkey with the future plan to register all twins born in Turkey for research purposes. The Turkish twin study will illuminate the etiology of health problems in the context of cultural differences, owing to the rich cultural diversity in Turkey. As a first step, we selected a sample of 618 twin individuals from 302 pairs and 14 singletons living in the Kirikkale and Ankara regions of Turkey and administered a questionnaire with items about nicotine use, psychiatric disorders, and information about the family, in addition to zygosity items. Our study provides valuable information regarding factors related to nicotine dependence, age, gender, social situation, and family structure.

AbstractIn this article we describe the design and implementation of a database for extended twin families. The database does not focus on probands or on index twins, as this approach becomes problematic when larger multigenerational families are included, when more than one set of multiples is present within a family, or when families turn out to be part of a larger pedigree. Instead, we present an alternative approach that uses a highly flexible notion of persons and relations. The relations among the subjects in the database have a one-to-many structure, are user-definable and extendible and support arbitrarily complicated pedigrees. Some additional characteristics of the database are highlighted, such as the storage of historical data, predefined expressions for advanced queries, output facilities for individuals and relations among individuals and an easy-to-use multi-step wizard for contacting participants. This solution presents a flexible approach to accommodate pedigrees of arbitrary size, multiple biological and nonbiological relationships among participants and dynamic changes in these relations that occur over time, which can be implemented for any type of multigenerational family study.

The Study of Dementia in Swedish Twins is a study of dementia in a defined population of twins. The goals included estimating heritability of Alzheimer's disease and identifying risk and protective factors in twin pairs discordant for the disease. The data, including not only diagnoses and age of onset but also extensive information about potential environmental risk factors, are now archived as Study ICPSR 25963 at National Archive for Computerized Data on Aging, Inter-university Consortium for Political and Social Research, at the University of Michigan, and available for researchers to use. Up to the time of archiving, 215 cases of dementia have been identified from a base sample of 2,394 individuals.

The Guangzhou Twin Registry is a population-based registry of twins residing in Guangzhou City. The initial registry database included 9,700 pairs of young twins recruited from the Official Household Registry of Guangzhou City. The registry is designed to provide a resource to identify the genetic and environmental causes of common diseases with an initial focus on eye diseases. From 2006 onward, phenotype and DNA collection have been completed for more than 1,200 twin pairs and their parents or siblings. Most of the young twins have come back for an annual examination of the progressive traits, such as refraction, ocular biometry, weight, and height. Genome-wide association scans have been completed recently. This article gives an update of the study design, cohort profile, previous findings, and future directions. Results from the Guangzhou Twin Project may contribute to the understanding of gene-environmental interplay for complex diseases in both adults and children.

AbstractWhile snus has been the focus of increasing public health interest, twin studies have examined neither sources of individual variation for its use nor the sources of resemblance between snus and cigarette use. Twins from the Norwegian Institute of Public Health Panel were assessed by self-report questionnaire for the initiation of regular use and maximal quantity used for snus and cigarettes. Twin modeling was performed using OpenMx on data from 2767 twins including 856 complete pairs. Fitting univariate twin models produced similar results for cigarette initiation and quantity with estimates of additive genetic, shared environmental and unique environmental effects of approximately 77%, 0% and 23%, respectively. Estimates of snus initiation and quantity were, respectively, approximately 53%, 26% and 21%. Joint analyses suggested that the genetic, shared environmental and unique environmental correlations between cigarette and snus initiation and quantity were +.82, 0 and +.42, respectively. However, these results could not be statistically distinguished from a model which postulated that resemblance between cigarette initiation and quantity resulted from genetic and unique environmental correlations of +.47 and +.43. Compared with cigarette initiation and quantity of use in Norwegian twins, the role of genes was less prominent and shared environment more prominent for initiation and quantity of use of snus. Joint analyses of both tobacco phenotypes suggested, but did not confirm definitively, that genetic risk factors for cigarette and snus use were similar but not identical, while shared environmental factors existed that were specific to snus use.

AbstractDr Nick Martin has made enormous contributions to the field of behavior genetics over the past 50 years. Of his many seminal papers that have had a profound impact, we focus on his early work on the power of twin studies. He was among the first to recognize the importance of sample size calculation before conducting a study to ensure sufficient power to detect the effects of interest. The elegant approach he developed, based on the noncentral chi-squared distribution, has been adopted by subsequent researchers for other genetic study designs, and today remains a standard tool for power calculations in structural equation modeling and other areas of statistical analysis. The present brief article discusses the main aspects of his seminal paper, and how it led to subsequent developments, by him and others, as the field of behavior genetics evolved into the present era.

AbstractNordic twin studies have played a critical role in understanding cancer etiology and elucidating the nature of familial effects on site-specific cancers. The NorTwinCan consortium is a collaborative effort that capitalizes on unique research advantages made possible through the Nordic system of registries. It was constructed by linking the population-based twin registries of Denmark, Finland, Norway and Sweden to their country-specific national cancer and cause-of-death registries. These linkages enable the twins to be followed many decades for cancer incidence and mortality. To date, two major linkages have been conducted: NorTwinCan I in 2011–2012 and NorTwinCan II in 2018. Overall, there are 315,413 eligible twins, 57,236 incident cancer cases and 58 years of follow-up, on average. In the initial phases of our work, NorTwinCan established the world's most comprehensive twin database for studying cancer, developed novel analytical approaches tailored to address specific research considerations within the context of the Nordic data and leveraged these models and data in research publications that provide the most accurate estimates of heritability and familial risk of cancers reported in the literature to date. Our findings indicate an excess familial risk for nearly all cancers and demonstrate that the incidence of cancer among twins mirrors the rate in the general population. They also revealed that twin concordance for cancer most often manifests across, rather than within, cancer sites, and we are currently focusing on the analysis of these cross-cancer associations.

The 4th International Network of Twin Registries (INTR) Consortium Meeting took place in Osaka, Japan, September 28–29, 2015. The venue was the Osaka Medical Center for Medical Innovation and Translational Research. An overview of presentations and other activities is provided. Next, 1930s research on familial fraternal twinning, preference for masculine faces, physical aggression and epigenetics, and a prenatal education program for parents of multiples are described. Current twin-related events include the 2016 Guinness Book of World Records (GWR), the oldest living male twins, newly reunited twins, the birth of panda twins and a controversial twin-based website.

AbstractTwin studies have demonstrated that personality traits show moderate genetic influence. The conclusions drawn from twin studies rely on the assumptions that twins are representative of the population at large and that monozygotic and dizygotic twins are comparable in every way that might have bearing on the traits being studied. To evaluate these assumptions, we used Multidimensional Personality Questionnaire (MPQ) data from three samples drawn from the Minnesota Twin Registry (totaling 12,971 respondents) to examine the effect sizes associated with mean differences on the 11 MPQ scales and 3 higher-order MPQ factors for singletons versus twins and MZ twins versus DZ twins. The singletons in the samples were family members of the participating twins. We also used ratios of scale variances to examine the significance of variance differences. The only mean or variance difference replicated across all three samples was greater Social Closeness (about .1 standard deviation) for twins than for singletons. This difference was obtained for both males and females. It would appear that, with respect to personality, twins are not systematically different from other people. Our results also highlight the importance of replication in psychological research because each of our large samples showed differences not replicated in other samples.

Fetal growth restriction in singletons has been shown to enhance fetal lung maturation and reduce the risk of respiratory distress syndrome due to increased endogenous steroid production. However, data on lung maturation in growth-discordant monochorionic (thus, identical) twins are lacking. Our objective was to compare the risk of severe neonatal morbidity between the larger and the smaller twin in monochorionic twins with birth weight discordance (BWD). We included in the study all consecutive monochorionic diamniotic pregnancies with severe BWD (≥25%) and two live-born twins delivered at our center (n = 47 twin pairs). We compared the incidence of neonatal morbidity, particularly respiratory distress syndrome (RDS), and cerebral lesions between the larger and the smaller co-twin. The incidence of severe neonatal morbidity in the larger and smaller twin was 38% (18/47) and 19% (9/47), respectively (odds ratio (OR) 2.66, 95% confidence interval (CI) 0.94–7.44) and was due primarily to the higher incidence of RDS, 32% (15/47) and 6% (3/47), respectively (OR 6.88, 95% CI 1.66–32.83). In conclusion, this study shows that the larger twin in monochorionic twin pairs with BWD is at increased risk of severe neonatal morbidity, particularly RDS, compared to the smaller twin.

Abstract'Healthy Twin' is a twin family study extension of the existing Korean Twin-Family Register. Healthy Twin recruits adult like-sex twins over the age of 30 and their adult family members. Healthy Twin protocols are primarily tailored to the study of the quantitative trait loci of complex traits as well as to the role of environment in the etiology of complex diseases. A full-length survey is underway, including questionnaires, health examinations and the collection of biological specimens. So far, 820 individuals (169 twin pairs and their families) have participated in the survey and 1068 individual twins (608 twin pairs) have replied to the mailed zygosity questionnaire as of July 2006. The first phase (2005–2006) of Healthy Twin will recruit 1550 individuals (including about 380 twin pairs), and the second phase a proposed 1500 to 2500 additional participants. We report study protocols and zygosity and the distribution of family size of the study participants.

Twins switched at birth are a rare subtype of twins who grow up apart. The frequency with which this event occurs, as well as selected life history events, twin relationships, and critical issues generated by the nine recorded cases are reviewed. This is followed by summaries of recent studies addressing classroom placement for twins, mirror syndrome in mothers, unusual twin types, and parenting stress. The article concludes with a review of human interest items from the general media, namely a triplet film, educational achievement, prison release, co-twin impersonation, and facial recognition technology.