Much progress has been made in twin research since our last special issue on twin registries (Hur, Y.-M., & Craig, J. M. (2013). Twin Research and Human Genetics, 16, 1–12.). This special issue provides an update on the state of twin family registries around the world. This issue includes 61 papers on twin family registries from 25 countries, of which 3 describe consortia based on collaborations of several twin family registries. The articles included in this issue discuss the establishment and maintenance of twin registries, recruitment strategies, methods of zygosity assessment, research aims and major findings from twin family cohorts, as well as other important topics related to twin studies. The papers amount to approximately 1.3 million monozygotic, dizygotic twins and higher order multiples and their family members who participate in twin studies around the world. Nine new twin family registries have been established across the world since our last issue, which demonstrates that twin registers are increasingly important in studies of the determinants and correlates of complex traits from disease susceptibility to healthy development.

Twins Research Australia (TRA) is a community of twins and researchers working on health research to benefit everyone, including twins. TRA leads multidisciplinary research through the application of twin and family study designs, with the aim of sustaining long-term twin research that, both now and in the future, gives back to the community. This article summarizes TRA’s recent achievements and future directions, including new methodologies addressing causation, linkage to health, economic and educational administrative datasets and to geospatial data to provide insight into health and disease. We also explain how TRA’s knowledge translation and exchange activities are key to communicating the impact of twin studies to twins and the wider community. Building researcher capability, providing registry resources and partnering with all key stakeholders, particularly the participants, are important for how TRA is advancing twin research to improve health outcomes for society. TRA provides researchers with open access to its vibrant volunteer membership of twins, higher order multiples (multiples) and families who are willing to consider participation in research. Established four decades ago, this resource facilitates and supports research across multiple stages and a breadth of health domains.

Neurodevelopment is sensitive to genetic and pre/postnatal environmental influences. These effects are likely mediated by epigenetic factors, yet current knowledge is limited. Longitudinal twin studies can delineate the link between genetic and environmental factors, epigenetic state at birth and neurodevelopment later in childhood. Building upon our study of the Peri/postnatal Epigenetic Twin Study (PETS) from gestation to 6 years of age, here we describe the PETS 11-year follow-up in which we will use neuroimaging and cognitive testing to examine the relationship between early-life environment, epigenetics and neurocognitive outcomes in mid-childhood. Using a within-pair twin model, the primary aims are to (1) identify early-life epigenetic correlates of neurocognitive outcomes; (2) determine the developmental stability of epigenetic effects and (3) identify modifiable environmental risk factors. Secondary aims are to identify factors influencing gut microbiota between 6 and 11 years of age to investigate links between gut microbiota and neurodevelopmental outcomes in mid-childhood. Approximately 210 twin pairs will undergo an assessment at 11 years of age. This includes a direct child cognitive assessment, multimodal magnetic resonance imaging, biological sampling, anthropometric measurements and a range of questionnaires on health and development, behavior, dietary habits and sleeping patterns. Data from complementary data sources, including the National Assessment Program — Literacy and Numeracy and the Australian Early Development Census, will also be sought. Following on from our previous focus on relationships between growth, cardiovascular health and oral health, this next phase of PETS will significantly advance our understanding of the environmental interactions that shape the developing brain.

The East Flanders Prospective Twin Survey (EFPTS) is a registry of multiple births in the province of East Flanders, Belgium. Since its start in 1964, over 10,000 twin-pairs have been registered. EFPTS has several unique features: it is population-based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are recruited at birth; basic perinatal data are recorded; chorion type and zygosity are established; since 1969, placental biopsies have been taken and frozen at –20°C for future research. Since its origin, the EFPTS has included placental data and allows differentiation of three subtypes of monozygotic twins based on the time of the initial zygotic division: the dichorionic–diamniotic pairs (early, with splitting before the fourth day after fertilization), the monochorionic–diamniotic pairs (intermediate, splitting between the fourth- and the seventh-day postfertilization) and the monochorionic–monoamniotic pairs (late, splitting after the eighth day postfertilization). Studies can be initiated taking into account primary biases, those originating ‘in utero’. Such studies could throw new light on the consequences of early embryological events and the gene–environment interactions as far as periconceptional and intrauterine environment are concerned.

Meta-analyses suggest that clinical psychopathology is preceded by dimensional behavioral and cognitive phenotypes such as psychotic experiences, executive functioning, working memory and affective dysregulation that are determined by the interplay between genetic and nongenetic factors contributing to the severity of psychopathology. The liability to mental ill health can be psychometrically measured using experimental paradigms that assess neurocognitive processes such as salience attribution, sensitivity to social defeat and reward sensitivity. Here, we describe the TwinssCan, a longitudinal general population twin cohort, which comprises 1202 individuals (796 adolescent/young adult twins, 43 siblings and 363 parents) at baseline. The TwinssCan is part of the European Network of National Networks studying Gene-Environment Interactions in Schizophrenia project and recruited from the East Flanders Prospective Twin Survey. The main objective of this project is to understand psychopathology by evaluating the contribution of genetic and nongenetic factors on subclinical expressions of dimensional phenotypes at a young age before the onset of disorder and their association with neurocognitive processes, such as salience attribution, sensitivity to social defeat and reward sensitivity.

The University of São Paulo Twin Panel (Painel USP de Gêmeos), based at the Institute of Psychology of the University of São Paulo, started formally in 2017. Our registry is new, but in only two years of formal existence, it comprises a volunteer sample of 4826 registered individuals (98% twins and 2% higher-order multiples), recruited at the University of São Paulo and by social media campaigns. Our main aim is to conduct and promote research with twins on psychological processes and behavior. The University of São Paulo is the largest higher education and research institution in South America, and the Painel USP de Gêmeos has great potential for fostering research on twin-related issues from a psychological perspective in Brazil and South America.

This paper is a revised and updated edition of a previous description of the Quebec Newborn Twin Study (QNTS), an ongoing prospective longitudinal follow-up of a birth cohort of twins born between 1995 and 1998 in the greater Montreal area, Québec, Canada. The goal of QNTS is to document individual differences in the cognitive, behavioral, and social-emotional aspects of developmental health across childhood, their early genetic and environmental determinants, as well as their putative role in later social-emotional adjustment, school, health, and occupational outcomes. A total of 662 families of twins were initially assessed when the twins were aged 6 months. These twins and their family were then followed regularly. QNTS now has 16 waves of data collected or planned, including 5 in preschool. Over the last 24 years, a broad range of physiological, cognitive, behavioral, school, and health phenotypes were documented longitudinally through multi-informant and multimethod measurements. QNTS also entails extended and detailed multilevel assessments of proximal (e.g., parenting behaviors, peer relationships) and distal (e.g., family income) features of the child’s environment. QNTS children and a subset of their parents have been genotyped, allowing for the computation of a variety of polygenic scores. This detailed longitudinal information makes QNTS uniquely suited for the study of the role of the early years and gene–environment transactions in development.

The Chinese National Twin Registry (CNTR), initiated in 2001, has now become the largest twin registry in Asia. From 2015 to 2018, the CNTR continued to receive Chinese government funding and had recruited 61,566 twin-pairs by 2019 to study twins discordant for specific exposures such as environmental factors, and twins discordant for disease outcomes or measures of morbidity. Omic data, including genetics, genomics, metabolomics, and proteomics, and gut microbiome will be tested. The integration of omics and digital technologies in public health will advance our understanding of precision public health. This review introduces the updates of the CNTR, including study design, sample size, biobank, zygosity assessment, advances in research and future systems epidemiologic research.

The Beijing Twin Study (BeTwiSt), which was established in 2006, is an ongoing study aiming to investigate the genetic and environmental etiology of adolescent psychopathology. Resting-state brain imaging datasets have been examined for same-sex twins, and other psychological traits and emotional and behavioral variables have been examined for all twins. Based on the registry, the main findings regarding the etiological mechanism underlying adolescent development, magnetic resonance imaging results, and genetic and environmental influences on other psychological traits have been published. This article summarizes the key findings in these three areas and discusses future plans for the BeTwiSt.

The primary aim of the Guangzhou Twin Eye Study (GTES) is to explore the impact that genes and environmental influences have on common eye diseases. Since 2006, approximately 1300 pairs of twins, aged 7–15 years, were enrolled at baseline. Progressive phenotypes, such as cycloplegic refraction, axial length, height and weight, have been collected annually. Nonprogressive phenotypes such as parental refraction, corneal thickness, fundus photo, intraocular pressure and DNA were collected once at baseline. We are collaborating with fellow international twin researchers and psychologists to further explore links with general medical conditions. In this article, we review the history, major findings and future research directions for the GTES.

The Danish Twin Registry (DTR) was established in the 1950s, when twins born from 1870 to 1910 were ascertained, and has since been extended to include twins from birth cohorts until 2009. The DTR currently comprises of more than 175,000 twins from the 140 birth cohorts. This makes the DTR the oldest nationwide twin register and among the largest in the world. The combination of data from several surveys, including biological samples and repeated measurements on the same individuals, and data from Danish national registers provides a unique resource for a wide range of twin studies. This article provides an updated overview of the data in the DTR: First, we provide a summary of the establishment of the register, the different ascertainment methods and the twins included; then follows an overview of major surveys conducted in the DTR since 1994 and a description of the DTR biobank, including a description of the molecular data created so far; finally, a short description is given of the linkage to Danish national registers at Statistics Denmark and some recent examples of studies using the various data resources in the DTR are highlighted.

The Twins Early Development Study (TEDS) is a longitudinal twin study that recruited over 16,000 twin-pairs born between 1994 and 1996 in England and Wales through national birth records. More than 10,000 of these families are still engaged in the study. TEDS was and still is a representative sample of the population in England and Wales. Rich cognitive and emotional/behavioral data have been collected from the twins from infancy to emerging adulthood, with data collection at first contact and at ages 2, 3, 4, 7, 8, 9, 10, 12, 14, 16, 18 and 21, enabling longitudinal genetically sensitive analyses. Data have been collected from the twins themselves, from their parents and teachers, and from the UK National Pupil Database. Genotyped DNA data are available for 10,346 individuals (who are unrelated except for 3320 dizygotic co-twins). TEDS data have contributed to over 400 scientific papers involving more than 140 researchers in 50 research institutions. TEDS offers an outstanding resource for investigating cognitive and behavioral development across childhood and early adulthood and actively fosters scientific collaborations.

The Children of the Twins Early Development Study (CoTEDS) is a new prospective children-of-twins study in the UK, designed to investigate intergenerational associations across child developmental stages. CoTEDS will enable research on genetic and environmental factors that underpin parent–child associations, with a focus on mental health and cognitive-related traits. Through CoTEDS, we will have a new lens to examine the roles that parents play in influencing child development, as well as the genetic and environmental factors that shape parenting behavior and experiences. Recruitment is ongoing from the sample of approximately 20,000 contactable adult twins who have been enrolled in the Twins Early Development Study (TEDS) since infancy. TEDS twins are invited to register all offspring to CoTEDS at birth, with 554 children registered as of May 2019. By recruiting the second generation of TEDS participants, CoTEDS will include information on adult twins and their offspring from infancy. Parent questionnaire-based data collection is now underway for 1- and 2-year-old CoTEDS infants, with further waves of data collection planned. Current data collection includes the following primary constructs: child mental health, temperament, language and cognitive development; parent mental health and social relationships; parenting behaviors and feelings; and other socioecological factors. Measurement tools have been selected with reference to existing genetically informative cohort studies to ensure overlap in phenotypes measured at corresponding stages of development. This built-in study overlap is intended to enable replication and triangulation of future analyses across samples and research designs. Here, we summarize study protocols and measurement procedures and describe future plans.

TwinsUK is the largest cohort of community-dwelling adult twins in the UK. The registry comprises over 14,000 volunteer twins (14,838 including mixed, single and triplets); it is predominantly female (82%) and middle-aged (mean age 59). In addition, over 1800 parents and siblings of twins are registered volunteers. During the last 27 years, TwinsUK has collected numerous questionnaire responses, physical/cognitive measures and biological measures on over 8500 subjects. Data were collected alongside four comprehensive phenotyping clinical visits to the Department of Twin Research and Genetic Epidemiology, King’s College London. Such collection methods have resulted in very detailed longitudinal clinical, biochemical, behavioral, dietary and socioeconomic cohort characterization; it provides a multidisciplinary platform for the study of complex disease during the adult life course, including the process of healthy aging. The major strength of TwinsUK is the availability of several ‘omic’ technologies for a range of sample types from participants, which includes genomewide scans of single-nucleotide variants, next-generation sequencing, metabolomic profiles, microbiomics, exome sequencing, epigenetic markers, gene expression arrays, RNA sequencing and telomere length measures. TwinsUK facilitates and actively encourages sharing the ‘TwinsUK’ resource with the scientific community — interested researchers may request data via the TwinsUK website (http://twinsuk.ac.uk/resources-for-researchers/access-our-data/) for their own use or future collaboration with the study team. In addition, further cohort data collection is planned via the Wellcome Open Research gateway (https://wellcomeopenresearch.org/gateways). The current article presents an up-to-date report on the application of technological advances, new study procedures in the cohort and future direction of TwinsUK.

The purpose of this review is to provide a detailed and updated description of the FinnTwin16 (FT16) study and its future directions. The Finnish Twin Cohort comprises three different cohorts: the Older Twin Cohort established in the 1970s and the FinnTwin12 and FT16 initiated in the 1990s. FT16 was initiated in 1991 to identify the genetic and environmental precursors of alcoholism, but later the scope of the project expanded to studying the determinants of various health-related behaviors and diseases in different stages of life. The main areas addressed are alcohol use and its consequences, smoking, physical activity, overall physical health, eating behaviors and eating disorders, weight development, obesity, life satisfaction and personality. To date, five waves of data collection have been completed and the sixth is now planned. Data from the FT16 cohort have contributed to several hundred studies and many substudies, with more detailed phenotyping and collection of omics data completed or underway. FT16 has also contributed to many national and international collaborations.

The German Twin Family Panel (TwinLife) is a German longitudinal study of monozygotic and dizygotic same-sex twin pairs and their families that was designed to investigate the development of social inequalities over the life course. The study covers an observation period from approximately 2014 to 2023. The target population of the sample are reared-together twins of four different age cohorts that were born in 2009/2010 (cohort 1), in 2003/2004 (cohort 2), in 1997/1998 (cohort 3) and between 1990 and 1993 (cohort 4). In the first wave, the study included data on 4097 twin families. Families were recruited in all parts of Germany so that the sample comprises the whole range of the educational, occupational and income structure. As of 2019, two face-to-face, at-home interviews and two telephone interviews have been conducted. Data from the first home and telephone interviews are already available free of charge as a scientific use-file from the GESIS data archive. This report aims to provide an overview of the study sample and design as well as constructs that are unique in TwinLife in comparison with previous twin studies — such as an assessment of cognitive abilities or information based on the children’s medical records and report cards. In addition, major findings based on the data already released are displayed, and future directions of the study are presented and discussed.

The Study of Personality Architecture and Dynamics (SPeADy) is a German research project that aims to investigate the sources of interindividual differences in intraindividual personality development. The main focus lies in the dynamic interplay between more stable core characteristics and more environmentally malleable surface characteristics, as well as between personality and life experiences over time. SPeADy includes a twin family study encompassing data from 1962 individuals (age: 14–94) of 682 families, including 570 complete twin pairs (plus 1 triplet set), 327 parents, 236 spouses and 145 children of twins. Data collection started in 2016 and data from the first wave are currently obtainable as open source. Available data comprise a broad range of personality variables, such as personality trait constructs, motives, interests, values, moral foundations, religiosity and self-related concepts. For the currently ongoing second wave of data collection, we added retrospective reports on major life events. Special features of this genetically informative study are the extended twin family data and its longitudinal design. Three assessment waves in 2 years’ intervals are planned until 2022. In this article, we briefly describe the design and contents of the SPeADy twin family study as well as some recent findings, future plans and open science issues.

Sub-Saharan Africa has the highest natural twinning rate in the world. Unfortunately, due to lack of adequate care during pregnancy, labor and postnatally, twin mortality in Sub-Saharan Africa also remains very high. Thus, it has been estimated that one in five twins dies during the childhood years. In spite of this, surprisingly few twin studies have been conducted in the region, making additional epidemiological data much needed. In 2009, we established one of the first twin registries in Sub-Saharan Africa at the Bandim Health Project in Guinea-Bissau. The registry had two main objectives. First, we wanted to describe the twinning rate and mortality patterns among newborn twins, including mortality risk factors and hospitalization patterns. Such studies can help the local clinicians improve twin health by identifying the most vulnerable children. Second, and in light of the rapidly increasing diabetes rates in Africa, we wanted to use the registry to particularly focus on metabolic disorders. Twins are often born with low birth weight, which according to the ‘thrifty phenotype hypothesis’ could predispose them to metabolic disorders later in life. Yet, no such ‘fetal programming’ data have previously been available from African twins despite the fact that nutritional patterns and influences from other factors (e.g., infections) could be markedly different here compared to high-income settings. In this article, we summarize the findings and current status of the Guinea-Bissau twin registry.

Since our last report on the voluntary Hungarian Twin Registry (HTR) in 2012, the number of pairs or multiplets included increased from 310 to 1044. Efforts to turn the registry into a population-based one are on the way. Nearly 128,000 twins living in Hungary (98,500 adults) will be mailed information on how to register on the new HTR website. Twins will be asked to invite their spouses and immediate family members. Meanwhile, strong cooperation through exchange programs has been developed with other foreign twin registries. Current research focuses on radiogenomics, musculoskeletal, cardiovascular and respiratory diseases, gut microbiome as well as basic molecular research and yielded new awards and further publications.

The Longitudinal Israeli Study of Twins (LIST) focuses on the developmental, genetic and environmental contributions to individual differences in children’s and adolescents’ social behavior. Key variables have been empathy, prosocial behavior, temperament and values. Another major goal of LIST has been to study gene–environment correlations, mainly concerning parenting. LIST includes 1657 families of Hebrew-speaking Israeli twins who have participated at least once in the study. Children’s environment and their development are assessed in a multivariate, multimethod fashion, including observed, parent-reported and self-reported data. The current article summarizes and updates recent findings from LIST. For example, LIST provided evidence for the heritability of human values with the youngest sample to date, and the first genetic investigation of adolescents’ identity formation. Finally, future aims of LIST are discussed.