Twin registries are foundational resources for research in behavioral and medical genetics, epidemiology, and public health. Over several decades, successive international reviews have documented their expansion in number, geographical coverage, phenotypic scope, molecular profiling, and participation in large-scale collaborations. This overview first traces this development through previous global overviews of twin registries and then examines record linkage as an increasingly important feature of contemporary twin research. Drawing on diverse twin registries, twin cohorts, multi-cohort resources, and international consortia, we describe how twin data are linked to population and administrative registers, health and educational records, biobanks, and environmental and geospatial data. Microdata linkage connects individual-level twin phenotypes with longitudinal administrative, clinical, educational, and molecular information, extending follow-up and supporting life-course analyses. Macrodata linkage incorporates contextual information on neighborhood environments, socioeconomic conditions, and policy settings, enabling individual and familial differences to be studied within broader social and environmental contexts. Comparison across resources demonstrates substantial variation in the scope and implementation of record linkage, from established national linkage systems to partial, domain-specific, and planned linkages, shaped by differences in legal frameworks, consent models, data governance, and secure access infrastructures. By placing current record linkage practices within the historical development of twin research resources, this overview shows how linkage extends the classical twin design through longitudinal, multi-domain, and contextual data, broadening its potential for research on health, behavior, and development across the life course.

To accompany the publication of the special issue, ‘Population-Level Data Linkages in Worldwide Twin Registries’, an online, interactive tool titled ‘Twin Registries Worldwide’ has been developed to provide an overview of twin registries across the globe. The tool currently offers a searchable table view that allows users to explore twin registries by country, registry name, recruitment period, sample size, phenotypic domains, availability of biological samples, and linkage to administrative or health-related databases.

Twin research is increasingly integrating administrative data, from hospital, birth, and education registers, to understand human behavior, health, and disease. This includes administrative data to supplement existing twin-cohort databases or to identify new twin cohorts from only administrative datasets. We conducted a rapid review to identify twin cohorts that were collecting linked administrative data. We searched the Medline database and the Atlas of Longitudinal Cohort Studies for such twin studies. This included twin studies either linking data to pre-existing cohorts or studies identifying twin cohorts in administrative data. We identified 117 manuscripts or profiles featuring 47 unique twin cohorts and two multicohort consortia using linked administrative data, from countries across most global regions. We summarize how these cohorts were established, the data collected, and the size of the cohorts. We then discuss opportunities, challenges, and future research in twin research using linked administrative data.

The East Flanders Prospective Twin Survey, established in 1964, is one of the longest running twin birth registries with known placentation in the world. It operates across 13 maternity hospitals in East Flanders, Belgium, and has already registered 10,787 twin pairs and 318 triplet sets. Using a standardized protocol, EFPTS prospectively collects perinatal and biological data at birth, including detailed placental examination, systematic clinical documentation, and biobanking. Longitudinal follow-up and integration with national health and environmental databases enable linkage of early-life data to later-life phenotypes. Enrichment through nationwide datasets, such as the Study Centre for Perinatal Epidemiology, Child and Family health centers, environmental databases (based on land cover data and national monitoring stations), the Belgian National Register, and genomic reference data, further expands research opportunities. With over 60 years of experience, the EFPTS provides a uniquely rich and reliable framework for investigating the (epi)genetic and environmental determinants of health and disease.

The Older Finnish Twin Cohort was established 50 years ago and includes twins born in Finland before 1958. Members of the cohort have responded to detailed questionnaires about their health, habits, and lifestyle up to four times, in 1975, 1981, 1991, and 2011. In 2019, the Finnish Parliament approved the Act on the Secondary Use of Health and Social Data, which enables wider use of data from national social and healthcare registers as well as various patient systems and social services. This data resource article describes the linkage of the Older Finnish Twin Cohort to numerous social and healthcare registers, alongside linked data from their families and the broader Finnish population born in 1945−1957, which serves as a reference population for generalizability and other analyses.

This report provides an overview of the current state of the German twin family panel TwinLife, including information on design and sample characteristics as well as a selection of assessed constructs. TwinLife is a register-based longitudinal panel of four birth cohorts of German-speaking monozygotic and dizygotic same-sex twin pairs, and their core family members, including parents and siblings, but also partners and children of twins, if available. Using address data provided by a representative set of residents’ registration offices from across Germany, twin families were identified through a multistage process. The twins were born in 1990−1993, 1997−1998, 2003−2004, and 2009−2010. They were about 5, 11, 17, and 23 years old at the time of the first survey. Designed to investigate the development of social inequalities over the life course, the TwinLife currently covers an observation period from 2014 to 2025. In the first wave, the panel included data on 4096 twin families from all parts of Germany, covering the full range of key socioeconomic indicators. Over the years, the core TwinLife project has been expanded by several satellite projects, such as the molecular genetic TwinSNPs and the TwinLife Epigenetic Change Satellite (TECS) project. TwinLife provides a unique database with a wide-ranging potential for research, since it combines a longitudinal panel with an extended twin family design, adding genetic, epigenetic and additional biological data as well as the possibility to match geographical information.

Twin registries worldwide increasingly function as large-scale research infrastructures, enabling standardized phenotyping across the lifespan, integration of biological and environmental data streams, and international cross-cohort collaborative research and replications. This development is also taking place in Germany. The GERman Twin Registry Under Development (GERTRUD; www.gertrud.info) was established in 2022 as the first nationwide research platform for recruiting twins and higher order multiples of all ages within Germany to support the large-scale genetically informative psychological, sociological, health, and neuroscience twin research at national and international levels. GERTRUD is being developed as a modular infrastructure that supports classical and extended twin family designs, combining annual core survey waves with optional embedded modules for intensive phenotyping (e.g., neuroimaging, smartphone-based assessments), biosampling, and linkage of participants’ residential context to external geographic datasets via geospatial information systems (GIS). To operate within Germany’s stringent data protection landscape, GERTRUD implements project-specific pseudonymisation, role-based access control, and contract-governed remote analysis access. This article describes GERTRUD’s governance and legal–technical framework, its multisource data architecture, and the potential for collaboration across Germany and internationally. Examples of early data implementations further illustrate that the continuously collected multimodal twin data constitute a critical asset, essential for successful harmonization, replication, and collaborative and integrative behavioral genetics research.

Over the past years, the Italian Twin Registry (ITR) has joined networks and consortiums for genetic-epidemiological research, and several opportunities have arisen for national and international collaborations involving data and tool sharing, as well as cohorts’ harmonization. These networking efforts have led ITR to face complex ethico-legal challenges within the framework provided by the General Data Protection Regulation (GDPR) since 2018 but have also represented the occasion to address critical aspects in ITR organization and activities, looking for long-term solutions. This article describes some examples of: ITR international collaborations implying data and tools sharing; ITR national collaborations aimed to replicate general-population findings; ITR integration with national and regional administrative records; ethico-legal criticalities experienced by ITR in multicohort collaborations, and initiatives to overcome the GDPR bottleneck. Regarding collected data, ITR currently contains information on 30,362 twins of different ages and geographical areas, of whom 7797 are being asked to reconfirm their enrollment, having turned 18 or over. In the coming period, these figures may increase rapidly, thanks to the ongoing access to the National Register of the Resident Population, for which ITR is working to solve privacy-related legal issues. This will enable a systematic enrolment of large and representative twin cohorts, making ITR a valuable tool for genetic-epidemiological research and for the monitoring of relevant health outcomes in the Italian population.

Record linkage projects provide powerful opportunities to enrich cohort data with administrative and clinical information from national registries. The Netherlands Twin Register (NTR) is a large population-based twin-family cohort with longitudinal data collection that has established multiple record linkage initiatives to expand research on genetic and environmental determinants of health, disease, behaviour, development, and fertility. In this article we summarize and discuss NTR record linkage projects. Clinical and pathology registries have provided essential data on chorionicity, reproductive history, neonatal thyroid function, and cancer diagnoses, facilitating studies on prenatal environment, disease heritability, and polygenic risk prediction for diseases. Linkage with health insurance data has enabled validation of medication use and health research. Collaborations with Statistics Netherlands allow linkage to nationwide population-based data via secure infrastructures such as Open Data Infrastructure for Social Science and Economic Innovations (ODISSEI), supporting genomewide association studies of, for example, healthcare expenditure. Finally, linkage to environmental exposure datasets has permitted exposomewide analyses of health and wellbeing. Together, these projects illustrate the feasibility, scientific value, and challenges of record linkage in the Dutch context, highlighting its role in advancing twin research, genetic epidemiology, and population health studies.

In behavioral genetics, divorce is typically analyzed as an individual-level outcome, even though marital dissolution can only be experienced by couples. In this article, we discuss how assortative mating complicates the study of couple-shared outcomes because individual-level effects can be confounded by effects of the spouse. We then show how chain-linking affines (i.e., in-laws) provides sufficient information to estimate spousal similarity for couple-shared outcomes, which we incorporate into an extended twin model that we use to test for sex differences and assortative mating for individuals’ liability to divorce. We linked the Norwegian twin register to the Norwegian population register and constructed 124,544 extended family units (1196 units with monozygotic twins) comprising 353,210 marriages entered between 1983 and 2008. We found that divorce was significantly correlated among affines, and that female relatives were more highly correlated than male relatives. The extended twin model estimated a strong correlation (r = .60, SE = .10) between female and male familial factors. Couples’ liability to divorce was attributed to 18% (SE = 5%) female and 10% (SE = 3%) male familial factors, with an additional 16% (SE = 4%) accounted for by their correlation. Estimates from a classic twin model were considerably higher. These findings show that spousal similarity is an important source of variation in divorce liability and that failing to model it can inflate estimates of individual-level effects. Overall, the analytical framework offers a blueprint for dissecting any couple-shared outcome into sex-specific and assortative components.

Populations of Arab ancestry remain underrepresented in global twin studies. These populations are described by distinct genetic characteristics, high consanguinity, and strong familial relatedness. To contribute to diversity in genetics and twin research, the Qatar Twin Registry (QTR) was established as the first national twin registry in the Arab world and the second in the Middle East and North Africa. The QTR is a prospective and retrospective cohort study developed by Qatar University in collaboration with the Qatar Precision Health Institute (QPHI). The registry integrates with Qatar’s national biobank and whole-genome sequencing infrastructure to enable large-scale twin-based research. Participants contribute comprehensive health, developmental, psychosocial, and lifestyle information, along with biological samples collected through clinic-based assessments and home visits. QTR marks a milestone in regional genetic and epidemiological research. Leveraging and linking to Qatar’s national biobank and genomic infrastructure, it establishes a sustainable framework for investigating heritability, gene–environment interactions, and complex diseases in an underrepresented Arab population, contributing to global precision medicine efforts.

The Murcia Twin Registry (MTR) has steadily expanded over two decades and has become a key resource for twin research in the Mediterranean region. The registry currently includes data from 3971 individual twins born between 1940 and 1999, as well as an associated biobank containing samples from 1586 participants. Its primary research focus is on health and health-related behaviors within a public health framework, covering areas such as lifestyle, health promotion, quality of life, and environmental factors. Across multiple waves of data collection, the MTR has compiled extensive and wide-ranging phenotypic data. These data can be further expanded and have strong potential for record linkage with other health databases, particularly those of the regional public health care system, including both primary and inpatient care. Efforts are also underway to establish record linkage with additional sources of information, such as the educational system. In the near future, the registry aims to expand its biobank and continue the collection of longitudinal data, as well as increasing the ability to collect additional data that could enrich the information from participants in the register.

The Swedish Twin Registry (STR), established in the late 1950s, is one of the world’s most comprehensive twin registries and a cornerstone for research on genetic and environmental determinants of health. STR includes data on Swedish-born twins since 1886, complemented by longitudinal questionnaires, clinical measures, and biobank samples. In 2019, STR became a national research infrastructure with a mission to provide broader data sharing, while in 2025, STR further transitioned into a legally defined role as a ‘certain research database’ under the new Swedish Act on Certain Research Databases. The new legislation provides a framework for collecting and storing personal data with broad consent for future research projects, without requiring ethical approval for the data collection per se. Instead, ethical review is conducted separately for each research project seeking to use STR data. This article aims to describe STR’s current role, governance, and legal context, highlighting implications for data access and biobank integration. The registry currently holds data on more than 160,000 twins, with ongoing ascertainment at 9 months and 9 years with follow-ups at ages 15, 18, and 24. Annual linkages to national health registers enrich longitudinal analyses of disease outcomes. A variable search engine is openly available via STR’s web portal while higher level metadata is provided through Swedish research data services. STR’s evolution illustrates both opportunities and challenges in balancing open science practices, legal compliance, and participant integrity within large-scale research infrastructures.

This study examines the long-term consequences of neonatal health using three birth anthropometric measures — weight, length, and head circumference — and analyzes their differential associations with outcomes across the life course. Using linked Swedish and Norwegian population registers with follow-up into adulthood, we compare birth measures of singletons and twins and further estimate within-twin-pair fixed-effects models that control for shared genetic and family environmental factors. Our samples comprise up to approximately 679,000 singletons and 10,400 twins in Sweden, and 347,841 singletons and 6,762 twins in Norway, followed into adulthood. We find that different birth measures capture distinct aspects of fetal development with varying predictive validity across outcome domains. Birth length emerges as the strongest predictor of health outcomes (infant mortality, adult height), consistent with its role as a cumulative indicator of skeletal growth throughout gestation. Birth weight retains stronger associations with cognitive and educational outcomes (grade point average [GPA], years of schooling) in conditional models. Head circumference shows weak and inconsistent associations across specifications. Using within-pair head circumference differences to classify twins as symmetrically versus asymmetrically growth restricted, we find that symmetric intrauterine growth restriction is associated with substantially worse long-term outcomes than asymmetric restriction. In Norwegian twin pairs, growth restriction is associated with lower general cognitive ability (measured at military enlistment) relative to the unaffected co-twin; this within-pair deficit is more than twice as large for symmetric as for asymmetric restriction. We also document significant intergenerational transmission of birth anthropometrics, suggesting both environmental and genetic contributions. Finally, our register-based design with near-complete population coverage minimizes selection bias and attrition while providing statistical power for within-pair analyses across multiple outcomes. Exclusive reliance on birth weight may obscure important information about early-gestation developmental insults.

The Twins Early Development Study (TEDS) is a longitudinal population study of over 10,000 twin pairs born in England and Wales between 1994 and 1996. As the twins enter their thirties, a primary focus of TEDS is to better understand the development of common physical and mental health problems and the relationship with the different social milestones of adulthood (e.g., employment, partnerships, and/or parenthood). With over 30 years of prospectively collected questionnaire and genetic data, the study is uniquely placed to answer questions about the health challenges facing young adults today. Incorporating linked medical records with the existing research data will provide a different data perspective on our twin’s health status and outcomes and support more equitable research by helping to address both response and attrition bias. This article provides an overview of the protocol to link TEDS participants to electronic health records collected by the UK National Health Service (NHS). It will outline the linkage process, characterize the available linked study sample and NHS datasets, and describe the legal basis for this work.

The Early Growth and Development Study (EGDS) began in 2002 as a longitudinal prospective adoption study of birth parents, adoptive parents, and adopted children (n = 361 adoptees). It expanded in 2007 to include a second cohort of adoptees (n = 200), and a third cohort of siblings (siblings reared by the birth mother at age 7 [n = 217 siblings in 2013] and additional siblings in both birth and adoptive family homes [n = 881 siblings in 2016]). Data are available in a national repository within the Environmental influences on Child Health Outcomes (ECHO) study and have been integrated into analyses with national and international cohorts. Birth and adoptive families were originally enrolled through a systematic recruitment approach that began with efforts to partner with all domestic adoption agencies in specific regions of the United States following the birth of a child. Longitudinal assessments are ongoing and occurred in 9-month intervals until the adoptees turned 3 years of age, and in 1- to 2-year intervals thereafter to age 21. Data collection includes child temperament, cognition, behavior, and physical health; birth and adoptive parent personality, mental and physical health, context, parenting, and marital relations; the prenatal environment; genetic, hormonal, and cardiovascular data; and geocoding. A unique aspect of the adoption-sibling design is its ability to detect environmental influences on development and test complex interactions and correlations between genetic, prenatal, and postnatal environmental influences on a range of outcomes. The sample and procedures are described, followed by an overview of multicohort findings and opportunities for integration with other registries.

The National Project on Achievement in Twins (NatPAT) is a twin project based in the United States (US) that began in 2017. Using a cohort sequential design, the overall goal of the initial project was to uncover salient factors, including genetic and environmental influences, which contribute to the co-development of reading and math performance during elementary school. In 2022, the focus of NatPAT pivoted towards a new focus on the COVID-19 pandemic’s short- and long-term impacts on children’s reading achievement. In addition, a genomics data collection began. New enrollment into the registry continues every year, but currently NatPAT follows 1997 twin pairs and their families as they progress through school. The project supports open science principles, with open materials and code, preregistration, and shared data. Here we present the goals of the project, summarize recent results, methods and materials, with a focus on the integration across many different data sources, and future directions of the project.

The Interplay of Genes and Environment across Multiple Studies (IGEMS) is a consortium of 21 twin studies from 5 countries (Australia, Denmark, Finland, Sweden, and United States) established to explore the nature of gene–environment interplay in cognitive, physical, and emotional health across the adult lifespan. The combined data from over 145,000 participants (aged 18 to 108 years at intake) has supported multiple research projects over the three phases of development since its inception in 2010. Phases 1 and 2 focused on launching and growing the consortium and supported important developments in data harmonization, analyses of data pooled across multiple studies, incorporation of linkages to national registries and conscription data, and integration of molecular genetic and classical twin designs. IGEMS Phase 3 focuses on developing appropriate infrastructure to maximize utilization of this large twin consortium for aging research.

The Painel USP de Gêmeos (University of São Paulo Twin Panel) is, based at the Instituto de Psicologia da Universidade de São Paulo. It was formally established in 2017 to advance research on fundamental psychological processes through twin study designs. Our relatively new registry comprises a volunteer sample of 8839 twin individuals, 70% of whom live in Brazil’s Southeast, the region with the highest twinning birth rate (10.64‰) of the country, within a national population of 213 million. Our collaborative research group has expanded to include partners from psychology, dentistry, and medicine at USP, as well as other Brazilian institutions, such as the Universidade Federal do Rio Grande do Norte, Universidade Federal da Bahia, and Universidade Federal do Espírito Santo. We are advancing biobehavioral research in Brazil through innovative methodologies, interdisciplinary collaboration, and international partnerships. All twin participants contribute to multiple studies associated with four datasets employing the same hierarchical identification system for participants and families: the Biorepository, the Physiological and Physical Repository, the Behavioral Repository, and Fonoteca Cesar Ades (FOCA). Future directions include expanding our twin registry across the five regions of Brazil, our research partnerships, promoting genetic literacy, and fostering public engagement.

The Hungarian Twin Registry (HTR) has operated as a population-based registry since 2021, following its establishment at Semmelweis University in Budapest. In recent years, the HTR infrastructure has been substantially expanded, including blood storage facilities, information technology systems, a phone bank and voicemail system, and administrative resources. Adult twins (aged ≥18 years) and relatives (parents, foster parents, or caregivers) of twins under 18 years of age can register via the registry’s website or by voicemail following a nationwide identification and notification process. The registration questionnaire consists of eight sections covering sociodemographic and anthropometric characteristics, smoking habits, and medical history, including diseases, surgeries, and therapies. By December 20, 2025, a total of 12,676 twin individuals and their parents or guardians had registered, including 7244 adult twins and 5432 parents or guardians of minors. Based on self-reported data, 44.1% of registered adult twins were monozygotic (MZ) and 55.9% dizygotic (DZ), while among registered children 23.5% were MZ and 76.5% DZ. Of the registered adult twins, 68.4% were female. The HTR offers substantial potential for linkage with national health databases, enabling enrichment of phenotypic and outcome data without additional participant burden. In particular, linkage with the Hungarian National Cancer Registry and other health-related databases is feasible under appropriate ethical approval and data protection procedures and may support long-term follow-up and disease-specific analyses. The article also summarizes current research projects conducted within the HTR, which has become the largest population-based twin registry in Central and Eastern Europe based on the number of registrations.