Twin family registries worldwide

For more than 65 years, twin registries have contributed to our knowledge about genetic and environmental influences on all aspects of human life. Since the establishment of the first twin registry in 1954 in Denmark, many twin registries have been founded worldwide. Here, we provide an overview of the spread of twin registries across the world. While the oldest and largest twin registries reside in Europe, twin registries also have a strong presence in the United States, Australia, and East Asia. New registries have more recently been established in Africa, the Middle East, and Latin America, ensuring that twin registries are present across six continents. Twin registries vary widely in their inclusion criteria: while some registries still focus on the twin pair, a number of registries have started to also include family members of twins. The total scope of twin registries is large, representing combined more than 1.3 million individuals. The longitudinal character of twin registries in combination with extensive data collection, by surveys but also via registry linkages and experimental settings, means that they represent a treasure trove of data. In addition, while at its roots twin registries used the difference in genetic similarity for monozygotic and dizygotic twins to estimate the heritability of traits, many twin registries now also collect genotypic and epigenetic information. The maintenance and expansion of twin registries across the world will continue to contribute to the enhancement of our knowledge of genetic and environmental causes of complex human behaviors and health.