A phenomenon in which an individual carries some genetic material originating from another individual (for example, from their co‑twin or mother).Ĭhorion. This impacts heritability estimation in the classical twin design.Ĭhimerism. Individuals with similar phenotypic characteristics, such as height or intelligence, are more likely to mate with each other than would be expected by chance. Forms the amniotic sac together with the chorion.Īssortative mating. A membrane that contains the amniotic fluid and provides a protective environment for an embryo by closely covering it. We conclude by discussing existing knowledge gaps in twin studies.Īmnion. Next, we turn to assumptions and advances in research methods and the study of twinning as a phenotype, which is important for the mothers of twins and for female fertility and may inform developmental biology.

We then summarize concordance and discordance in twin pairs for major psychiatric and somatic disorders, arguing that no other design is as informative for studying penetrance and genetic prediction of disease risk. We begin by introducing the classical twin design. We reflect on methods for achieving progress in fields that have traditionally valued twin studies of individual differences, and we explore which fields offer new opportunities for twin research and how insights from other disciplines can inform twin studies. In this Review, we examine the use of twin studies as researchers become interested in new phenotypes, including those based on omics technologies. They can also extend the classical twin design to model causal relations between exposures and traits, to describe differences within discordant twin pairs, to estimate genetic and cultural components of inheritance, and to account for measured genetic variables and environmental exposures. Researchers can apply the classical twin design to estimate the influence of genes for any trait they can measure. Because MZ twins are genetically identical and DZ twins share 50% of their genes on average, any additional similarity between identical twins should be related to genes, provided that the twins share the same environments 1, 2. The classical twin design compares trait resemblances in monozygotic (MZ) twins to those in dizygotic (DZ) twins. Since long before the advent of human genomics, researchers have relied on twin studies to answer these questions. This has important implications for public understanding of genetic risk prediction tools, as the accuracy of genetic predictions can never exceed identical twin concordance rates.īehavioural scientists are often interested in understanding how human traits, such as intelligence, educational attainment, depression and anxiety, are influenced by inherited and environmental factors. We provide an updated overview of twin concordance and discordance for major diseases and mental disorders, which conveys a crucial message: genetic influences are not as deterministic as many believe.

We ask whether the results of existing twin studies are representative of the general population and of global diversity, and we conclude that stronger efforts to increase representativeness are needed. Here we review recent developments in twin studies, recent results from twin studies of new phenotypes and recent insights into twinning. The twin design is also a valuable tool for studying causality, intergenerational transmission, and gene–environment correlation and interaction. In the classical twin design, researchers compare trait resemblance in cohorts of identical and non-identical twins to understand how genetic and environmental factors correlate with resemblance in behaviour and other phenotypes.

Twin models female