Maximizing the Value of Twin Studies in Health and Behaviour

In the classical twin design, researchers study trait resemblances and differences in cohorts of identical (monozygotic) and non-identical (dizygotic) twins to understand how genetic and environmental factors correlate with behaviour and other phenotypes. The twin design has also become a valuable tool for studying causality, intergenerational transmission, gene-environment correlation, and interaction. Here we review recent developments in twin studies, results from twin studies for newly emerging phenotypes, what we have learned about twinning, and look at whether twin studies are representative of the general population and of global diversity. We provide an updated overview of twin concordance and discordance for major diseases and disorders, which conveys a crucial message: genetic influences are not as deterministic as is often believed. This has important implications for genetic risk prediction tools, as genetic predictions can only go as far as the monozygotic twin concordance rates.