Twin cohorts provide a unique advantage for investigating the role of genes and environment in the aetiology of complex traits, such as psychiatric disorders. Twin data, collected in large-scale studies in Europe, including in the Netherlands, are exceptionally useful for genetic studies, because they supply information that is unobtainable in regular family or case-control studies. The purpose of the twin design is to solve problems that can arise in the analysis of human study samples that are poorly designed in an epidemiological sense, that suffer from ascertainment bias and lack of longitudinal follow-up and that are contaminated by noise created by environmental variation throughout development, including fetal life and childhood.
Variation in vulnerability for psychiatric disorder may be caused by differences in genotype and by non-genetic differences ('environment') between individuals. The relative contributions of genotype (g) and environment (e) to variation can be assessed with the classical twin design, in which phenotypic resemblance in monozygotic (mz) twins who are genetically identical (or nearly identical), is compared to phenotypic resemblance in dizygotic (dz) twins, who share on average 50% of their segregating genes. A larger resemblance in mz than in dz twins suggests that genetic factors make a contribution to phenotypic individual differences. The variance decomposition for a single trait can be generalized to longitudinal and multivariate data. In such data, the comorbidity of traits can be decomposed into genetic and non-genetic sources. We illustrate this approach using longitudinal data collected from young and adult Dutch twins with regard to a number of phenotypes (mood disorders, adhd and borderline disorder personality features).