The beauty, logic and limitations of twin studies

Abstract

During the last fifty years more than 2700 twin studies have been published, examining the etiology of ahigh number of traits. Twin studies enable investigation of both genetic and environmental effects, andthereby also examination of causal factors involved in human traits and disorders. The beauty of twinstudies resides in the potential of studying the unobserved by the logic of a design. The aim of this article isto outline central theoretical foundations and possible limitations, and to review selected key findings. Wedescribe the inherent fundamentals of the classic and extended twin designs. The logic of the main analyticapproaches is outlined, and the principles of univariate biometric modelling described. Next, we reviewdifferent multivariate models, including the Cholesky, correlated factors, common factor, common pathwayand phenotypic causality models. Additionally, the cotwin-control approach, representing a naturalexperimental design, and mimicking a counterfactual situation, is outlined. Central assumptions, threatsand limitations of the twin design are discussed. In particular, we address the issue of missing heritability,non-random mating, the equal environment assumption and gene-environment correlations. Finally, wereview some selected findings from the field of behavior genetics and twin studies.