During the last fifty years more than 2700 twin studies have been published, examining the etiology of a high number of traits. Twin studies enable investigation of both genetic and environmental effects, and thereby also examination of causal factors involved in human traits and disorders. The beauty of twin studies resides in the potential of studying the unobserved by the logic of a design. The aim of this article is to outline central theoretical foundations and possible limitations, and to review selected key findings. We describe the inherent fundamentals of the classic and extended twin designs. The logic of the main analytic approaches is outlined, and the principles of univariate biometric modelling described. Next, we review different multivariate models, including the Cholesky, correlated factors, common factor, common pathway and phenotypic causality models. Additionally, the cotwin-control approach, representing a natural experimental design, and mimicking a counterfactual situation, is outlined. Central assumptions, threats and 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, we review some selected findings from the field of behavior genetics and twin studies.
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