Estimating heritability of psychological traits using the classical twin design; a gentle introduction to concepts and assumptions

Heritability is a concept that is often misunderstood. The term is used somewhat differently by researchers from how it is used in common parlance. This paper is a gentle introduction to the scientific concept of heritability, and to how it can be estimated for psychological traits in humans through analyses of data from monozygotic and dizygotic pairs of twins. The paper then explores some of the assumptions of the classical twin design, and presents calculations of the consequences of breaking these assumptions. Lastly, the paper introduces multivariate twin modeling, with a focus on how heritability of traits can be impacted by causal effects from other traits, and how twin designs can be informative when making causal inferences.