Classical Twin Model

Simplified representation of the classical twin model. In behavioral-genetic models the three sources of influences on individual differences are commonly labeled by the letters A, C, and E, respectively, stemming from Additive genetic influences (also known as heritability, and sometimes represented by an h2 instead on an A), Common environmental influences (also known as shared environmental influences), and non-shared Environmental influences (and measurement Error). Note that the latter are by definition uncorrelated between twins. See for a detailed representation of the classical twin model for example Figure A.9 in Knopik et al. (2017). rMZ = monozygotic twin correlation; rDZ = dizygotic twin correlation.