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. 2011 Sep 1;34(9):1179–1188. doi: 10.5665/SLEEP.1234

Figure 1.

Figure 1

ACDE twin model. Individual differences in sleep reactivity (depicted with rectangles to denote an observed variable) are modelled as being due to multiple types of influences, represented with circles to denote that they are unobserved: additive genetic (A), dominant genetic (D), shared environmental (C), and nonshared environmental (E). The Twin 1 with Twin 2 A correlation is set to 1.0 for monozygotic (MZ) twins because they share all of their genes, and 0.5 for dizygotic (DZ) twins because they share on average half of their segregating genes by descent. The D correlation is set to 1.0 for MZ twins and 0.25 for DZ twins. The C correlation is set to 1.0 for both types of twins because both are raised together. The E correlations are set to zero because nonshared environment is uncorrelated, by definition. In a traditional twin design, D and C cannot be simultaneously estimated; one is set to zero depending on the pattern of MZ and DZ correlations (C influences tend to make the DZ correlations greater than half the MZ correlations, while D influences tend to make DZ correlations less than half the MZ correlations). Fitting this model to the covariance matrices relating Twin 1's scores to Twin 2's scores in each zygosity group provides estimates of these effects.