Next, we used the classical twin model to partition the total variance in cannabis withdrawal into additive genetic (A), common (or shared) environmental (C), and residual/non-shared environmental (E) variance. A denotes the variance resulting from the sum of allelic effects across all segregating genes. C refers to environmental influences shared by family members and may include shared home environment, parental style and uterine environment. E includes environmental factors not shared by twin pairs (e.g. idiosyncratic experiences), stochastic biological effects, and also measurement error. These variance components can be estimated using twin data because identical (monozygotic, MZ) twins share all their genes, while nonidentical (dizygotic, DZ) twins share on average half their segregating genes. A, C, and E influences predict different patterns of MZ and DZ twin pair correlations, and structural equation modeling is used to determine the combination of influences that best matches the observed data.
