Variance components from the biometric twin model are reported in Figure 2 as a series of correlation matrices with superimposed heatmaps. The phenotypic correlation matrix is denoted “P.” As the participants age, CPD becomes more stable over time. The correlation between age 14 and age 17 CPD is .55; the correlation between age 20 and age 24 CPD is .78. Note that the A, C, and E matrices are scaled such that the A, C, and E matrix, if summed element-wise, give the P matrix. The “A” matrix denotes that portion of the phenotypic matrix due to the additive effect of genes. It follows the same pattern as the phenotypic matrix, with genetic effects becoming more stable over time. Shared environmental effects are stronger at younger ages and generally taper over time. Unshared environmental effects contribute largely only to variance at assessments, but also contributes increasingly to stability of CPD across assessments, perhaps as a result of the highly addictive nature of cigarette smoking.
