Fig. 2.
A bivariate biometric dual change score model to examine genetic and environmental influences on the relationship between age changes in two measures (Y and X). Error variance (σu) is assumed to be constant at each age within each factor; αy and αx represent constant change related to the slope factors ys and xs; βy and βx represent proportional change in Y and X; cross-trait coupling is indicated by γyx and γxy. The model includes estimates for intercepts (y0 and x0), mean intercepts (μy0 and μx0), and mean slopes (μys and μxs). The model includes genetic and environmental effects specific to the each intercept (ax0 and ex0, ay0 and ey0) and specific to each slope (axs and exs, ays and eys). The paths from ax0 and ex0 to xs and from ay0 and ey0 to ys generate the decomposition of the correlation between the intercept and slope for X and Y