Fig. 1.

A univariate biometric dual change score model to examine genetic and environmental influences on age changes in Y. Error variance (σ_u) is assumed to be constant at each age; α represents constant change and is related to the slope factor y_s; β represents proportional change. The model includes an estimate for intercept (y₀), mean intercept (μ₀), and mean slope (μ_s). The model includes genetic and environmental effects specific to the intercept (a₀ and e₀) and specific to the slope (a_s and e_s). The paths from a₀ and e₀ to the slope (y_s) generate the decomposition of the correlation between the intercept and slope