for this age group reduced to ζe 12. For the CAGE at age 18–20, the unique environmental variance equaled ße 12 × ζe 12 + ζe 22 and for age 21–23 ße 22 × (ße 12 × ζe 12 + ζe 22) + ζe 32. The unique environmental covariance over age is modeled to run via the transmission terms only and can therefore be separated from variance due to measurement error that is modeled to influence the variances but not the covariances. For model identification purposes, a constraint has to be applied to the measurement error variance at the outer categories (Neale and Cardon 1992). In this study, variance due to measurement error was constrained to be the same for all age groups (submodel 3a). Whether unique environmental influences were purely age-specific was tested by fixing the transmission terms at zero in submodel 3b.
