We fit a bivariate Cholesky model (see Neale and Cardon 1992) with measurements taken at ages 12 and 14 entered as separate variables. In a longitudinal analysis, the bivariate model permits the estimation of both the genetic and environmental influences at each age and the correlations between genetic and environmental factors across ages, which is important for examining whether same or different genetic and environmental factors influence the trait at different ages. Since our sample included both male and female twins and preliminary analyses suggested some differences between male and female twin correlations, we investigated potential sex differences in genetic and environmental effects on the DD phenotype by allowing genetic and environmental parameters to differ between males and females.
