To generate the developmental trajectory estimates, we applied a method to estimate longitudinal development from all available repeated observations using linear mixed models (van den Oord et al., 2009). This method first determines the optimal functional form of the developmental trajectory, then generates individual trajectory estimates based on the best fitting model, using best linear unbiased predictors (BLUPs) (Pinheiro & Bates, 2000). As this approach takes advantage of all available repeated assessments, it results in more precise estimates than traditional approaches that estimate development using two assessments (i.e., change scores) (Willett et al., 1998). Further, as estimates are based on mixed model trajectory slopes, they are more robust to attrition than competing approaches (Laird & Ware, 1982). This method of estimating outcomes from longitudinal phenotypic assessments is well-established in psychiatric GWAS (Adkins et al., 2011; Clark et al., 2012; McClay et al., 2011a).
