In line with molecular data (Chen et al., 2004), and previous imaging-genetic studies (Egan et al., 2001), we modeled COMT Val158Met genotype in an ordinal manner (i.e. 0/1/2 Val allele “dosage”). Mixed model regression was chosen over traditional methods to examine cortical thickness development (i.e. repeated measures ANOVA, fixed effects regression models) because it permits the inclusion of multiple measurements per person, missing data, and irregular intervals between measurements (Pinheiro and Bates, 2000). Linear (rather than non-linear) age-terms were used in all analyses because (i) preliminary analyses within this dataset established that higher order age terms were not better able to predict variance in CT than linear age, and (ii) our previous work has demonstrated that over the age range included in this study—the predominant effect of age is linear (Shaw et al., 2008).
