Each item could be answered with a true/false response and to avoid response set bias the items were phrased in such a way that for some items a true and for others a false answer adds to the subscale score. Scale scores were calculated by summing the item scores for each scale. For the complete sample (i.e. including the non-genotyped individuals), we then performed the following data preparation procedure. Missing items were replaced with the sample mean score on the specific item. Individuals with missing values on more than 25% of the scales’ items were treated as missing for that scale. Scale scores were then transformed by taking the arcsine of the square root, in order to minimize departures from normality (Eaves, Eysenck, & Martin, 1989; Freeman & Tukey, 1950). Finally, scores were corrected for age, age2, sex, sex*age, and sex*age2 effects and all scales were standardised separately for each sex to a mean of 0 and a standard deviation of 1.
