The following data cleaning procedure was performed separately for each subsample. The personality scale scores were calculated by summing the relevant item scores, reverse scoring where necessary. (Note that, for consistency, the rating scale used in the YFS study was converted to a 0–1 measure by converting the item scores as follows: 1=0, 2=0.25, 3=0.5, 4=0.75, and 5=1.0.) Missing items were imputed with the sample mean score on the item. Personality scale scores for individuals with more than 25% missing values on that scale were assigned as missing. To minimize departures from normality the scale scores were then angular transformed (Freeman and Tukey 1950; Eaves et al. 1989), as was also done in Keller et al (2005). Last, scale scores were corrected (by regression) for sex, age, age2, sex*age, and sex*age2 effects and each scale was standardised separately per sex. Note that because all individuals in the NFBC sample were 31 years old we only corrected for sex effects in that cohort.
