at 42oC in a 420 µl volume containing 1 µg/ml Proteinase K, 6 mM Tris-Cl pH 7.5, 6 mM EDTA, 0.3% Na Sarkosyl, 1.2 M guanidine HCl, and 0.5 M ammonium acetate. DNA was purified from the digest by standard chloroform extraction and EtOH precipitation, and resuspended in de- ionised water. COMT genotypes were determined by restriction fragment length polymorphism method. A 109- base-pair polymerase chain reaction product was generated using the primers Comt1 nt 1881 5´ CTCATCACCATCGA GATCAA 3´ and Comt2 nt 1989 5´ CCAGGTCTGACAAC GGGTCA 3´. The Val and Met alleles were discriminated by digesting the PCR product with NlaIII, followed by agarose gel electrophoresis and visualization by ethidium bromide staining. Psychological measures Early life stress questionnaire (ELSQ) Early life stress was measured using the ELSQ, which was developed for use in an international cohort (Gordon et al 2005; McFarlane et al 2005; Paul et al 2005). The ELSQ is based on the Child Abuse and Trauma Scale, which has been shown to have strong internal consistency, test-retest reliability, and validity, as it correlates with adult outcome and psychopathology (Sanders and Becker-Lausen 1995). The measure consists of 19 events shown to have psychological impact in childhood in previous studies (eg, Sanders and Becker-Lausen 1995). Participants reported whether they experienced a broad range of specific early life stressors and the age at which it occurred. Specific events covered on the questionnaire are listed in Table 1. For the current study, participants were grouped based on the number of early life stressors that they reported (ie, “low early life stress” = 0 early life events, and “high early life stress” = 2 or more early life events). NEO-Five Factor Inventory (NEO-FFI) The NEO-FFI, a 60-item self-report questionnaire, was used to assess personality. The NEO-FFI is a short form of the NEO-Personality Inventory-Revised (NEO-PI; Costa and McCrae 1992), a measure of the Five Factor Model of Per- sonality composed of Neuroticism, Extraversion, Openness to experience, Agreeableness, and Conscientiousness scales. Sample From the larger dataset, we identified all individuals over the age of 18 who had data for the relevant genetic and psychological measures. This resulted in a sample of 486 Neuropsychiatric Disease and Treatment 2006:2(2) COMT polymorphism and personality Table 1 Events on the early life stress questionnaire 1. Emotional abuse 2. Physical abuse 3. Sexual abuse 4. Domestic violence 5. Severe family conflict 6. Neglect 7. Divorce 8. Separated from family 9. Death in family 10. Major illness in family 11. Fire destroyed home 12. War 13. Natural disaster 14. Major personal illness 15. Hospitalization/surgery 16. Bullied 17. Premature birth 18. Adoption 19. Describe other events individuals. Participants had an average age of 38.52 (SD = 16.6; range 18–82), an average education of 14.5 years (SD = 2.6; range 8–18), and the sample was 53% male. Seventy-three percent of the sample was collected in Australia, 15% was collected in the US, and 12% was collected in Europe. The majority of the sample was of European descent (83.1%). Approximately 5% reported having “mixed” ethnicity, 3.1% were Asian, 2.1% were Middle Eastern, and the remaining 7% were either Indian, African, indigenous American, indigenous Australian, or Pacific Islander. Approximately 20% of the sample had a Val/Val genotype and 37% percent had a Met/Met genotype and allele frequencies were in Hardy-Weinberg equilibrium. This is commensurate with the frequency of the low enzyme activity allele reported in previous psychiatrically screened Caucasian control samples (Karayiorgou et al 1997). Data analysis Univariate comparisons of demographic characteristics between COMT genotype groups were carried out using independent samples t-tests for continuous variables and χ2 tests for categorical variables. To examine the comparability of the Val/Met allele frequencies across subsamples collected on each of the three continents, a χ2 test was conducted. Analysis of variance (ANOVA) was used to examine differences in personality dimensions on the NEO-FFI between the low (Met/Met) and high (Val/Val) COMT genotype groups. MANOVA was used to examine the potential interaction between COMT, sex, and personality. A second MANOVA was carried out to examine the possible 221