Although combining data from studies using different personality assessments may enable GWAS of personality in sample sizes large enough to detect common loci with small effects on personality, this is a challenging undertaking. A naïve meta-analysis that would use simple sum scores is unlikely to be effective, given the modest correlation between dimensions measured in different instruments (for example, De Fruyt et al.6 showed the maximum correlation between dimensions of the TPQ and the FFM to be 0.54). Alternatively, one might attempt to map in a meta analysis not the sum scores themselves, but the scores from a principal components analysis that would combine information across scales within the same instrument, and potentially account for more of the phenotypic variance. A more sophisticated approach would be to employ item response theory (IRT40) to estimate the unmeasured, latent trait thought to be evaluated by personality assessments. van den Berg et al.41 applied IRT to the attention problems subscale of the Young Adults Self Report questionnaire42 assessed in a sample of individuals from the Netherlands Twin Registry. Heritability of the estimated latent
