the purpose of a GWAS meta-analysis in mind. The second objective was to test for measurement invariance across cohorts, and the third objective was to establish the heritability of the harmonized Neuroticism and Extraversion scores in the six participating twin cohorts. Sex differences in the genetic background of Neuroticism and Extraversion were studied, as well as the contribution of non-additive genetic factors. The contribution of non-additive genetic factors to variation in personality traits has been extensively discussed in the literature (Keller et al. 2005), but their assessment requires a large sample (Posthuma and Boomsma 2000). Lastly, we studied the theoretical increase in power of finding a quantitative trait locus due to the harmonization of phenotypes in two large cohorts.
