This study included data obtained from 160,958 individuals from 23 cohorts, of which 6 were twin cohorts. Neuroticism and Extraversion were assessed by 9 different personality inventories; 7 cohorts assessed more than one inventory. The first objective was to determine the feasibility of the IRT approach in linking Neuroticism and Extraversion item data from different inventories: to what extent do the different inventories measure the same constructs? For instance, Harm Avoidance correlates moderately high with Neuroticism (r = 0.5–0.6) (De Fruyt et al. 2000). Therefore, we expect that mapping item data from Harm Avoidance with Neuroticism will be less perfect than mapping Neuroticism item data from other personality inventories (e.g. EPQ versus NEO neuroticism). We expect that this is even more the case for mapping Reward Dependence with Extraversion. Here we determine to what extent cross-inventory mapping is feasible, for 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
