Applying the IRT method to Neuroticism and Extraversion is especially relevant for the field of behavior genetics, as these personality traits are correlated with numerous other traits and disorders, not only phenotypically but also genetically (Heath et al. 1994; Hopwood et al. 2011; Klein et al. 2011; Markon et al. 2005; Samuel and Widiger 2008). For example, Neuroticism is highly related to a variety of psychiatric disorders, including major depression and borderline personality disorder (Distel et al. 2009; Kendler and Myers 2009), and Extraversion is associated with alcohol use (Dick et al. 2013). Earlier GWA studies of personality (De Moor et al. 2010; Service et al. 2012; Shifman et al. 2008; Terracciano et al. 2010; van den Oord et al. 2008) focused on single inventories, hence hampering sample size, and few, if any, genome-wide significant loci were detected. Large sample sizes are needed, which can be achieved by pooling results from multiple inventories.
