Integrating personality disorder with basic personality science.

The work by Kendler et al. (1) in this issue of Acta Psychiatrica Scandinavica replicates the phenotypic correlation between borderline personality disorder (BPD) and the trait of neuroticism from the fivefactor model of personality (FFM; 2) within a large twin sample. This study also extends previous work by using behavioural genetics to examine the underpinnings of the relationship between BPD and the FFM. This study assessed four maladaptive trait components relevant to BPD (i.e., affective lability, insecure attachment, identity problems and cognitive dysregulation) and found they displayed a reasonably unidimensional structure. Furthermore, genetic analyses suggested that the four BPD traits were highly heritable and that a majority of their variance was explained by a common factor. In particular, this common genetic factor was most clearly defined by the trait of affective instability, consistent with the view that BPD can be viewed as primarily a disorder of dysregulated and labile affect. Kendler et al. (1) also found that each of the four BPD traits, as well as factor common among them, was most strongly related to the FFM trait of neuroticism, but obtained secondary relationships with the traits of low agreeableness and low conscientiousness. Similarly, the genetic variance of the latent BPD factor was strongly related to all three of these normal personality domains, but the unique variance accounted for by the individual BPD traits evinced relationships that were specific to FFM neuroticism. Based on these results, Kendler et al. conclude that a common genetic liability accounts for a substantial portion of the association between BPD and the normal personality traits from the FFM. This study adds appreciable evidence to support the dimensional view of BPD and personality pathology, in general. Throughout the history of psychiatric nomenclatures, personality disorders (PDs) have been conceptualized as a group of categorical syndromes that are distinct from one another and from normal personality. However, researchers have increasingly highlighted the limitations of the current categorical approach including problematic heterogeneity, substantial co-occurrence, arbitrary boundaries with normality, and failure to adequately describe the pathology treated in clinical practice (3). The alternative, dimensional viewpoint is that PDs, including borderline, can be dismantled into component traits (e.g., affective dysregulation and impulsivity) as Kendler et al. did within the current study. Furthermore, research has suggested that these PD components are maladaptive variants of the same traits from the FFM that describe normal personality functioning (4, 5). The FFM, which includes the five broad, bipolar traits of neuroticism (vs. emotional stability), extraversion (vs. introversion), openness (vs. closedness to experience), agreeableness (vs. antagonism), and conscientiousness (vs. impulsivity), has extensive validity evidence and is considered the predominant model for describing normal personality. While continued research is needed, these results provide continued empirical support for a dimensional understanding of personality pathology. This is particularly relevant and important to the field in light of the fact that the DSM-5 and ICD11 will likely include a dimensional trait model in the upcoming revision.