Abstract

Background The relationship between borderline personality disorder (BPD) and type-II bipolar disorder (BDII) is not clearly understood. Nevertheless, in clinical practice and research, most efforts focus on establishing a categorical distinction between the two. We propose using personality traits as a more informative strategy to describe them. Methods Five-Factor Model personality traits were measured in 73 individuals with either BPD or BDII. Latent class cluster analysis was applied to the sample. Results A three-cluster model resulted the best fit to the data, where all clusters had high neuroticism and low extraversion scores but differed widely on the other traits. The clusters' boundaries did not match the categorical diagnosis. Conclusions Our sample showed significant heterogeneity on personality traits, which can have a relevant effect on the outcome of each disorder and that was not captured by the categorical diagnosis. Thus, we advocate for a multivariate approach as a better way to understand the relationship between BPD and BDII.