This study compared borderline personality disorder (BPD) and bipolar 2 disorder (BP 2 disorder) with respect to reported childhood trauma and Five-Factor personality traits using the Childhood Trauma Questionnaire (CTQ) and the NEO Five-Factor Inventory (NEO-FFI). Participants were 50 men and women, aged 18-45, with DSM -5-diagnosed BPD and 50 men and women in the same age group with DSM -5-diagnosed BP 2 disorder. Participants could not meet criteria for both BPD and BP 2 disorder. Borderline participants had significantly higher scores on the neuroticism subscale and significantly lower scores on the agreeableness subscale of the NEO-FFI. After correction for multiple comparisons, there were no between-group differences on CTQ scores. Study results suggest that BPD and BP 2 disorder differ primarily with respect to underlying temperament/genetic architecture and that environmental factors have only a limited role in the differential etiologies of the two disorders.