Which one impacts the other?: The relationship between change in borderline personality disorder severity and change in posttraumatic stress disorder severity among individuals in dialectical behavior therapy.

Borderline personality disorder (BPD) and posttraumatic stress disorder (PTSD) commonly co-occur. Limited research, however, has examined the association between fluctuations in the severity of one disorder with fluctuations in the severity of the other disorder. In a sample of N = 240 self-harming individuals with BPD undergoing 6 or 12 months of Dialectical Behavior Therapy (DBT), we used a cross-lagged panel model with 3-month intervals to evaluate the temporal relationship between BPD severity and PTSD severity during DBT and throughout follow-up. We similarly examined the relationship in the sub-sample meeting criteria for both disorders (BPD + PTSD; N = 81). Results indicated that decreases in PTSD severity at one timepoint corresponded with decreases in BPD severity at the subsequent timepoint, and that there was minimal evidence for the reverse relationship. Findings were consistent across our analyses in both the full BPD sample and the BPD + PTSD subsample. In the context of DBT, addressing PTSD might help reduce subsequent BPD severity, but addressing BPD features might not have similar effects on PTSD severity.