 Hypotheses: 1 Patients in the DBT-6 arm will show equivalent reductions in the frequency and severity of self-harm across the treatment phase and one-year post treatment follow-up phase compared with patients in the DBT-12 arm. 2 Patients who present with high rates of self-harm and impulsive behaviours will have reductions in the frequency and severity of self-harm behaviours that are comparable in the DBT-6 arm and the DBT-12 arm, over the course of both the treatment phase and the 1-year post treatment follow-up. Patients who present with co-occurring post-traumatic stress disorder, generalized anxiety disorder, or cluster A or C personality disorders will exhibit lesser reductions in the frequency and severity of self-harm behaviors in the DBT-6 arm compared to the DBT-12 arm, over the course of both the treatment phase and the 1-year post treatment follow-up. 