A central component of Dialectical Behavior Therapy (DBT) is the teaching of 
specific behavioral skills with the aim of helping individuals with Borderline 
Personality Disorder (BPD) replace maladaptive behaviors with skillful behavior. 
Although existing evidence indirectly supports this proposed mechanism of 
action, no study to date has directly tested it. Therefore, we examined the 
skills use of 108 women with BPD participating in one of three randomized 
control trials throughout one year of treatment and four months of follow-up. 
Using a hierarchical linear modeling approach we found that although all 
participants reported using some DBT skills before treatment started, 
participants treated with DBT reported using three times more skills at the end 
of treatment than participants treated with a control treatment. Significant 
mediation effects also indicated that DBT skills use fully mediated the decrease 
in suicide attempts and depression and the increase in control of anger over 
time. DBT skills use also partially mediated the decrease of nonsuicidal 
self-injury over time. Anger suppression and expression were not mediated. This 
study is the first to clearly support the skills deficit model for BPD by 
indicating that increasing skills use is a mechanism of change for suicidal 
behavior, depression, and anger control.