The present study examined BPD individuals' DBT skills use throughout treatment, and its relationship to primary outcome variables. Four main findings emerged. First, all participants reported using at least some DBT skills about 50% of the time before treatment started. Second, participants treated with DBT reported using skills throughout treatment significantly more than participants in the control condition. Third, DBT skills use fully mediated the likelihood of suicide attempts and partially mediated the likelihood of NSSI to occur. Fourth, skills use fully mediated the change over time of certain indicators of emotional distress (Anger Control and depression). These findings provide further validation for the DBT model by indicating that an increase in DBT skills mediates some of the outcomes reported in DBT RCTs (see Lynch, Trost, Salsman, & Linehan, 2007a
Although DBT is based on the assumption that remediating skills deficits will drive a decrease in maladaptive behavior, research thus far has at most indicated an association between skills deficit change and maladaptive behavior change. This study is the first to clearly support the skills deficit model for suicidal behavior in BPD by showing that increasing skills use is a mechanism of change for suicide attempts. The partial mediation found for nonsuicidal self injury partially supports the skills deficit model, and suggests that additional factors may play into the change seen in this behavior throughout treatment. For example, research indicates that thought suppression is a partial mediator for the relationship between emotional reactivity and NSSI (Najmi, Wegner, & Nock, 2006) and is also associated with self harm frequency (Chapman, Specht, & Cellucci, 2005
). BPD treatments may reduce thought suppression through at least directly assessing problems, which may in turn influence the change seen in NSSI above and beyond skills use.
Skills use also fully mediated the decrease in depression over time. This finding is compatible with the theory proposed by Jacobson, Martell, and Dimidijan (2001)
who suggested that the mechanism of change for depression is behavioral activation. DBT conceptualizes the lack of behavioral activation as a skills deficit and therefore teaches principles of behavioral activation within the emotion regulation module (Linehan, 1993b
). There have now been a number of studies that indicate DBT's efficacy in treating depression (Harley et al., 2008
; Lynch et al., 2003
; Lynch et al., 2007b
). Furthermore, an increase in emotional processing was found to be related to a reduction in depression only for DBT participants but not for control participants in a treatment resistant group with major depression (Feldman et al., 2009
). Thus, while DBT has been shown to be effective in reducing depression in BPD and depression populations, the mechanism of change was to date unclear. Our finding suggest that the use of DBT skills is an active ingredient behind changes in depression.
Skills use also fully mediated the increase in anger control over time. The STAXI Anger Control scale assesses how successful a person is in regulating cues that could trigger anger and is very close to the anger regulation taught in the DBT emotion regulation module (Linehan, 1993b
). The full mediation of Anger Control improvement signals that skills training may indeed help clients to better regulate the emotion of anger. Similar to depression, prior studies have shown a positive effect of DBT treatment on anger dysregulation (e.g., Linehan et al., 2008
). This finding emphasizes that the mechanism of change behind such findings may be use of DBT skills.
No evidence for mediation was found for changes in Anger In (anger suppression) or Anger Out (anger expression) which may seem puzzling, especially in light of the other findings. However, it is consistent with the DBT skills deficit model because the items in both scales may refer to both functional (“I keep things in”; “I express my anger”) and dysfunctional (“I pout or sulk”, “I do things like slam doors”) behaviors. In DBT skills training, clients learn how to tolerate and problem-solve unjustified anger, how to accept justified anger, how to stand up for themselves if needed, how to be mindful of the current emotion and how to accurately express how they are feeling (Linehan, 1993b
). Keeping anger to oneself may be effective behavior (e.g., the individual is mindful of the emotion while aware that expressing it may hurt a relationship) or a target for change (e.g., individual bottles up and explodes periodically in rage attacks). Similarly, expressing anger may be a behavior to increase if emotional numbness or lack of assertiveness is the problem behavior, or to decrease if anger is not justified by the context and appears out of control. DBT skills are aimed at increasing the client's effectiveness in handling inter- and intra- personal situations. The fact that the Anger In and Out scales are not necessarily tied to effectiveness may explain the null findings in this study.
In addition to the mediation findings, results also indicate that DBT is effective in teaching DBT skills, which further supports the studies of Lindenboim and colleagues (2007)
and Stepp and colleagues (2008)
. Besides strengthening their finding that skills use increases with the provision of DBT, this study further indicates that skills use increases more in DBT than in other treatment conditions. Thus, being treated with DBT versus a control treatment results in more skills used in distressing situations, even when DBT-specific language does not confound measurment. Moreover, the fact that the change seen in DBT at the end of treatment was maintained at the four month follow up (unlike in the control conditions) also indicates that DBT is effective in ensuring that generalization of skills use occurs. The important question that remains to be addressed is whether the increase in skills use is particular to the DBT skills taught or whether it represents an increase in DBT skills and self efficacy in general.
It is also interesting to note that when DBT skills were reframed into non-DBT specific language, participants reported using some of the skills even before treatment started. While some people reported using skills only a few times, the majority reported using DBT skills about half the time at pretreatment. Also, skills use and not skills training mediated treatment outcomes. This leads to important questions about whether skills training is the most effective way to increase skills use, whether teaching all the skills is necessary for successful DBT outcomes, or whether the skills training modules could be shortened to incorporate only novel skills.
Although the results are generally supporting of the DBT model, several potential limitations should be noted. Even though there is no reason to believe that there was a systematic over-reporting or under-reporting of skill practice by the entire sample, it is certainly possible that some individuals exhibited either the former or the latter tendency. In a study comparing an assessment of retrospective coping over the past week using the RWCCL and an abbreviated daily coping version, Smith, Leffingwell and Ptacek (1999)
reported that only 25% of the variance was shared between the retrospective account and the modified daily version. Since the present analyses used an adapted version of the RWCCL that was assessed over the previous month, retrospective bias may indeed be an important flaw in data accuracy. However, since the statistical model used controls for intra-individual factors, retrospective bias likely had a smaller effect since it can be expected that it did not fluctuate over time within the same individual. A possible future improvement would be to control for retrospective bias by using ambulatory monitoring and assessing DBT skills use on a daily basis. An additional limitation was that the measure used a checklist format that focused on how often a specific skill was practiced in particular distressing situation. The format offered advantages since participants from both DBT and control treatments could offer comparable information about their DBT skills use. Nevertheless, important descriptors of the skills practice such as intensity of use, the appropriateness of the skill used, and the quality of the skill used were not assessed.
It is important to also highlight the statistical approach we took to examine our hypotheses. We considered skills use to be a universal treatment mechanism and assessed its impact on the relationship between time in treatment and various outcomes. The DBT model could have also been assessed using a moderated mediation statistical approach, in which treatment condition (DBT or control) could have been considered the moderator and skills use would have been the mediator (Kraemer et al., 2002
). Our decision not to use a moderated mediation approach was based on the fundamental question for the current study being not whether DBT performs better than another treatment, but what processes drive changes in suicidal behavior, depression and anger over time. Moderated mediation investigates the mechanisms through which a treatment performs better than another, not about the mechanism through which change happens over time in treatment in a clinical population (Doss & Atkins, 2006
). While DBT may have indeed targeted skills training more explicitly, therapy in general targets engaging in effective behavior that leads to change which is in essence skills use. In this regard using skills or using effective behaviors in difficult situations can be thought as a universal mechanism of change in psychological treatments. Illustrative of this, all clients used some skills before treatment started, and varied in skills use throughout treatment, regardless of the treatment they were receiving.
Nevertheless, our approach has some limitations. First, it may be that skills use is a mechanism of change unique to DBT. In this case, our analysis may underestimate the mediation effect by adding data from treatments where this mechanism was not present. Future analyses should investigate whether skills use is a universal or a unique mechanism of change in treatment. Second, an assumption that is commonly made for standard mediation analysis methods, such as the one used here, is that there is no extraneous variable which influences both the mediator and the outcome (Robins and Rotnitzky, 2005
). In the case of our analysis, it is possible to have had unmeasured confounds influencing the results. An interesting avenue for future research would also be to validate the DBT model using a causal inferences approach, which would account for this limitation (MacKinnon et al., 2007
). Although there have been some examples of these models used in longitudinal data (e.g., Lin et al., 2008
), proper models to accommodate unmeasured confounds coupled with repeated measures are an active area under current research.
An additionally important avenue for future research is therefore to replicate these findings with different samples and statistical techniques. Furthermore, future research should also assess how use of particular skills mediates outcomes that are directly targeted in skills training. Some questions that could be answered from such research include: Does use of emotion regulation skills actually improve emotion regulation? Are distress tolerance skills and/or other skills responsible for the improvement in behavioral self-control seen in DBT? Answers to these questions could lead to the refinement of DBT and other treatments to maximize their effectiveness.