BackgroundEmotion dysregulation is core to many biopsychosocial models of Borderline Personality Disorder (BPD) and is often targeted as part of their associated psychological therapies. Several distinct specialist psychotherapies are thought to be effective for people diagnosed with BPD but it is unclear whether they share common change mechanisms. Some evidence suggests that Mindfulness Based Interventions improve competency in emotion regulation as well as trait mindfulness, which are both plausibly associated with good treatment outcomes. It is unclear whether the association between the severity of BPD symptoms and emotion dysregulation is mediated by trait mindfulness. Would improvement in trait mindfulness mediate an association between lower severity of BPD symptoms and fewer problems of emotion dysregulation?MethodsOne thousand and twelve participants completed online, single time-point, self-report questionnaires.ResultsAs predicted, the severity of BPD symptoms was significantly and positively associated with emotion dysregulation with a large effect size (r= .77). Trait mindfulness mediated this relationship as the 95% confidence interval for the indirect effect did not cross zero (size of direct effect = .48 and size of indirect effect = .29 [.25, .33].ConclusionsThe relationship between the severity of BPD symptoms and emotion dysregulation was confirmed in this dataset. As hypothesised, this relationship was mediated by trait mindfulness. Process measures of emotion dysregulation and mindfulness should be included in intervention studies for people diagnosed with BPD to understand if improvements in these factors are a universal occurrence with good response to treatment. Other process measures should also be explored to identify other factors involved in the relationship between BPD symptoms and emotion dysregulation.