Why might bipolar disorder and borderline personality disorder be bonded?

The prevalence of comorbid bipolar disorder (BD) and borderline personality disorder (BPD) is distinctly higher in community samples than would be expected if the two conditions are independent. While there have been multiple explanations suggested for their interdependence, no clear model has been established. This paper reviews a broader set of explanations than considered previously, where relevant prevalence studies of the conditions are reported, previous explanations overviewed, and additional potential linkage causes are considered. It was found that there is unlikely to be any single determinant of the comorbid presence of BD and BPD. The most likely candidates are the artefactual impact of transdiagnostic features, with true comorbid status reflecting both pleiotropic genetic influences and environmental factors. Measurement errors in diagnostic assignment emerging from transdiagnostic features are likely to have clouded previous studies and therefore the interpretations. Comorbid BD/BPD is likely to be distinctly more common than estimated by clinicians, and clarification of the reasons why this is may well assist clinical management.