The scientific motivation for longitudinal study of individuals with mental illness has increasingly become an imperative, if we are to translate biological insights into discoveries that benefit individual patients. For studies intended to reveal differences between patients and healthy individuals at a group level, characterizing patients during periods of profound changes in mental status is a sensible approach, since it maximizes biological signal while minimizing the need to keep track of patients over time.
However, because patient clinical state in borderline personality disorder (BPD) is inherently unstable, findings from such traditional case-control studies are fundamentally limited in generalizability by the clinical heterogeneity of the patient sample and a host of state factors that can confound group-level comparisons. Indeed, these confounding factors likely contribute to the lack of convergence in the BPD literature with respect to biological understanding, since few studies even attempt to measure and account for these critical factors. To be sure, longitudinal studies in patients with severe personality psychopathology pose significant logisticaland technical challenges, since keeping track of patients in different clinical states is logistically challenging, and especially when intensive characterization is required, have until recently been cost-prohibitive. And yet longitudinal studies are essential for gaining an understanding of the biological and experience-dependent factors that exacerbate and ameliorate mood and cognition in individuals with BPD.
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