Have you ever “trauma-dumped” to a friend? Lost your keys and thought “I’m so ADHD”? Or been diagnosed with “pathological tourism”? If so, you may have witnessed, or participated in, diagnostic drift. Just as “tourism” means something very different today than in the late 19th century, many clinical diagnoses also shift in meaning over time. Diagnostic labels are fundamental tools in clinical psychology, intended to support help-seeking, treatment, and self-understanding. Yet their meanings expand and contract over time, shaped by clinical practice, research trends and cultural discourse. This drift is linked to rising rates of certain diagnoses and the growth of self-diagnosis, but there is still no shared framework for explaining how and why diagnostic meanings change.

This project addresses that gap by formalising patterns of diagnostic drift across three studies. A systematic literature review maps existing research and synthesises core findings, and expert interviews clarify mechanisms, typical trajectories of drift, and how they might be simulated. Drawing on these results, the final study uses theory construction methodology to develop an agent-based model of how diagnostic meanings evolve through interactions between patients and clinicians. The thesis offers a structured starting point for future work, making assumptions explicit, highlighting testable mechanisms, and providing shared language for discussing the consequences of diagnostic drift.