TRANSD recommendations: improving transdiagnostic research in psychiatry.

361 the gold standard of treatment evaluation. However, on their own, they do not provide evidence that a psychological therapy works through the mechanisms that it claims. The effect could result from the expectation of the therapy working (placebo effect), or through simply talking to a professional. Again, if we follow the successful examples of other sciences, such as chemistry, physics and engineering, the most robust test of a theory is to build and assess a working model of a process. This tradition started with Galileo, continued with prototyping in machine design, and today is typically carried out within computer simulations. If the model behaves the same way as the real system under natural conditions, then the theory informing the model must be correct. There is no a priori reason why this should not apply as well to human behaviour as it does to the theory of aerodynamics informing airplane design, for example. Our clinical research team uses Method of Levels (MOL) as a transdiagnostic intervention which we disseminate widely. This therapy is based on perceptual control theory, a general theory of behaviour drawn from control engineering. Its key principles of control, conflict and reorganization have been assessed through testing computational models against behavioural data. In sum, transdiagnostic psychiatry is well established, but to understand its transformative potential requires adopting the appropriate scientific approach. Future reviews need to evaluate a broad literature including general psychopathology and shared neuropsychological pathways, and to separate the evaluation of treatment and process studies. Treatment research needs to consider the multiple perspectives of different stakeholders when determining how to index evidence for the potential benefits of a transdiagnostic approach. Process research, on the other hand, needs to be theory driven, hypothesis-led, and ideally emulate the model-testing paradigms of other sciences. A transdiagnostic approach of this kind has the potential to generate a genuine, interdisciplinary, paradigm shift in psychiatry and mental health.