Major developments in methods addressing for whom psychotherapy may work and why.

Significant progress has been achieved in the last decades in studying two central questions in psychotherapy research: what treatment works for which patient and why does treatment work. This paper delineates central developments in the methods used to study each of these questions. Through targeted examples, the paper discusses several phenomena and trends in psychotherapy research. Regarding the question of what works for whom, the discussion focuses on the progress from the search for one moderator to guide clinical decision-making to the search for a set of such moderators and their interactive effects, to best answer this question. To answer the question why treatment is effective, the paper reviews the progress from a single snapshot of a process variable to approaching causality, that is, temporal relationships, higher dependability, and closer attention to the dynamics of change in process variables. Finally, methodological developments made it possible to combine these two questions so as to better capture the richness and complexity of therapeutic work. Two central products of this integration are discussed and demonstrated through the case of the working alliance.