Lumping and Splitting: Toward a Taxonomy of Placebo and Related Effects.

The placebo effect is closely related to many other constructs, including most prominently, conditioning and expectancy, but also natural history, regression to the mean, priming, mindset, context effects, the meaning response, specific and non-specific clinical effects, placebo-related effects, the patient-clinician relationship, and the common factors in psychotherapy. How are these various terms related to one another? To what degree do they overlap, and to what degree do they diverge? To form a better theoretical understanding of these constructs and to foster improved empirical research, is it better to lump these terms together in some fashion? Or will progress best be served by maintaining the splits between the constructs? Or would it perhaps be most effective to employ some mixture of lumping and splitting? I will address these and related questions with two major goals: (1) to delineate and clarify the relationship between these various terms and (2) to suggest some possible re-alignments in the way in which we conceptualize the relationships among these constructs that might prove useful in fostering research on placebo and related effects. In addition, clarifying the interconnections between the placebo effect and other related terms has the potential to spark innovative cross-fertilizations between related areas of research.