Rethinking placebo in psychiatry: how and why placebo effects occur†

The range of placebo mechanisms can be bewil­ dering, but evolution provides an overarching theoretical framework that permits the integration of the different levels of explanation. Tinbergen’s (1963) classification, for example, divides the causes of behaviours such as placebo responses into proximate mechanisms and ultimate mechan isms (Box 1). This framework allows the integration of physiology, neuroscience, social and developmental psychology, psychodynamics, anthropology and evolutionary biology. We will start by considering the proximate mechanisms, which are subdivided into: causa­ tion, which includes immediate psychological and physiological stimuli, meanings, learning and the neurochemical substrate; and development (ontogenesis) – how behaviour is influenced by early experience and how it changes with age (Fig. 1).