Causality in psychotherapy research: Towards evidential pluralism

Identifying causal relationships is at the heart of all scientific inquiry, and a means to evidence base practices and to guide policymaking. However, being aware of the complexities of interactions and relationships, scientists and academics are cautious about claiming causality. Researchers applying methods that deviate from the experimental design generally abstain from causal claims, reserving them for designs that adhere to the evidential ideals of empiricism (e.g., RCTs), motivated by the Humean conceptions of causality. Accordingly, results from other designs are ascribed lower explanatory power and scientific status. We discuss the relevance of also other perspectives of causality, such as dispositionalism and the power perspectives of various realist approaches, which emphasize intrinsic properties and contextual variations, as well as an inferentialist/epistemic approach that advocates causal explanations in terms of inferences and linguistic interaction. The discussion will be illustrated by the current situation within psychotherapy research and the APA Policy Statement on Evidence-Based Practice. The distinction between difference-making and causal production will be proposed as a possible means to evaluate the relevance of designs. We conclude that clarifying causal relationships is an ongoing process that requires the use of various designs and methods and advocate a stance of evidential pluralism.