Novel psychological theories are often conceived in a general or heuristic form that can benefit from development and granulation through context-specific theory testing. Here, a theory-testing single case study methodology, adapted from an approach developed in the field of psychoanalysis, is presented. The study exemplifies this methodology though an interrogation of the explanatory value of a relatively new child development theory, the theory of epistemic trust, in the context of the relationship between a foster carer (“John”) and a young person in their care (“Buster”). Using in-depth interview material, the ways and extent to which the theory of epistemic trust could aid understanding of this fostering relationship are examined. We discuss the implications for the development of the theory of epistemic trust and the applications of these findings to social work contexts. The strengths and limitations of this theory-testing case study approach are explored
