Abstract
This paper examines the concept of allegiance effects in psychotherapy research from a constructivist approach. After considering their role in outcome and process research, a constructivist explanation of them is proposed. It is also suggested that traditional ways to control them, while necessary and sound, may not be enough. Alternatively, a call for methodological pluralism in psychotherapy research is made, especially regarding the inclusion of qualitative, hermeneutic, phenomenological and discovery oriented case studies that privilege the voice of clients and not only the researchers favoured constructs.