The More, the Merrier? Membership Expansion and Incumbents' Boundary Work Divergence in the Platformization of Belgian Philanthropy

When actors emerge on the periphery of a field, incumbents either engage in protective boundary work to enforce the field's membership criteria, or opt for membership expansion by adapting these criteria to accommodate peripheral actors. Less explored is the divergence configuration where a minority of incumbents pursue expansion whereas the majority adopt a protective strategy. Since the inclusion of peripheral actors may challenge membership criteria (i.e., the symbolic boundary) and shift the resource distribution and social hierarchy (i.e., the social boundary), how minority incumbents induce membership expansion against the majority's protective stance is an intriguing question. Drawing on a qualitative field‐level case study of Belgian philanthropy, we examine incumbent foundations' responses to the rise of ‘social‐mission platforms.’ We identify four mechanisms through which minority incumbents can overcome the majority's initial opposition and bring about support to membership expansion: affirming divergent expansive posture, leveraging definitional ambiguity, demonstrating comparative reinforcement, and facilitating shared buy‐in. We further show how each mechanism bridges the social and symbolic boundaries through the combined (re)actions of the diverging incumbents and the peripheral actors. Our findings extend understandings of membership expansion as a contested, multi‐actor process and unpack the interaction of social and symbolic boundaries in shaping field evolution.