Recently, intergovernmental organizations (IGOs) have adopted policies that engage the private sector in the implementation of their development mandates. Despite the implications of these changes, the subject is among the least conceptualized. By applying a theory-guided process-tracing (TGPT) methodology, this paper examines the process of change at the United Nations Development Programme (UNDP). It advances a constructivist argument and evaluates whether this change could be viewed as a norm-driven change, where norms of corporate social responsibility (CSR), in the process of interaction and learning, have obtained an intersubjective quality and redefined the role of the private sector in the context of organization’s objectives. The paper evaluates this argument in light of the alternative assumptions of the principal-agent model, the bureaucratic culture literature, and rational choice institutionalism. It highlights the implications of this research in empirical, analytical, and theoretical terms for further studies and concludes that, without a due assessment of the intervening effect of norms on policies, the causal claims of other theories are seriously challenged.