Effectuation and causation decision-making logics are noted to be major alternative approaches to international network formation. However, knowledge is lacking on how and the conditions under which the two approaches contribute to post-entry performance of international new ventures (INVs). We integrate the theory of effectuation and institutional development logic to explain how effectuation and causation approaches to international network formation individually and jointly contribute to post-entry performance under varying conditions of home market institutional support. We test our proposed framework on primary data from 228 INVs in a sub-Saharan African economy. Results suggest that greater uses of both effectuation and causation approaches to international network formation are associated with stronger post-entry performance. More interestingly, results show that the joint effect of the two international network formation approaches on post-entry performance is amplified under conditions of low home market institutional support. Our findings provide theoretical and managerial insights on the importance of complementing effectual and causal reasoning in international network formation in weak home market institutional environments.