In emerging ecosystems, resource-constrained new ventures often face a dilemma in deciding how to best form relationships with other ecosystem participants. This study explores when managers of new ventures choose to engage in selective revealing, that is, sharing specific pieces of information with a broader audience, with the purpose of building ecosystems with other new ventures and incumbent firms. We test this phenomenon using a choice-based conjoint (CBC) experiment that evaluates 440 decisions made by managers from new ventures on whether to reveal information to incumbent firms and other new ventures (or not) in emerging ecosystems. We base our argumentation on effectual and causal decision-making logic: While causal logic has a goal-oriented and predictive character, effectual logic focuses on means-orientation and flexibility. The results of our experiment suggest that managers of new ventures tend to reveal information when incumbent firms commit resources ex-ante to the emerging ecosystem (over when incumbent firms seek to deploy resources flexibly), which complies with a causal logic. However, when interacting with other new ventures, managers of new ventures choose to reveal information when other new ventures emphasize ecosystem learning (over individual learning), which mirrors an effectual logic. Our study therefore illuminates decision factors of ecosystem actors that foster ecosystem building independent from static firm characteristics. Moreover, we provide empirical evidence that new ventures, in interaction with incumbent firms, deviate from the entrepreneurial decision-making logic of effectuation and rather turn to a causal logic.