This paper tracks economists’ rising, yet elusive and unstable interest in collective decision mechanism after World War II. We replace their examination of voting procedures and social welfare functions in the 1940s and 1950s in the context of their growing involvement with policy-making. Confronted with natural scientists’ and McCathythes’ accusations of ideological bias, positive studies emphasizing that collective decisions mechanisms were unstable and inefficient, and normative impossibilities, economists largely relied on the idea the policy ends they worked with reflected a “social consensus.” As the latter crumbled in the 1960s, growing disagreement erupted on how to identify and aggregate those individual values which economists believed should guide applied work, in particular in cost-benefit analysis. The 1970s and 1980s brought new approaches to collective decision: Arrow’s impossibility was solved by expanding the informational basis, it was showed that true preferences could be revealed by making decision costly, and experimentalists and market designers enabled these mechanisms to be tested in the lab before being sold to those public bodies looking for decision procedures that emulated markets. In this new regime, the focus paradoxically shifted to coordination, revelation and efficiency, and those economists studying collective decision processes were marginalized.