Pré-publication, Document de travail: When policymakers and private agents use models, the economists who design the model have an incentive to alter it in order infuence outcomes in a fashion consistent with their own preferences. I discuss some consequences of the existence of such ideological bias. In particular, I analyze the role of measurement infrastructures such as national statisticall institutes, the extent to which intellectual competition between di¤erent schools of thought may lead to polarization of views over some parameters and at the same time to consensus over other parameters, and .nally how the attempt to preserve in.uence can lead to degenerative research programs.