We show, against skeptics, that however latent it may be in everyday life, the ability to reason effectively about politics can readily be activated when conditions are right. We justify a definition of deliberative reason, then develop and apply a Deliberative Reason Index (DRI) to analysis of 19 deliberative forums. DRI increases over the course of deliberation in the vast majority of cases, but the extent of this increase depends upon enabling conditions. Group building that activates deliberative norms makes the biggest difference, particularly in enabling participants to cope with complexity. Without group building, complexity becomes more difficult to surmount, and planned direct impact on policy decisions may actually impede reasoning where complexity is high. Our findings have implications beyond forum design for the staging of political discourse in the wider public sphere.