Abstract: Citizen participation in decision-making has been widely lauded as a method for improving outcomes in international development. While there are normative reasons to encourage more inclusive decisionmaking processes, costly and time-intensive group decision-making processes are often justified on the grounds that they may also improve outcomes. Deliberative discussion, in particular, is believed to be more transformative than a mere aggregation of individual preferences, leading perhaps to more socially optimal decision making and subsequent behavior. Prior work confirms that deliberation results in shifts of opinion, but it has had little to say about the quality of the resulting decisions, which are difficult to assess in a field setting. I report the results from a laboratory experiment with 570 subjects in Nairobi, directly testing the effect of participation in deliberative group decision-making on collective outcomes. Participants are asked to engage in a group effort task to earn compensation toward a shared group fund. Randomly assigned treatments vary according to whether decision-making over the task to be completed occurs through (1) external assignment, (2) a majority vote, or (3) consensus through deliberative discussion. I find that participation in group decision-making involving deliberation (but not a simple majority vote) does improve collective outcomes. This effect is achieved primarily through better strategic decision making that minimizes the costs associated with contributions. Deliberation is also associated with changes in preferences, greater levels of agreement with decision outcomes, and greater perceived fairness. Evidence for behavior change is weaker, but there may be a positive effect mediated by preference change.