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Walking and other forms of physical activity have many health benefits, but people often fail to follow through on their own goals of being more active. To address gaps in current understanding of how to design technology-supported physical activity interventions, we conducted a randomized field experiment of a commitment device: making public announcements. In a control condition, weekly commitments were kept private. In two treatment conditions, they were announced on Facebook and by email. In one of the two, the announcements also included results: whether the previous week's commitment was kept. We find that, with or without public results, these posts can elicit supportive replies from the poster's social networks. People in both public announcements conditions were less likely to make commitments. We conclude that the prospect of public accountability may suppress the making of commitments in a …