Abstract: Routine default threatens the foundations of the United States court system. We study the problem of routine default by human defendants, using the Boston Municipal Court’s debt collection docket as our laboratory. Arbitraging various non-law literatures, we designed interventions consisting of two forms of mailings. We study the effectiveness of our two mailings in a randomized control trial that includes a no-intervention control group. We find no difference in effectiveness as between our two mailings, but that both roughly double the rate at which defendants participate in their lawsuits. Results are statistically significant. We discuss implications of our findings.