We study response behavior of New York City parking-ticket recipients, analyzing administrative data on 6.6 million tickets issued to 2 million individuals over two years. Using variation in the timing of reminder letters, we find evidence consistent with significant forgetting. But we find large differences across individuals, and, importantly, those with a low baseline propensity to respond to tickets–a natural nudge target–react least to reminders. These low-response types, who incur significant late penalties, disproportionately come from already disadvantaged groups. They do react strongly to more incentive-based interventions. We caution against designing nudge policy without analyzing effect heterogeneity.
