This paper analyzes a randomized experiment to shed light on the role of information and social interactions in employees’ decisions to enroll in a Tax Deferred Account (TDA) retirement plan within a large university. The experiment encouraged a random sample of employees in a subset of departments to attend a bene!ts information fair organized by the university, by promising a monetary reward for attendance. The experiment multiplied by more than !ve the attendance rate of these treated individuals (relative to controls), and tripled that of untreated individuals within departments where some individuals were treated. TDA enrollment !ve and eleven months after the fair was signi!cantly higher in departments where some individuals were treated than in departments where nobody was treated. However, the effect on TDA enrollment is almost as large for individuals in treated departments who did not receive the encouragement as for those who did. We provide three interpretations—differential treatment effects, social network effects, and motivational reward effects—to account for these results.