Recent genopolitics and political psychology research suggests individuals' biological differences influence political participation. The interaction between individual differences and environments has received less attention, not least because of the confound of self‐selection into environments. To test the interaction between innate predispositions and an exogenous environmental influence, we conducted a field experiment during the 2010 California midterm elections. We randomly assigned subjects to receive a postcard mobilization treatment designed to induce an emotional response to the degree of political contention in the election. We tested the possibility that subjects who are genetically predisposed toward negative affectivity will be less likely to vote after treatment exposure. To our knowledge, this is the first field experiment in political science to measure genetic moderation of a treatment, and it suggests experimental approaches can benefit from the inclusion of genetically and other biologically informative covariates.