Is there more sincere voting in multi-member districts than in single-member districts? Existing research on this question is inconclusive, at least in part because it is difficult with observational data to isolate the effect of district magnitude on voting behavior independently from voters’ preferences or the number of parties. Hence, we investigate this issue in a laboratory experiment, where we vary district magnitude while keeping the distribution of voters’ preferences and the number of parties constant. We find that voting for the preferred party (sincere voting) increases with district magnitude. This is consistent with existing findings from observational data. We also discover a surprising result: a high incidence of “frontrunner” voting, which cannot be easily explained by existing research.