Why Do Vote-by-Mail Elections Boost Voter Turnout?

Despite growing evidence that Vote-by-Mail (VBM) elections boost voter turnout, previous research lacks a unified explanation for why VBM elections do so, making assessing the election reform challenging. We theorize that VBM has a dual effect of mobilizing uninformed voters by notifying them about upcoming elections (notification effect) and making voting more convenient for all voters (convenience effect). Our theory implies that the VBM effect for frequent voters is larger in midterm elections than in presidential elections, while in presidential elections, the VBM effect is larger for infrequent voters than for frequent voters. Using 27 million individual voting records from Colorado, North Carolina, and New Mexico and a difference-in-differences design with exact matching, we find supportive evidence for our expectations. Our results offer insight to assess the impact of VBM elections on voter turnout as well as fruitful topics for future research.