Andrew Menger, Bob Stein, and Greg Vonnahme have an interesting paper that is now forthcoming at American Politics Research, “Reducing the Undervote With Vote by Mail.” Here’s the APR version, and here’s a link to the pre-publication (ungated) version.
The key result in their analysis of data from Colorado is that they find a modest increase in ballot completion rates in VBM elections in that state, in particular in higher-profile presidential elections. Here’s their abstract:
We study how ballot completion levels in Colorado responded to the adoption of universal vote by mail elections (VBM). VBM systems are among the most widespread and significant election reforms that states have adopted in modern elections. VBM elections provide voters more time to become informed about ballot choices and opportunities to research their choices at the same time as they fill out their ballots. By creating a more information-rich voting environment, VBM should increase ballot completion, especially among peripheral voters. The empirical results show that VBM elections lead to greater ballot completion, but that this effect is only substantial in presidential elections.
This is certainly a topic that needs further research, in particular, determining how to further increase ballot completion rates in lower-profile and lower-information elections.