No idea if this is common knowledge, but in many parts of the world including probably the US, mail in ballots are (slightly)
left leaning (Barber and Holbein, 2020, based on a multiple decade covering 2018 dataset iirc, though their findings weren't quite statistically significant) - usually because of it being employed by people with
more resources funnily enough (
Berinsky et al 2001,
Kousser and Mullin 2007), though from what I understand in the states, the effect isn't all that great. Expect any state that counts mail in ballots late to have more left leaning votes at the very end. Then again, people prefer more exciting explanations over huge datasets I suppose.
This difference is irrelevant in most countries, but even a 0.7% overall difference can shift quite a lot in a system that employs a winner-takes-all approach (something I'd consider a shortcoming, but that is a matter of debate ofc).
Barber, M., & Holbein, J. B. (2020). The participatory and partisan impacts of mandatory vote-by-mail.
Science advances,
6(35), eabc7685.
Berinsky, A. J., Burns, N., & Traugott, M. W. (2001). Who votes by mail?: A dynamic model of the individual-level consequences of voting-by-mail systems.
Public Opinion Quarterly,
65(2), 178-197.
Kousser, T., & Mullin, M. (2007). Does voting by mail increase participation? Using matching to analyze a natural experiment.
Political Analysis, 428-445.
