There was this story in the LA Times, “Sierra County (where everyone votes by mail) is serious about elections.” Here’s the relevant info from the story:

The unofficial voter turnout in the conservative, rural enclave was 53.6% — higher than any other county in the Golden State, nearly double the statewide figure and more than three times the turnout in Los Angeles County.

Sierra had a 10-point lead over No. 2 Alpine County, which clocked an impressive 43.4% turnout.

…

It helps, of course, that Sierra and Alpine counties have all-mail balloting. And that Foster had election reminders put up in every Sierra County post office. And that there’s no home mail delivery to speak of, so everyone has to go to the post office and walk by the signs urging them to vote.

I’ll leave it for Paul to tell us about academic research linking all-mail balloting to turnout …