Stanford study: (1) vote-by-mail does not appear to affect either party’s share of turnout; (2) vote-by-mail does not appear to increase either party’s vote share; and (3) vote-by-mail modestly increases overall average turnout rates
http://www.andrewbenjaminhall.com/Thompson_et_al_VBM.pdf

“In our data, we confirm important conventional wisdom among election experts: vote-by-mail offers voters considerable convenience, increases turnout rates modestly, but has no discernible effect on party vote shares or the partisan share of the electorate.”

“conclusions support the conventional wisdom of election administration experts and contradict many popular claims in the media.”

Found author thread. Thank you @andrewbhall https://twitter.com/andrewbhall/status/1250104293866098688?s=21