I see no mention of whether they adjusted for age and urban vs rural settings. Dense metropolitan cores, where most economic activity happens, tend to be both younger and highly diverse ethnically as the jobs there attract migrant flows. It is unsurprising that younger people in dense metropolitan "the fewer people vote and the less they volunteer, the less they give to charity and work on community projects". That's what retired people do.
And to be completely transparent about where I'm coming from: I'm the father of two mixed-race children and all this talk of "homogeneous societies" reeks to me of plain old racism that will hurt my kids. And I grew up in one of those European "homogeneous" countries that right-leaning racists love to talk about, too.
> You are the one who hurt your kids, and it’s on the rest of us now to stop you from also hurting society. Homogenous societies are a good thing and always have been.
Thank you for supporting my thesis that praise for "homogeneous" societies is nothing but a dog whistle for plain old racism. I hope my children won't have to suffer people like you IRL.
Edit: It is hilarious that people like you praise places like the country where I was born and the country where my wife was born, but when we marry and have mixed-race children then suddenly you label our family as a menace to society.
https://wcfia.harvard.edu/publications/downside-diversity