Here’s the chart on university subject segregation
Paper summary here
cepr.org/voxeu/column...
This is not just a cross country thing: within individual countries, kids from richer (typically more gender progressive) families have fewer mixed gender friendships too
They speculate that as families and societies get richer, children socialize more through structured activities (clubs/hobbies) rather than with neighbors/family, and this ends up being more gender segregated
And this in turn might impact what subjects people choose to study