> I wonder if the cultural existence of the 'introvert vs extrovert' idea has reinforced this to a degree. I'm too young to know, but I doubt this was a discussion in the 80s.

That doesn't mean it didn't exist then, it means it either wasn't socially acceptable or wasn't understood (or both) then. See also the graphs of left-handedness over time, which were basically flat at the actual level of left-handedness except for a big dip during the century or so when left-handed people were commonly forced to hide it and use their right hands.