When I was in elementary school, we always had a table at the back where the advanced students would do more difficult stuff than the rest of the class while not being completely isolated. The table was always me and 5 or 6 girls. When we graduated high school, I was the top-ranked boy - and the 22nd-ranked student overall. I just took it completely for granted that girls were smarter than boys (although I did perceive the very strong anti-intellectual culture among boys which seemed more impactful than native abilities).

It wasn't until I went to college that I started encountering the belief that men were fundamentally smarter than women, even though every college and university I've attended had more women than male students and the women had much better academic performance. That was my first taste of the power of group delusion.