This is kind of a personal one, but up until about five years ago I used to believe that cramps and PMS and things were imaginary. I thought girls talked themselves into it, or used it all as an excuse to get away with all sorts of crap a man could never do.
I'm 23 and female; it just took me a while to grow up. I was very, very wrong.
A childhood belief about periods

others believed things like this
- When I was little, I couldn't wait to grow up so I could grow a mustache. I am female.— Laura
- When I was little I thought that only little girls had nipples, and they went away when you grow up. — Cutie
- When I was little I thought girls had "man parts" as well, but it just took longer for them to grow.— person
- i used to believe that when you had cramps your appendix would blow up.— Avery
- i thought that when it was your birthday you grow and then you didn't grow until a years time.— hello
- When I was little, I thought the tooth fairy was a personal friend of my mom since she was a dentist.— Jess