Not my own belief, but I had a boyfriend who used to think that all the women in the world had their periods at the same time.
A childhood belief about periods

others believed things like this
- i used to believe that women wore bras because their periods happened in their boobies.Anon
- a male friend of mine used to think that only women who had been pregnant had periods— oiman
- my boyfriend used to think that Hanna and Barbera were two women that did all the cartoons— Helen
- I used to belief that women had babies by pressing their belly buttons!— amy
- I used to belief that the Muslim women who wore their garment wore it because they were bald.Anon
- I convinced my boyfriend that paprika flavored crisps were made from donkeys...he was 20 at the time.Anon