system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Where does the stereotype for "stupid" blondes come from and why?
gpt
There's an old saying: "God gives with one hand, and takes with the other."

The idea is that people generally have looks, or brains, but not both.  Blondes are considered by some to be more attractive ("Blondes have more fun") so it's assumed they are not as intelligent.  There's also the aspect of a lot of men being threatened by an intelligent woman/preferring a more "docile" or unchallenging but attractive one.  

A dumb blonde, that is to say someone who is attractive, but doesn't bring much else to the table has the connotation that it would be a good thing as a mate.  So it becomes a stereotype to subtly indicate what is actually supposed to be desirable.