There's actually a pretty extensive article in Wikipedia that addresses the origins of the myth.
If it is a myth.
Here are more opinions and answers from other WikiAnswers contributors:
I personally think that most blondes are pretty smart. Not only smart but hot, funny, kind, and trustworthy.
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Excuse me, but I am a blonde and I get straight A's. Blondes in general are not stupid even though some get their "dumb blonde" moments.
Being blonde doesn't make you dumb, that is just a untrue stereotype.
No, it is highly impossible for a person to get fired for calling someone a dumb blonde.
to have a blonde moment is to do or say something really dumb
Yes because they are mentally challenged and they are dumb
Yes, Bushra Shamim is a blonde because she is dumb like a dumb blonde and has no idea what is going on in her life except for her obsession over Justin Bieber and Music.
dumb blonde
Is that a squirrel on your face?
because why
Jacob tells Rosalie three dumb blonde jokes.
A dumb blond is a girl or guy with blonde hair that is either stupid or forgetful A dumb brown is a girl or guy with brown hair that is either stupid or forgetful Kazzc Kaz
Christianity
wave at her