Why are blondes considered dumb?

Blondes are dumb. Blondes are stupid. Blondes have no brains.
Those are some of the ideas about blondes that you may have heard before. You also hear it in blonde jokes, although some blonde jokes say the opposite. Of course you know that they are not true. Well, they may be true for some blondes, just as for some people belonging to any other category. Stupid people exist everywhere. But they are of course not more likely to be blonde than to have any other hair color.
So why are blondes considered blonde?
The real reason may be that blondes are considered attractive. Historically, blondes were always rare, and what's rare is easily seen as attractive. Being attractive, some blondes in the 20th century have been known as social climbers by using their attractiveness. Simply put, it was more easy for a blonde to sleep her way to the top. If you were not blonde, you needed other talents to do the same.
The image was made more strong by actresses who liked to play a dumb blonde. Think Marilyn Monroe. In her movies, she made the image of the dumb blonde much more strong.
Recently, there have been a number of blonde celebrities who were perceived as dumb, probably because they really are. Most noted are Jessica Simpson and Anna Nicole Smith. They didn't do the image of blondes much good. And let's also not forget the likes of Pamela Anderson.
Finally, blonde jokes have made the image of dumb blondes more strong. People like to tell blonde jokes, and this reinforces the ideas about blondes.
I hope this help you to better understand why blondes are considered dumb.