We are all overly familiar with this. The dumb blonde is the woman (though, male characters sometimes get this trope) that is brainless, beautiful, busty, and blonde-haired. She’s either sexually promiscuous or a pure virgin. There is no middle ground. If it’s a dumb blond male, then he is typically brutish, a jock, and either lovable or despicable. Again, there is no middle ground.
Why this is bad: First of all, it completely obliterates a subset of people based on their hair color. News flash: intelligence is not actually tied to hair color. I promise. It often falls into this trap of reducing women – often a love interest – to someone who can only care about stereotypical “female” activities and will always need a man to save her (literally or metaphorically). The dumb blond males, on the other hand, are also victims of stereotyping, typically enjoying typically masculine activities. Both characters are completely diminished and often given no chance at redemption.
How you can fix it: Two words: Legally Blonde. In probably the single best example of trope subversion, this movie took the concept of a dumb blonde and put it on its head. Elle Woods is competent and intelligent, but she is also blonde, loves to shop, and loves the color pink. The writers didn’t subvert the trope by making her hate all feminine pastimes in the end. She still loves to dress her tiny dog in sweaters, but she’s also a damn good lawyer. Subverting tropes, especially this one, doesn’t mean your character has to be the opposite of who they are. Your blond dumb jock doesn’t subvert the trope by becoming “cultured” and never playing football again. He subverts it by being intelligent or wise and still clinging to the things he likes.
Bottom Line: We already pass judgments on people in real life based on things as ridiculous as hair color; we don’t need to bring it in to our writing. Keep your characters multilayered and complex. Even if they’re dumb, that doesn’t mean they have to fall into the same stereotype as thousands of other characters.
So go on, write and break that trope.