Why is there a dichotomy between being pretty and being smart (for women)

It’s interesting to note that in the media, you often get the pretty girl portrayed as the shallow, self-centered airhead, and it’s the homely, bookwormish girl that ends up getting the guy (who previously dated the pretty girl, but now obviously has seen the light). Why is it that there’s often this divide between being pretty and being smart?

Or you have this idea that you can’t be girly and still be taken seriously, which is where you get women that proclaim with pride: “I was a tomboy! I didn’t buy into the Barbie BS! I don’t care about clothes and all those frivolous things”, as if caring about said frivolous things somehow precludes being mature, intelligent, a feminist, et cetera…as if people have to choose, “I’m going to be smart or I’m going to be pretty.”

I was musing on this today and wondered what Fluther might have to say about it. I know as a kid I felt that I couldn’t be both (I must’ve internalized it from somewhere). What about you, ladies and gentlemen? Experienced it? Any theories about why?