Perhaps whilst at school, where boys see you as a less attractive prospect for a relationship, and more a target for bullying (I think, because they are intimidated). I have found that this effect lessens once at university, where you meet people more like yourself, and where intelligence is not something to be ashamed of. I can only make guesses about later in life, since I don't have direct personal experience of that, but I suppose it depends what direction the woman takes with her career. I've heard that men are intimidated by highly intelligent women, but some men are the opposite. My view is that everyone likes to say they want an intelligent partner, but they also want their own ego to be flattered.
I don't know if the OP is talking about (presumably) herself in the 3rd person - I imagine she might be, since she's choosing to post anon - but I must say, don't dumb yourself down because you think it will make you more attractive to men. If you're still at school, don't worry. You wouldn't want to go out with any of the losers there anyway!