The store Forever 21 sells a magnet that reads, "I'm too pretty to do math." You can read about it (with excellent commentary) on the blog Sociological Images.

This reminds me of the sad fact that even though more girls than boys initially signed up for my higher level high school calculus course, all but three of those girls dropped it when they found out how difficult the class would be. I almost dropped it myself, but the teacher convinced me to stay. I am so glad she did.

In my years studying math and tutoring math, I've found that girls more often than boys have low self-esteem about their ability to do math, and they tend to just quit when the going gets tough. There is nothing different about girls' and boys' ability to do math, and you can be sure that looks have nothing to do with it.