You sit men and women down and give them a maths test, and they will do fairly equally. Then you set up the same test, but with different people, and make them tick a box to say whether they are a man or a woman, and the women do significantly worse in the maths test than they did previously in a group set.
From my personal experience, because I'm in a relationship, on paper I would never have imagined - I'm an Essex girl, maths geek who likes football, and I've ended up with a Russian ballroom dancer, and I guess the things you think are important, especially when you're younger, turn out not to be.