However, in amongst all that was this interesting little article in the Guardian today:
Girls think they are cleverer than boys from age four, study finds
So, girls may or may not be naturally more intelligent than boys (or vice versa). The chances of the growth of the achievement gap being evolutionary at that speed seems unlikely though, so there must be some other cultural reason for it like this.
We generally have a strange habit of labelling children from an early age, and then being surprised when they fulfil our expectations. The idea of the gender gap being another result of this, seems all to plausible.
For a world where people push for equality, we seem utterly rubbish at actually treating people equally as individuals rather than just filing them in their stereotype box.