Why aren't girls as good as boys at math

They are. The usual reason given for the fact that boys always win the top prizes is that the overall math ability of both sexes may be identical, but boys have a different-shaped curve, because, it is said, intelligence (and math ability) are on the X chromosome, so girls get an average of her two Xs, but boys get only whatever is on the one X they get from their mother, so boys are more likely to be found at the extremes.

I used to find this plausible, if unappealing, till I discovered that in Iceland, girls are more often at the top. This article in the Proceedings of the National Academy of Sciences and discussed in ieee Spectrum says there are other places this is true, including Thailand, the UK, and Asians in America.

This seems to me to be definitive.

Yesterday I read that, for the first time, there are more new PhDs granted to women than to men in the US. Woman have been on top for BAs for some time.