Originally Posted by CCN
Girls have spares Xs and boys don't, which is why girls are less extreme (the two Xs balance each other out).
This has recently been de-bunked, using an elegant twin-study method: http://link.springer.com/article/10.1007%2Fs10519-012-9552-z?LI=true

That is, boys do seem to have greater variance in cognitive traits, but the X-chromosome hypothesis of why that would be so appears to be false.