Wasn't there some research regarding the X chromosome and general intelligence recently?
I read something about this... there are something like
140 (can't find it anywhere I but I think that's what I read) genes associated with cognition on the X, and not nearly as many on the Y. Girls have spares Xs and boys don't, which is why girls are less extreme (the two Xs balance each other out). So statistically (supposedly) there are more boys with extremely high and extremely low IQs than girls... more boys diagnosed with autism, ADHD, etc etc.
Also worth mulling over is the idea that because of this link, much of cognition comes from the mother. Not sure how true this is... there are lots of gifted dads out there who have passed down their talents to their kids... but supposedly for a son, because he only gets one X, if mom is gifted, he's more likely to be as well (unless... mom has a strong X and a weak and he gets her weak).
(Have I just revealed my layperson's ignorance about genetics? ;p )
EDIT - just did a quick google:
http://hmg.oxfordjournals.org/content/14/suppl_1/R27.full"X-linked genes play a disproportionate role in the development of human intelligence. Why should there be such a concentration on this particular chromosome (1)? Zechner et al. (2) suggest that the X-chromosome has been engaged in the development of sexually selected characteristics for at least 300 million years and that natural selection has favoured the development of X-linked genes that are associated with higher cognitive abilities. In particular, males are more likely than females to be influenced by haplotypes that are associated with exceptionally high abilities. For an equivalent reason, they are also more likely to show deficits in mental abilities than females because of the impact of deleterious mutations carried in haploid state. The hypothesis offers an explanation for the higher male variance in many aspects of cognitive performance (3)."
