The sex chromosomes: keys to the evolution of human intelligence?

Recent surveys of educational attainment and schooling suggest that girls are outperforming boys all the way through from primary school to A-levels. The reason for this discrepancy, though, remains unknown. Many explanations have been suggested, ranging from the fact that there are simply more boys who fall into the category of disadvantaged" to boys lacking interest, having shorter attention spans, or not being encouraged to take learning seriously by male role-models.

But could there be a simpler explanation? It is no secret that a number of educational and mental disorders have a higher incidence in males than in females, usually because the genes which influence them are linked to the X-chromosome. Imagine a couple, of whom one parent has an X-chromosome gene linked to a particular disorder. If this parent passes on the "defective" gene, this is more likely to affect boys, who have only one X-chromosome (and hence no "good" copies of the gene to counterbalance the defective one) than girls, who will have one defective and one, usually dominant, normal copy of the gene. As a result, the couples' male children may display characteristics associated with the disorder, while female ones typically will not ("normal" genes are no always dominant, but this is often the case).

What does this have to do with human intelligence more generally? Well, if a genes' malfunction is associated with mental disorder, it's function is clearly important to mental functioning. The concentration of these genes on the X-chromosome, moreover, suggests that this chromosome might play an important role in the evolution of human intelligence, and warrants further investigation. A paper by Zechner et al. (2001), published in the journal Trends in Genetics, has done just that, and has found that, in fact, the X-chromosome has a concentration of genes associated with cognition that is over three times higher than that for any other chromosome, even when the authors corrected for possible bias in their data collection.

And a link between the sex chromosomes and intelligence raises some interesting possibilities for its evolution. Zechner et al. (2001) discuss one of these particularly. They note that a link to the X-chromosome may implicate sexual selection in the evolution of intelligence. Sexual selection occurs when individuals of one sex "select" mates on the basis of particular characteristics and/or there is competition between members of the same sex for mates or resources. Both these types of selection (called intersexual and intrasexual selection respectively) can cause the evolution of traits which are unrelated to fitness in the traditional sense; like the male peacock's tail, which is a drain on physical resources, but provides its owner with a reproductive advantage.

Many sexually selected traits, moreover, differ between the sexes. In intersexually selected traits (where one sex chooses mates on the basis of their characteristics), moreover, the possession of a trait in the selected sex must be linked to a preference for it in the other (selecting) sex. Many such traits are linked to the X-chromosome (Zechner et al. 2001). Sexual selection can also occur much faster than natural selection, providing a potential explanation for the speed with which human intelligence seems to have evolved in Homo sapiens - perhaps there is no adaptive explanation for our brains at all, and they are the result of selection for intelligent mates by women...?



References

Zechner, U., Wilder, M., Kehrer-Sawatzki, H., Vogel, W., Fundele, R., & Hameister, H. (2001). A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution? Trends in Genetics, 17 (12), 697-701 DOI: 10.1016/S0168-9525(01)02446-5