Gallup recently released a poll confirming a preference for male children. Specifically, men displayed this preference while women responded with near indifference. It would be easy to write this off as a purely cultural phenomenon --machismo, labor demand, etc. --, but I hypothesize a potential genetic explanation unique to the influence of the Y-chromosome.
Gallup makes passing mention of the theory of evolutionary advantage to having male children --presumably that males are capable of being more genetically prolific than females--, but this theory fails to account for the discrepancy between the preferences of men versus the preferences of women. On a whole-genome level, a woman would benefit genetically just as much as a man as the result of a prolific male child.
The Y chromosome is somewhat of a genetic island. 95% of the Y is unable to recombine with the X during meiosis. The Y is thus passed on virtually intact through the generations, and it is immediately apparent in newborn children whether the Y is present or not. A father with all female offspring will see his Y lineage disappear.
The X, on the other hand, is not similarly unique. The mother's two X chromosomes would have recombined during meiosis, and male children will also receive an X chromosome from the mother, avoiding the genetic dead-end of the Y chromosome in female offspring.
This Y chromosome "selfish gene" explanation thus avoids the criticism regarding the "units" of natural selection in that the genotype and the phenotype are essentially synonymous in this unique circumstance.
The "selfish Y" hypothesis is at least as credible as the original theory of evolutionary advantage, and has the added benefit of accounting for the discrepancy of preferences found between men and women.