According to an unattributed, but very professional-sounding, quote from a commenter on the story (who is both trustworthy and determined, as evidenced by his/her screen name: "Frodo Baggins"):
"The authors looked at some of the cultural factors that might be expected to explain the difference between the two societies. Males are likely to receive more education in the patrilineal society, and the authors found that introducing education as a factor in their analysis accounted for a third of the difference. Male ownership of the home also had a large effect; the gender gap is only a third the size in homes that are not owned solely by males."
But why would girls in the West have spacial reasoning skills less impressive than than those of the Western boys? They have access to education and are allowed to own their own homes and inherit property.
One possible explanation for the gap can be found in the t-shirt recently discontinued by J.C. Penny because of a totally unpredictable backlash from consumers:
Do not despair, though. I firmly believe the gender gap will be eradicated not by helping girls develop news skills, but by taking away the ones the boys already have. Another commenter on the "The Mary Sue" website led me to the following page, which details the exciting bibs now available for both boy and girl babies. While it's true that the "high chair hottie" bib for the discriminating girl baby might seem to stress the importance of her looks, the "flirt", "hunk", and "single" bibs for boys do much the same thing for him. Soon both girls AND boys will be too pretty to do homework, and true equality between the genders will be achieved.