It's an artifact of the 90s. At the time, double-y chromosome abnormalities where known and not well understood. One theory was that it may link to more aggressive behavior and criminal activity. The movie was attempting to be edgy and scientifically current. A few years later, it was discovered that double-y has no impact on a person. They are no more aggressive. And this is the first I'm hearing of them being taller! Anyway, who cares? I applaud the film for trying to be on top of things for the times. It's funny to me that the book bug hunt says double-x because that's a huge error, and would make them all female.