@oristarA,
If you consider aptitude for a certain ability as a curve from low to high with most people being in the middle, the "high end" refers to the very top, the most capable in the field. For example, if you were talking about the population of people who can run a mile in under four and a half minutes, there are a lot more men than women because men are stronger. Sumner was suggesting that inherent differences in men and women related to math and science meant that at the very top of the field there were many more men available. The controversial part is his assertion that men are
inherently better than women at math and science.