Although the authors of the article leave a bit of room for attributing a lack of upward mobility to circumstantial disadvantage, you seem to be operating from an almost eugenicist point-of-view with the assumption that position is almost entirely based on inherited ability.

Those studies don't prove anything but I believe that culture and circumstance play a large part in determining mobility.

The funny thing is that in ten or twenty years when artificial super-intelligence arrives, everyone, even rich intellectuals, will be, relatively speaking, mentally challenged, compared to the AIs. Please review this video to get a good idea of what I am talking about: http://www.youtube.com/watch?v=I_Juh7Xh_70 (Unless we augment our intelligence with implanted technology of some sort.)