If you start at the bottom there is a lot of up to rise into and no down to further sink. Same at the opposite end: all the nepotism and Ivy Leagues in the world won't be able to keep some billionaire spawn from sinking and there are no higher ranks to rise into. Their average mobility can only be negative. I guess I'd still prefer being a failing trust fund baby over the upward mobility of rags to better rags, despite a considerable delta in sense of achievement (rags to better rags might actually be happier with themselves).