> Only about 20 percent of people in the middle fifth would rise into the top fifth over the course of their lives. In an equal system, the middle fifth would

> Only about 20 percent of people in the middle fifth would rise into the top fifth over the course of their lives. In an equal system, the middle fifth would be expected to migrate into any of the other four fifths equally. Which means exactly 20 percent of them would head into the top as often as 20 percent would head into the bottom. This is almost identical to the notorious Manager's Complaint: that 40% of sick days are Mondays and Fridays, insinuating employees aren't really sick because "so many" of them extend the weekend. But with a perfect random distribution of sickness (which is an acceptable assumption), precisely 20% would be on Monday and another 20% would be on Friday.