>>Yell, yes but for upward social mobility either people must also go down... or you need ever greater number of poor immigrants to undertake the work of the bottom quintile.<<
False.
The bottom "quintile" means nothing more than "that equal fifth of the population whose wealth is lowest". There is absolutely no requirement that anybody move down at all.
In fact what you see in developing countries is the definition of lowest wealth quintile move steadily upwards. Whereas in 1810 that bottom fifth might have made 0-$25000 adjusted for inflation, today it might hit $30,000 for a family of four.
My point in posting it is to show that the United States enjoys --or has until now, enjoyed - a broad freedom of citizens to define their own circumstances.
Wealth is different of course to the movement of money. I could make 1 million dollars a year, rent a car, rent a house, own just 2 suits, and spend all my money in restaurants, on holidays, on dating women, etc. I'd make a million but have no wealth. The 40% who have just 0.3% of the wealth don't all make a million bucks a year, that's pretty certain. However, just because they have no 'wealth' it doesn't mean they are cash poor - just asset limited.
>>The 1968 model can be explained quite sensibly. I am 25. I have just finished my degree and have just started. $10000 job. By 1978 I am 35 and make $25000 and part own my home. By 1988 I am on $50000 have paid my mortgage and am about to buy a Lexus. By 1998 I am 55, make $90000 and have an investment portfolio.
It isn't actually modelling social mobility really. The study is a fail unless you take only 1 year holds and record where they end up.<<
Possibly...but what's the complaint against it, again? If it's possible to do that, why worrya bout where you start?
The core joke of Hitchhiker's Guide to the Galaxy is that of course no civilization would develop personal computers with instant remote database recovery, and then waste this technology to find good drinks.
Steve Jobs has ruined this joke.