Does anybody have some sources? (I'd guess it would be interesting to look at societies in great turmoil (e.g. England after 1066, French Revolution, Communist revolution in China) or societies with more equality of wealth than the US (e.g. Scandinavian countries). They may well have more mobility (at least in a purely technical sense).)
Sources: http://www.americanprogress.org/issues/2006/04/b1579981.html "By international standards, the United States has an unusually low level of intergenerational mobility: our parents’ income is highly predictive of our incomes as adults. Intergenerational mobility in the United States is lower than in France, Germany, Sweden, Canada, Finland, Norway and Denmark. Among high-income countries for which comparable estimates are available, only the United Kingdom had a lower rate of mobility than the United States.
From The Economist, http://www.brendan-nyhan.com/blog/2005/01/the_economist_o.ht... "would-be Horatio Algers are finding it no easier to climb from rags to riches, while the children of the privileged have a greater chance of staying at the top of the social heap."
As far as I can tell, that study measured mobility by quintile. If the United States has a wider variance in income, the quintiles will be much wider, hence more difficult to move from one quintile to another.
It seems reasonable to assume that the social welfare states of Western Europe and Canada have less variance in income distribution, and therefore it's easier to move from one quintile to another because the quintiles are smaller. Saying that there's greater mobility between quintile in Western Europe really only reflects the smaller variance in incomes in Western Europe.
To set up an accurate cross-national comparison, one would have to construct a unified set of incomes for all people across all the studies countries. Then measure mobility of individuals between quintiles of the unified data set. Only that type of design would control for the different variance found in different national-economic contexts.
Or, in other words, apparently everyone else in this thread apart from you. How is one supposed to tell that "we have insane mobility compared to any other society at any time in human history" was meant to mean everyone, worldwide?
Indeed, how can that possibly be true (what do you think is the mobility rate out of the poorest 10% of the world population?), or make sense (given that it requires that you regard, say, the US, China, Afghanistan and Burundi as parts of a single "society")?
(And, FWIW, I am still having grave difficulty believing that you really meant "the world" when you said "we".)
I do agree that measured by income the world has entered a golden age. But I do not know whether mobility in income quantiles is at its peak: You can have a lot of mobility at a very low level of income, too. And in the stone age people were probably more alike in income, so that quantiles were closer together and more easily shuffled just by random changes. (Not that this is a good or bad thing per se.)