This, says Clark, suggests that there was complete social mobility, if we look at a long enough time-span.
How can we reconcile this with the fact - reported in a new paper (pdf) from the OECD - that the UK now has low social mobility?
One possibility is that there’s no inconsistency, just different time periods. Let’s say that the coefficient on father’s earnings of a son’s earnings is 0.5; this is consistent with Miles Corak’s estimate. In other words, if a man’s income is 100% more than the average, his son’s income will be 50% more. Then the coefficient over five generations will be 0.5 to the power five. Which means that the man’s great great great grandson will get only 3% more than the average. Over five generations, then, inherited advantage will, for all practical purposes, vanish.
There is, though, a more sinister possibility - and here I‘m speculating wildly. Today, a major determinant of someone’s economic success is their education. This, though, is highly heritable not only because rich parents get their kids into the best schools, but also because cognitive skills are partly inherited. However, in the past there were many other routes to wealth: predation, winning a lord’s favour, skill on the battlefield and so on. If these were less heritable than education, mobility might have been greater.
A further question is: why should we care if there is social mobility over four or five generations? Those who worry about a lack of social mobility do so because individuals’ life chances are blighted by the misfortune of who their parents happen to be. Is it really a consolation to these individuals that their as yet unborn great-grandchildren won’t suffer such disadvantage? Surely, we worry about injustice because of its effect upon existing individuals. If so, the claim that there is social mobility over the long-run is irrelevant.