I’m open to the possibility that variables besides policy,
population, and science belong in a multiplicative growth model.  But labor
quality – the variable that Clark emphasizes in A Farewell to Alms – just
isn’t very important.  Sure, you can’t build a modern economy on
chimpanzee labor.  But the average human from the poorest parts of the
planet has high productivity as long as he works in the United States – a
point that Clark interestingly grants:

[T]he only policy the West could pursue that will ensure
gains for at least some of the poor of the Third World is to liberalize
immigration from these countries.  We know a good deal about the economic
consequences for migrants from the historical record of countries like Britain,
the United States, Canada, Australia, and New Zealand, which had large flows of
immigrants in the modern era.  That records shows that migrants, particularly
those from very-low-income countries, have been able to achieve enormous income
gains through migration.

It’s a devastating admission: If mere relocation
to the First World causes such massive increases in wages and productivity, how
can the First World’s superior labor quality be the key to its prosperity?