Leaning against this force, however, is migration. Right now, the richest places are not the most populous.
Should it become relatively easy to migrate, people will move from countries that are populous but poor to others that are rich.
As migration swells the population of rich places, their long-run dominance is assured because of the link between population size and innovation.
But if there is very little migration, then the populous but poor countries will out-innovate the small but rich ones,
and make their way up the income league table. The process is not quick; the authors reckon that convergence takes about 400 years.
In practice, rich places tend not to allow much migration from poor ones.
That could change, but assuming that it does not, the model delivers a striking forecast: half a millennium from now,
Asia and sub-Saharan Africa will have become great engines of productivity.
Stranger things have happened. A millennium ago real output per person was significantly higher in China than in Britain.
To predict that a European backwater would lead the world into the most transformative economic epoch in history would have seemed like madness.
Over very long time horizons the world's poorest places can indeed become the world's richest, even if it does not happen often.
Still, if Britain did not have the upper hand over China 1,000 years ago, it did soon after, at least in terms of real output per person.
By 1400 incomes in Britain were meaningfully larger than in China (and higher still in the Netherlands and Italy),
according to work by Stephen Broadberry of Oxford University,
Hanhui Guan of Peking University and David Daokui Li of Tsinghua University.
By 1700 the diverging trajectories of China and north-west Europe were clear
(though it was anything but obvious just how much further apart they would become).
In other words, population over the past millennium has not been destiny.
If China's and India's masses did not raise them to prosperity during the past 600 years, what reason is there to believe the future will be different?