To grasp the stakes of economic growth, start with the arithmetic of compounding.
Over two generations an economy growing at about 1% a year will not even double in size;
one growing at 7% will expand roughly 30-fold.
South Korea and Ghana show what such differences can mean.
In 1960 the two had similar incomes.
Today South Koreans browse Gangnam's luxury boutiques and sip lattes in Garosu-gil's trendy cafés,
while the typical Ghanaian still lives on less than $4 a day.
Even a few percentage points make an enormous difference over time:
fewer children die, people live longer, education spreads and daily life becomes more comfortable.
Little wonder that Robert Lucas, a Nobel-prizewinning economist, remarked that
once you start thinking about economic growth it is hard to think about anything else.
These days it is not difficult to think about other things.
Economists have played their part by drifting away from the question.
In the 1990s they tried to explain growth by comparing entire countries,
asking whether prosperity came from accumulating more educated workers and machines
or from something harder to measure.
The answer was usually "something else" (productivity, ideas, technology and so on).
That diagnosis was illuminating but not especially helpful.
By the early 2000s a backlash was under way.
Development economists turned away from nations and towards villages and households.