Nepal, April 16 -- It is predictable that India will start to grow at a much faster rate than China, simply because its workforce still has a lot of growing left to do

MANCHESTER - Last month, I wrote about the growing divide between economic theory and real-world economic conditions, and reminded readers that economics is still a social science, despite whatever loftier ambitions its practitioners may have. Nonetheless, when it comes to the specific question of what drives economic growth in the long term, one can still offer rigorous predictions by focusing on just two forces.

Specifically, if one knows how much a country's working-age population will grow (or shrink), and how much its productivity will increase, one can predict its f...