Wars, internal strife and malaria keep nations poor until most of them eventually graduate to middle-income status.
What starts then is the race to get rich, an arduous marathon that only a few countries have managed to finish.
While demographic forces determine the intensity of the initial growth spurt, the winners tend to be those that build stamina along the way by paying attention to institutions, infrastructure and fiscal policies.
India and China, the two most populous nations to have ever attempted the middle-income marathon, offer striking contrasts - not in the tempo of growth, but in their endurance.
The two nations were equally poor in 1978 when China ended its economic isolation - a decade before India chose to open up its economy.
Image: Prime Minister Manmohan Singh, right, with Chinese Premier Li Keqiang before a meeting in New Delhi in this file photo.
Text: Andy Mukherjee, Reuters