Amid the non-stop political fireworks over border control and migration policy, we pay surprisingly little attention to the global economic forces that drive the mass movement of people.

The sheer scale of wealth and income inequality is fundamental. All the talk about the stellar economic growth rates in some emerging economies, and the rapid formation of a global middle class has distracted some attention from the huge disparities that still exist between rich and poor countries.

Illustration: Kerrie Leishman

World Bank economist Branko Milanovic points out that the difference in gross domestic product per person between rich nations such as the US and Australia, and some of the poor countries of Africa, is about 50 to one. That's up from a ratio of 10 to one in 1960. Even in fast-growing Asian countries such as India, Bangladesh and Indonesia, hundreds of millions of people have not benefited much from the economic changes.

A recent report by investment bank UBS that compared prices and earnings across the world showed the real hourly wage rate of a bus driver (adjusted for the cost of living) was about $US20 in Sydney but only $US3 in India's biggest city, Mumbai. The same report found the weighted hourly wage for 15 common professions in Sydney was 12 times higher than in Jakarta. The gulf in average incomes is the main motivation for migration.