An estimated 5-6 million migrants are on the move every year in India, travelling from some of its poorer states to newly emerging urban zones, according to a report on migration in cities released by the World Economic Forum on Wednesday.

Citing a recent study undertaken as part of Economic Survey 2017, the report said the rate of inter-state migration doubled between 2001 and 2011.

India’s migration flows are driven by deep and persisting economic inequities between states. Bihar, for example, has a per capita income roughly equivalent to Somalia (approximately $520) and a birth rate of 3.4 children per woman. On the other hand, Kerala has a per capita income that is four times more (approximately $2,350) and a birth rate of 1.6 children per woman, which puts the state on par with Denmark, the WEF report stated.

However, India’s migration story is complicated because cities like Faridabad, Ludhiana and Surat have over 55% of migrants, whereas the rate in Agra and Allahabad is below 15%.

Globally, an estimated 244 million people are international migrants who move between countries. Internal migration is three times that number, affecting the lives of far more people, although it is given much less attention in political debates and planning processes, according to the report.