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Internal migrants, estimated to constitute about 30% of the population, contribute 10% to the country's GDP with employment having become the biggest reason behind migration, a UNESCO report has said.
The report considers internal migration as being a key factor behind prosperous cities, boosting economic activity and growth.
Citing various sources, it estimated that following Census 2011, the number of migrants may have increased to about 400 million from 309 million in 2001.
The report, however, says that most of the million-plus cities have recorded significant decline in population growth, hinting that they have become "less welcoming" to migrants.
Against the projected 400 million migrants in India, their global number was 740 million in 2009.
The report entitled 'Social Inclusion of Internal Migrants in India' says women form an overwhelming majority of migrants in the country - 70.7% as per Census 2001 and 80% according to NSSO (2007-08) data.
It said 91.3% women in rural areas and 60.8% in urban areas were migrants, putting such high numbers down to marriage.
About 30% of the migrants in the country belong to the 15-29 years age group.
The report says migrants are often victims of politics based on "vote banks along ethnic, linguistic and religious lines" and face political and administrative exclusion and discrimination.
Women migrants face a double discrimination for reasons particular to their gender, it adds.
Migration, it says, is an integral part of development and the rising contribution of cities to India's GDP would not be possible without them.
"Internal migrants contribute cheap labour for manufacturing and services and, in doing so, contribute to the national GDP, but this is not recognised. Far from being a drain and burden, they are in fact providing a subsidy," the report says.
"Migrants are looked upon as outsiders by the host administration and as a burden on systems and resources. Their right to the city is denied on the political defence of the 'sons of the soil' theory which aims to create vote banks along ethnic, linguistic and religious lines," it says.
The UNESCO report has pushed for promoting positive political discourse and avoiding a prejudiced negative portrayal of internal migrants.
