Are night lights on earth captured by satellites from outer space a good way to measure inequality?
Economists Praveen Chakravarty and Vivek Dehejia certainly believe so. They acquired images grabbed by satellites from the US Air Force Defence Meteorological Satellite Programme. These satellites circle the earth 14 times a day and record lights from the earth's surface at night with sensors. They superimposed a map depicting India's districts on their images, allowing them to develop a unique data set of luminosity values, by district and over time.
Using data generated by the night lights, they studied of 387 of 640 districts in 12 states. These districts account for 85% of India's population and 80% of its GDP. Some 87% of parliamentary seats are in these districts. Using the novel methodology, the economists documented income divergence in India.
Most of India is dark at night because there is little economic activity going on. But the delicate tracery of lights as seen from space also showed that the states are becoming more unequal between and within them.
Why inequality in India is at its highest level in 92 years
Some 380 districts in 12 states were on average just a fifth as bright as the big cities of Mumbai and Bangalore.
Also, 90% of all the districts are just a third as bright in the night as the top 10% of all districts. And the ratio has worsened between 1992 - a year after India embraced economic reforms - and 2013.
While the pre-1991 years show a modest trend towards convergence of income between different states, the years after show widening divergence. By 2014, the economists found, the average person in the three richest states (Kerala, Tamil Nadu, Maharashtra) was three times as rich as the average person in the three poorest states (Bihar, Uttar Pradesh and Madhya Pradesh).
"What we find is that both across states and across districts with each state, this is a wide, and widening disparity in economic activity. No, it is not that the rich getting richer and the poor getting poorer, but that the poor are not getting richer fast enough to close the gap with the already rich," says Dr Dehejia, a senior fellow at the Mumbai-based think tank IDFC Institute. In other words, less developed states are falling behind the more prosperous states.
"This is a level of regional inequality unprecedented in large federal states in contemporary world economic history."
So why are disparities within India rising even as standards of living are going up? Some economists believe it could be due to poor governance and lack of adequate skills for jobs in poorer areas. But, in the end, it remains a puzzle.
The economists find this worrying. In a large and complex country like India, they warn, this could "easily lead to social disharmony as populous, poorer regions attempt to extract greater redistribution from less populous, richer regions - whether within or across states".