There is a story that is commonly told in Britain that the colonisation of India – as horrible as it may have been – was not of any major economic benefit to Britain itself. If anything, the administration of India was a cost to Britain. So the fact that the empire was sustained for so long – the story goes – was a gesture of Britain’s benevolence.

New research by the renowned economist Utsa Patnaik – just published by Columbia University Press – deals a crushing blow to this narrative. Drawing on nearly two centuries of detailed data on tax and trade, Patnaik calculated that Britain drained a total of nearly $45 trillion from India during the period 1765 to 1938.

It happened through the trade system. Prior to the colonial period, Britain bought goods like textiles and rice from Indian producers and paid for them in the normal way – mostly with silver – as they did with any other country. But something changed in 1765, shortly after the East India Company took control of the subcontinent and established a monopoly over Indian trade.

Here’s how it worked.