It is an old question: why are some countries rich and others poor? Sir Partha Dasgupta, a hugely accomplished economist, born in Dhaka and educated in Delhi and Cambridge, is as well qualified as anyone to come up with an answer – which he did, delivering this year's Royal Economic Society public lecture.

Dasgupta began by inviting his audience, many of whom were A-level students, to consider the lives of two girls – Becky, an American, and Desta, an Ethiopian.

Becky lives in a country with a gross domestic product per head of $46,000, life expectancy of 78 years and near-universal adult literacy. GDP per head in Desta's country is $780; life expectancy is 53 years, adult literacy 36 per cent, and most women spend about 15 years bearing or taking care of children, with average fertility of more than five live births per woman.