Want poor families in north India to immunise their children? Offer the parents a small bag of lentils as an incentive and vaccination rates make a startling jump. Does charity always work? Poor Africans, for example, do not value the mosquito nets given free. Massachusetts Institute of Technology economists Esther Duflo and Abhijit Banerjee tell these stories in their new book Poor Economics. Anoo Bhuyan met them at the ThinkFest conference held in Goa in November. Edited excerpts of the conversation