“WHAT charity will give me the biggest bang for my buck?” asks Elie Hassenfeld. In 2007 the former hedge-fund manager co-founded GiveWell, a non-profit organisation set up to answer that question for the growing number of donors who want to know how much good their cash will do before deciding which charity to entrust it to. GiveWell researches charities active in fields where there is strong evidence that great good can be done for a modest cost, such as cutting the incidence of malaria and treating children for parasites. The most effective are published in a list of “best buys”.