A movement founded at the University of Oxford in 2009 has now captured the imagination – and the wallets – of some of the brightest and most successful across elite Western academic and business circles. Effective altruism, a 21st-century data-driven take on the philosophy of utilitarianism, claims we must give our time and money only to those causes which can be proven to increase the greatest amount of pleasure to the most people. Why has this eccentric community grown so fast, has it become unmoored from its original intentions, and what perverse incentives arise when we try to distil ethics into an algorithm?