If humans are so clever, why do we smoke and gamble, or take drugs, or fall in love? Is this really rational behaviour? And how come your idiot boss is so overpaid? In fact, the behaviour of even the unlikeliest of individuals – prostitutes, drug addicts, racists and revolutionaries – complies with economic logic, taking into account future costs and benefits, even if we don’t quite realise it. We are rational beings after all.