Ethical choices are rarely black or white and individuals are not very good at assessing the purity of their own motivations, writes the Economist's Bartleby. In a new book about behavioural biases, “You’re About To Make A Terrible Mistake”, Olivier Sibony of HEC business school in Paris writes that “as soon as there is any ambiguity about a judgment…we reason in a way that is selective enough to serve our interests and yet plausible enough to convince others (and ourselves) that we are not intentionally distorting the facts.”