Why do so many countries so often wander far off the path of fiscal responsibility? Concern about budget deficits has become a burning political issue in the United States, has helped persuade the United Kingdom to enact stringent cuts despite a weak economy, and is the proximate cause of the Greek sovereign-debt crisis, which has grown to engulf the entire eurozone. Indeed, among industrialized countries, hardly a one is immune from fiscal woes.

Clearly, part of the blame lies with voters who don’t want to hear that budget discipline means cutting programs that matter to them, and with politicians who tell voters only what they want to hear. But another factor has attracted insufficient notice: systematically over-optimistic official forecasts. read more