In retrospect, the ongoing revolt in Tunisia seems almost inevitable. There were so many reasons for anti-government street action: high youth unemployment (31 per cent), a corrupt ruling family (the president’s wife was widely known as “shopper-in-chief”), and extreme political repression (the World Bank says only Syria is harsher in the region). But hindsight is different from foresight.

Equity investors did not expect anything untoward. The Tunis stock market index rose 19 per cent in 2010, its eighth straight positive year. Last April, S&P hailed the “gradual pace” of political liberalisation. A triple-B (investment grade) rating reflected a system that “has yielded political and social stability”. In September, the World Bank wrote of “remarkable progress on equitable growth, fighting poverty and achieving good social indicators”.

The general failure to predict the Tunisian political tsunami could spring from inattention. The country is very small, lacking commodity wealth, and with a gross domestic product of only $40bn (about the same as Brussels). More likely, though, the experts missed the imminent collapse in Tunis for the same reason that similar upheavals, such as in Tehran and Moscow, came as a surprise: radical political change is unpredictable. Unpopular governments can stay in power for years despite good reasons to fall. They can also change and become more popular – or suddenly fail.