IN 1381, a mob of angry Essex peasants revolted against the poll tax, and marched on London, destroying tax registers and records as they went. The Essex men wanted an end to their serfdom and the right to rent land at fourpence an acre. King Richard II, just 14 years old, bowed to their demands and the mob dispersed, although not before invading the Tower of London, trespassing on the royal bedchambers, and killing the Archbishop of Canterbury.

The conflict between mutinous masses and self-preserving elites is the theme of this ambitious, even audacious, book by Daron Acemoglu, of the Massachusetts Institute of Technology, and James Robinson, of Harvard University. Their aim is to figure out when such struggles result in democracy, and when that democracy endures. The peasants' revolt of 1381 was not such a case. Once the uprising had ebbed, the young monarch reneged on his promises to the Essex men, rounded up the surviving ringleaders and had them executed.

As this sorry episode shows, people power is fleeting; sullen discontent flares only rarely and briefly into forceful dissent. This is obviously a problem for rebels, who find it hard to sustain a revolt. But, as this book points out, it also poses a conundrum for the elites who might want to appease them.

It is a king's or dictator's prerogative to change his mind. As the sovereign, he can overrule everyone, including himself. Thus after an insurrection peters out, the sovereign cannot hold himself to any concessions he might have made when the rebels were at his bedroom door. Dissidents should anticipate this. If they do not want to meet the same fate as the Essex men of 1381, they should not settle for the ruler's sops, but push instead for outright revolution while they still can.

This dilemma sets up the book's main thesis: democracy is a solution to the elite's commitment problem. By agreeing to a peaceful extension of the franchise, the elites institutionalise their concessions, and the masses lock in their power before it drifts away. Institutions, such as universal suffrage, are harder to overturn than policies, such as rent of fourpence an acre. It is easy, all too easy, for the elite to renege on a promise to the people; rather harder for it to mount a coup against them.

Uncircumscribed authority can be a handicap. It makes it impossible for the sovereign to make a lasting concession even if he wants to. Such paradoxes are familiar to students of game theory, a subdiscipline of economics on which the authors draw heavily. Game theory sheds light on strategic encounters, in which each player's move must take account of the others'; it is a good way to explore the machinations and manoeuvres of politics.

The authors hope to convert readers to their method as well as their argument. The pace of the book suffers from this laudable pedagogical purpose. They tell the reader what they are going to say, say it, then tell the reader they've said it. Their technical apparatus, like scaffolding, is no doubt a great help in building their theory. But an awful lot of scaffolding is left out on show, which is a great help to anyone teaching this book, but a distraction for those who just want to admire the edifice.

And there is much to admire. True to their title, Messrs Acemoglu and Robinson offer a unified theory of both democracy and its opposite. Some Latin American countries have swung between the two with metronomic regularity. Argentines won universal male suffrage as early as 1912, but lost it to a coup in 1930. Democracy was restored in 1946, overthrown in 1955, re-introduced in 1973, subverted in 1976 and cemented, one hopes, in 1983.

In the authors' eyes, the demise of democracy is a near mirror image of the fall of dictatorship. Anxious incumbents try to buy off the military, much as Richard II sought to placate the Essex men. Chile's Salvador Allende, for example, raised army pay and benefits. But in the long run, a democracy cannot commit itself to serve the interests of anyone but the swing voters. The military elite know this, and take their chances when they can. Allende was duly deposed in 1973.

The book takes its title from “The Social Origins of Dictatorship and Democracy” by Barrington Moore, an American sociologist who died last month. The conclusion of that treatise has been summed up as: “No bourgeoisie, no democracy.” Messrs Acemoglu and Robinson agree that the middle class can be a midwife for democratic rule, not least because the elite is more willing to cede power to merchants than to the mob. They also agree with Moore that agrarian societies tend towards authoritarianism. Land is easier to expropriate than capital; and peasants are easier to repress than factory workers. Thus feudal lords fear democracy more than capitalists do; and have an easier time suppressing it.

Such bold generalisations and pithy dictums have fallen out of fashion since Moore wrote his classic in 1966. One more recent scholar counted no fewer than 27 different factors that are said to promote democracy. This book is entirely free of such intellectual indecision. The authors are brutal wielders of Occam's razor, and the 27 factors have been chopped down to a coherent handful. This may leave a lot out, but what historians bemoan as simplistic, economists tend to celebrate as parsimonious. According to two scholars cited in this book, even to look for a general theory of democratic reform requires great temerity. Happily, Messrs Acemoglu and Robinson have temerity in spades.