This is a book review of the Economic Origins of Dictatorship and Democracy by Daron Acemoglu and James A. Robinson.
The authors' main thesis is that the interplay between elites and common citizens determines the transition from nondemocracy to democracy. Elites prefer nondemocracy, while citizens prefer democracy. Both elites and citizens want right (beneficial to each group) policies not only for today but also tomorrow (sustainability). Political institutions, which are durable today and tomorrow, can act as a negotiating mechanism and help in regulation and persistence of policies over time.
In nondemocracy, the elites have de jure political power and, if there are no checks on their power, the elites would generally choose the policies that are most beneficial to them (e.g. low taxes and tariffs and no redistribution to the poor or middle classes). However, sometimes, nondemocracy is challenged by the vast majority of citizens (e.g. through revolution the poor could have a temporary de facto political power). But, this power is transitory as the majority could have it today but unlikely to have it tomorrow. So the majority group could use this transitory power gained possibly through a revolution and try to change the system to their benefit (today and tomorrow) creating major losses to the elites along with significant collateral losses to the society as a whole. The elites in order to prevent such a revolutionary situation could make (empty) promises such as pro-majority policies would be followed in the future but within the existing political system (these are often not credible). To make them credible, some power is transferred to the majority, which is how democratization begins in fits and starts and takes time.
The reason that the transition does not generally take place (e.g. Argentina) is that the elites controlling the current political system do not want to create political institutions/mechanisms that extend voting rights to the citizens or transfer political and economic power to the citizens. Democratic transition will not occur unless there is a threat of revolution (implying that citizens need to be organized, otherwise the transition is delayed indefinitely). The strength and nature of civil society organizations is important to the creation and consolidation of democracy.
Later in the book, a third group, the middle class, who are materially more comfortable and educated, is introduced as a driver of change during transition of power from the elites to the great mass of citizens. The middle class acts as a buffer between two extremes, ensure consolidation of democracy by limiting redistribution (e.g. Costa Rica and Colombia). The lack of middle class is said to be a problem in El Salvador and Guatemala, where democracy is not being consolidation.