John Gerring, Brendan Apfeld, Tore Wig, and Andreas Tollefsen’s new book does an excellent job of presenting two new arguments specifying how geography has affected the development of democracy: first, that access to the sea fostered early and modern forms of democracy in general, and second, that it aided the spread of modern democratic institutions from Europe to the rest of the world.

According to the authors, the presence of natural harbors, in particular those with ocean access, had a series of positive implications for the development of democracy. First, such harbors made long-distance trade possible and allowed for the migration of people, technologies, and ideas. This fostered economic development and modernization more generally. Second, sea access favored navies over standing armies. Navies are of little use in crushing internal opposition, and they are often associated with traders and their political aims (i.e., democracy). Third, ocean states tend to be smaller, and if a state is extended across oceans, it is likely to govern its overseas territory via indirect rule. Being spread across large swatches of land made it harder for people to coordinate against the ruler, and it gives people fewer exit options. This makes rulers less likely to offer concessions to their subjects. Finally, port cities, often containing many diaspora communities, are open societies that tend to be more diverse than inland cities and the countryside. Exposure to diversity leads to greater acceptance of differences. In addition, rulers of port cities had to offer freedoms to attract new subjects and to keep their existing population. In short, the authors argue that harbors favor democracy over autocracy.

However, this is only the first part of the story that the book tells. The second part traces how European migrants brought modern democratic institutions to the places that they settled (initially only for themselves). This story is also related to harbors in two important ways. The uniquely widespread access to the ocean in Europe promoted the development of modern democracy. And once Europeans set out to settle and colonize other parts of the world, natural harbors were more likely to be discovered and used as staging points for migration.

The early impact of European settlement was extremely negative: Indigenous inhabitants were barred from political participation, denied civil and property rights, and often put into servitude or simply killed. In some places, however, democracy was extended to the Indigenous population over time. The European settlers themselves expected to enjoy political rights that were comparable to home institutions when abroad. These demands were most likely to foster actual democracy in areas with a higher proportion of Europeans for three reasons. When Europeans predominated, they had less reason to fear losing political control to the Indigenous population. More Europeans implied greater exposure to the idea of democracy via schools, churches, media, and contact with settlers. Areas with additional settlers also saw higher levels of modernization. Thus, a higher share of Europeans historically implies more democracy today.

This brief outline does not do justice to the book, which has a lot more to say about the specific ways that oceanic access and subsequent European settlement affected political (and economic) development. The Deep Roots of Modern Democracy is a compelling read because it not only breaks new theoretical ground but also presents rich and diverse empirical evidence. It provides persuasive and extensive statistical analyses that draw on global, national, and subnational evidence. Moreover, it carefully and systematically considers alternative explanations of democracy, and it contains qualitative evaluations of its arguments based on historical accounts of regime change from across the globe. In sum, the reader is convinced that the arguments rest on solid empirical ground.

However, as with any other academic work, it can be criticized. First, the relationship between the two arguments is not well integrated empirically. According to the authors’ causal models (pp. 6 and 233), areas outside Europe with natural harbors were more likely destinations for Europeans. But the book contains no attempt to correlate harbor distance (the preferred explanatory variable in models that test the first part of the argument) with the share of the population with European ancestry outside Europe (the preferred explanatory variable in models that test the second part of the argument). Second, as the authors themselves acknowledge (pp. 220–21), the argument—that areas where European settlement was easier saw additional institutional diffusion—was made before by, for instance, Daron Acemoglu, Simon Johnson, and James A. Robinson (“Colonial Origins of Comparative Development: An Empirical Investigation,” American Economic Review 91, 2002) and Jacob G. Hariri (“The Autocratic Legacy of Early Statehood,” American Political Science Review 106, 2012). Thus, this part of the book can perhaps be seen more as an elaboration of mechanisms present in earlier work. And the focus on how sea access and sea power favored democracy, of course, goes back to classical Greece, where it was forcefully formulated by Aristotle, who argued that the Athenian navy manned by the poor oarsmen was a force for political equality (see also Carles Boix, Political Order and Inequality: Their Foundations and their Consequences for Human Welfare, 2015). The Deep Roots of Modern Democracy, of course, presents a more developed argument subject to increased empirical scrutiny than these earlier arguments.

Finally, to mitigate issues of causal identification, the authors instrument the share of the population with European ancestry using the Indigenous population density in 1500 (p. 298). Yet, the exclusion restriction assumption is not likely to hold, because early agglomeration has been related to a reinforcing process of institutional development within Europe (e.g., Scott Abramson, and Carles Boix, “Endogenous Parliaments: The Domestic and International Roots of Long-Term Economic Growth and Executive Constraints,” International Organization 73 [4], 2019), or to state formation and technological advancement before colonization outside of Europe (e.g., Oana Borcan, Ola Olsson, and Louis Putterman, “State History and Economic Development: Evidence from Six Millennia,” Journal of Economic Growth 23, 2018). These processes are unlikely to be captured fully by the controls. However, the authors do recognize the difficulties of specifying the correct data generation process when the analysis spans so many centuries and areas of the world (see pp. 303–4).

These points of criticism are relatively minor. The Deep Roots of Modern Democracy is without a doubt a major contribution to our understanding of why countries manage to introduce democratic institutions. It shows that countries with access to natural harbors tend to be more open to trade, migration, technologies, and new ideas, thereby reaping benefits in the form of economic development and democratization. The book ends by looking forward. Openness and connections have made some parts of the world richer and more democratic than others. However, this inequality need not remain. As the authors note (p. 394), the impact of geographical differences has attenuated over time due to advances in logistics and communication. This might foster a convergence if future technical advances favor the ruled over the ruler.