Moore's central thesis is that political outcomes—whether a country develops democracy, fascism, or communism—are determined by the class relationships that emerge during the transition from agrarian to industrial society. He famously argued that 'no bourgeoisie, no democracy,' meaning that a strong, independent commercial class (bourgeoisie) is a necessary condition for democratic development. Where the bourgeoisie was weak or co-opted by the landed aristocracy, fascism or authoritarianism resulted; where the peasantry was revolutionary, communism emerged. This class-configurational framework challenged both Marxist determinism and the modernization theory that assumed all countries would eventually converge on liberal democracy.

Moore identifies three historical routes to modern political order. The first is the bourgeois-democratic route, exemplified by England, France, and the United States, where the bourgeoisie gained power through revolutions, balanced with landed interests, and established liberal democracy. The second is the fascist route, seen in Germany and Japan, where the landed aristocracy and bourgeoisie formed a coalition to suppress the working class and peasantry, producing authoritarian nationalism. The third is the communist route, as in Russia and China, where a weak bourgeoisie and a revolutionary peasantry led to a state-led, anti-capitalist transformation. Each route produced distinct political institutions.

Moore departed from orthodox Marxism by rejecting economic determinism: he argued that class relations are shaped by political and historical contingencies, not just by the mode of production. He also emphasized the autonomous role of the state and the peasantry, which Marx had largely ignored. At the same time, he rejected modernization theory's assumption that economic development inevitably leads to democracy, showing that industrialization can produce fascism or communism depending on class configurations. His approach was historically specific, comparative, and sensitive to the role of political struggle in shaping developmental outcomes.

Moore treated the peasantry as a central revolutionary actor, countering the Marxist emphasis on the industrial proletariat. He argued that peasants could be revolutionary when they faced economic exploitation and political oppression from the landlord class, and when the state was weak or collapsing. In Russia and China, peasant revolts were key to the communist revolutions. In France, peasants played a crucial role in the bourgeois revolution. Where peasants were successfully co-opted or suppressed—as in Germany and Japan—they did not generate revolution and instead contributed to fascist outcomes. Moore's focus on agrarian politics was a major departure from urban-centered theories.

Moore's method in *Social Origins* became a model for comparative historical analysis. He compared six countries (England, France, the US, Germany, Japan, Russia, and China) to identify patterns of causation, treating each case as a distinct historical trajectory. He combined narrative historical description with systematic comparison, showing that small-N studies could yield powerful causal generalizations. His work legitimized historical case-study research in comparative politics and inspired a generation of scholars, including Theda Skocpol, who extended his state-centric approach to revolutions. His method remains a cornerstone of historical-institutionalist research.

Moore's insistence that democracy is not inevitable but contingent on class relations has been validated by the uneven history of democratization. His emphasis on the bourgeoisie as a democratic agent has been supported by many studies. However, critics argue he underestimates the role of the state as an autonomous actor (a gap Skocpol later addressed). His framework is also Eurocentric and struggles with post-colonial cases where class structures were shaped by imperialism. Moreover, his analysis of Germany has been challenged by historians who emphasize longer-term cultural factors. Nonetheless, his core insight—that agrarian class relations shape political futures—remains foundational for comparative political development.

A common misconception is that Moore is a deterministic Marxist, when in fact he explicitly rejected economic determinism. He argued that political outcomes are shaped by historical struggles and contingent alliances, not by inevitable economic laws. Another misconception is that he believed the bourgeoisie is the *sole* agent of democracy; he actually argued it was necessary but not sufficient—peasants, state structures, and international pressures also mattered. He also did not argue that democracy was the only desirable outcome; he analyzed all three paths objectively, without prescribing a preferred route. His work is more nuanced than the 'bourgeoisie thesis' caricature suggests.