Lipset's most influential contribution is his systematic demonstration that democracy's stability depends on specific social and economic conditions—not simply on constitutions or elite pacts. In his 1959 article 'Some Social Requisites of Democracy' and *Political Man* (1960), he showed that wealthy, industrialized, and educated societies are more likely to sustain democracy because they generate larger middle classes, reduce inequality, create more pluralistic civil societies, and develop a political culture of tolerance and moderation. This 'modernization theory' became the dominant framework for understanding democratization for decades and remains a touchstone for contemporary empirical research.

Lipset argued that democratic stability depends on the structure of social divisions. When societal cleavages—class, religion, ethnicity, region—are 'cross-cutting,' individuals hold multiple, overlapping identities that moderate political conflict. A worker who is also a devout Catholic and a regional nationalist has cross-pressures that prevent any single identity from becoming totalizing. In contrast, 'reinforcing' cleavages, where the same people are poor, ethnic minorities, and religious outsiders, produce intense, zero-sum conflict that threatens democratic institutions. This insight has been foundational for understanding ethnic conflict, party system development, and democratic consolidation.

Lipset revived and systematized the Tocquevillian idea of American exceptionalism, defining it as a distinctive set of values—liberty, egalitarianism, individualism, populism, and laissez-faire—that he called 'the American creed.' He argued these values explain why the U.S. never developed a major socialist party, why its welfare state is more modest than Europe's, and why its politics are uniquely moralistic and anti-statist. Crucially, he treated exceptionalism as 'a double-edged sword': the same values that foster dynamism and innovation also produce higher crime rates, weaker social solidarity, and resistance to universal healthcare.

Moore and Skocpol focused on class structures and state breakdowns as drivers of regime change, emphasizing historical conjunctures and revolutionary moments. Lipset, by contrast, focused on the aggregate social and economic conditions that make democracy sustainable over time. He was less interested in the *transition* to democracy than in its *consolidation*—asking not 'how do revolutions happen?' but 'under what conditions do democracies survive?' His approach was quantitative, cross-national, and rooted in survey research and aggregate data, whereas Moore and Skocpol used comparative historical analysis of small numbers of cases.

Lipset argued that democracy's stability depends on widespread belief in its 'moral title to rule'—its legitimacy. A regime is legitimate when citizens accept its right to govern, even when they disagree with specific policies. Legitimacy is built through effective performance, especially early in a regime's life, and through mechanisms that moderate conflict intensity. When legitimacy erodes, even minor crises can trigger breakdown. This concept linked Lipset's work to Weberian sociology and distinguished his approach from purely institutional or economic explanations of democratic survival.

A common misconception is that Lipset argued economic development *causes* democracy in a deterministic, linear fashion. In fact, he was more nuanced: he argued that development creates the *conditions* that make democracy more likely—a larger middle class, higher education, more pluralistic civil society—but he did not claim that wealth guarantees democracy. He also emphasized that legitimacy and effective institutions are crucial mediating factors. Moreover, he recognized that poor countries can become democracies if they develop these associated characteristics. His theory is probabilistic, not deterministic, and he repeatedly revised it in light of new evidence.

Lipset's 1959 article, 'Some Social Requisites of Democracy,' was the seventh most-cited article in the century-long history of the *American Political Science Review*. It provided the first systematic, cross-national empirical test of the relationship between economic development and democracy, using data from Europe and Latin America. The article's core finding—that democracy correlates strongly with wealth, industrialization, education, and urbanization—has been replicated in hundreds of subsequent studies using more sophisticated methods. It remains the foundational text of modernization theory and the starting point for virtually all empirical research on democratization.