Though the democratic peace theory was not rigorously or scientifically studied until the 1960s, the basic principles of the concept had been argued as early as the 1700s in the works of philosopher Immanuel Kant and political theorist Thomas Paine. Kant foreshadowed the theory in his essay Perpetual Peace: A Philosophical Sketch written in 1795, although he thought that a world with only constitutional republics was only one of several necessary conditions for a perpetual peace. Kant's theory was that a majority of the people would never vote to go to war, unless in self-defense. Therefore, if all nations were republics, it would end war, because there would be no aggressors. In earlier but less cited works, Thomas Paine made similar or stronger claims about the peaceful nature of republics. Paine wrote in "Common Sense" in 1776: "The Republics of Europe are all (and we may say always) in peace." Paine argued that kings would go to war out of pride in situations where republics would not (Levy & Thompson 2011; Paine 1945, p. 27). French historian and social scientist Alexis de Tocqueville also argued, in Democracy in America (1835–1840), that democratic nations were less likely to wage war.[b]
Dean Babst, a criminologist, was the first to do statistical research on this topic. His academic paper supporting the theory was published in 1964 in Wisconsin Sociologist (Babst 1964); he published a slightly more popularized version, in 1972, in the trade journal Industrial Research (Babst 1972). Both versions initially received little attention.
Melvin Small and J. David Singer (1976, pp. 50–69) responded; they found an absence of wars between democratic states with two "marginal exceptions", but denied that this pattern had statistical significance. This paper was published in the Jerusalem Journal of International Relations which finally brought more widespread attention to the theory, and started the academic debate. A 1983 paper by political scientist Michael W. Doyle contributed further to popularizing the theory. Rudolph J. Rummel was another early researcher and drew considerable lay attention to the subject in his later works.
Maoz and Abdolali (1989) extended the research to lesser conflicts than wars. Bremer (1992) and Maoz and Russett (1993) found the correlation between democracy and peacefulness remained significant after controlling for many possible confounding variables. This moved the theory into the mainstream of social science. Supporters of realism in international relations and others responded by raising many new objections. Other researchers attempted more systematic explanations of how democracy might cause peace (Köchler 1995), and of how democracy might also affect other aspects of foreign relations such as alliances and collaboration (Ray 2003).
There have been numerous further studies in the field since these pioneering works.[c] Most studies have found some form of democratic peace exists, although neither methodological disputes nor doubtful cases are entirely resolved (Kinsella 2005).