Proponents of democratic peace theory argue that electoral democracies are hesitant to engage in armed conflict with other identified democracies. Different advocates of this theory suggest that several factors are responsible for motivating peace between democratic states. Individual theorists maintain "monadic" forms of this theory (democracies are in general more peaceful in their international relations); "dyadic" forms of this theory (democracies do not go to war with other democracies); and "systemic" forms of this theory (more democratic states in the international system makes the international system more peaceful).
In terms of norms and identities, it is hypothesized that democracies are more dovish in their interactions with other democracies, and that democratically elected leaders are more likely to resort to peaceful resolution in disputes (both in domestic politics and international politics). In terms of structural or institutional constraints, it is hypothesized that institutional checks and balances, accountability of leaders to the public, and larger winning coalitions make it harder for democratic leaders to go to war unless there are clearly favorable ratio of benefits to costs.
These structural constraints, along with the transparent nature of democratic politics, make it harder for democratic leaders to mobilize for war and initiate surprise attacks, which reduces fear and inadvertent escalation to war. The transparent nature of democratic political systems, as well as deliberative debates (involving opposition parties, the media, experts, and bureaucrats), make it easier for democratic states to credibly signal their intentions. The concept of audience costs entails that threats issued by democratic leaders are taken more seriously because democratic leaders will be electorally punished by their citizens from backing down from threats, which reduces the risk of misperception and miscalculation by states.
The connection between peace and democracy has long been recognized, but theorists disagree about the direction of causality. The democratic peace theory posits that democracy causes peace, while the territorial peace theory makes the opposite claim that peace causes democracy. Other theories argue that omitted variables explain the correlation better than democratic peace theory. Alternative explanations for the correlation of peace among democracies include arguments revolving around institutions, commerce, interdependence, alliances, US world dominance and political stability. There are instances in the historical record that serve as exceptions to the democratic peace theory.
History
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 18th century 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. 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. 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. Herbert Spencer also argued for a relationship between democracy and peace. At the Geneva Congress of the League of Peace and Freedom in 1867, Giuseppe Garibaldi declared that "democracy alone can provide a guarantee against the evils of war".
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; he published a slightly more popularized version, in 1972, in the trade journal Industrial Research. Both versions initially received little attention.
Melvin Small and J. David Singer 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.
Maoz and Abdolali extended the research to lesser conflicts than wars. Bremer, Maoz and Russett 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, and of how democracy might also affect other aspects of foreign relations such as alliances and collaboration.
There have been numerous further studies in the field since these pioneering works. Most studies have found some form of democratic peace exists, although neither methodological disputes nor doubtful cases are entirely resolved.
Definitions
Research on the democratic peace theory has to define "democracy" and "peace" (or, more often, "war").
Defining democracy Democracies have been defined differently by different theorists and researchers; this accounts for some of the variations in their findings. Some examples:
Small and Singer define democracy as a nation that (1) holds periodic elections in which the opposition parties are as free to run as government parties, (2) allows at least 10% of the adult population to vote, and (3) has a parliament that either controls or enjoys parity with the executive branch of the government.
Doyle requires (1) that "liberal regimes" have market or private property economics, (2) they have policies that are internally sovereign, (3) they have citizens with juridical rights, and (4) they have representative governments. Either 30% of the adult males were able to vote or it was possible for every man to acquire voting rights as by attaining enough property. He allows greater power to hereditary monarchs than other researchers; for example, he counts the rule of Louis-Philippe of France as a liberal regime.
Ray requires that at least 50% of the adult population is allowed to vote and that there has been at least one peaceful, constitutional transfer of executive power from one independent political party to another by means of an election. This definition excludes long periods often viewed as democratic. For example, the United States until 1800, India from independence until 1979, and Japan until 1993 were all under a dominant-party system, and thus would not be counted under this definition.
Rummel states that "By democracy is meant liberal democracy, where those who hold power are elected in competitive elections with a secret ballot and wide franchise (loosely understood as including at least 2/3 of adult males); where there is freedom of speech, religion, and organization; and a constitutional framework of law to which the government is subordinate and that guarantees equal rights."
Non-binary classifications The above definitions are binary, classifying nations into either democracies or non-democracies. Many researchers have instead used more finely grained scales. One example is the Polity data series which scores each state on two scales, one for democracy and one for autocracy, for each year since 1800; as well as several others. The use of the Polity Data has varied. Some researchers have done correlations between the democracy scale and belligerence; others have treated it as a binary classification by (as its maker does) calling all states with a high democracy score and a low autocracy score democracies; yet others have used the difference of the two scores, sometimes again making this into a binary classification.
Young democracies Several researchers have observed that many of the possible exceptions to the democratic peace have occurred when at least one of the involved democracies was very young. Many of them have therefore added a qualifier, typically stating that the peacefulness apply to democracies older than three years. Rummel argues that this is enough time for "democratic procedures to be accepted, and democratic culture to settle in." Additionally, this may allow for other states to actually come to the recognition of the state as a democracy.
Mansfield and Snyder, while agreeing that there have been no wars between mature liberal democracies, state that countries in transition to democracy are especially likely to be involved in wars. They find that democratizing countries are even more warlike than stable democracies, stable autocracies or even countries in transition towards autocracy. So, they suggest caution in eliminating these wars from the analysis, because this might hide a negative aspect of the process of democratization. A reanalysis of the earlier study's statistical results emphasizes that the above relationship between democratization and war can only be said to hold for those democratizing countries where the executive lacks sufficient power, independence, and institutional strength. A review cites several other studies finding that the increase in the risk of war in democratizing countries happens only if many or most of the surrounding nations are undemocratic. If wars between young democracies are included in the analysis, several studies and reviews still find enough evidence supporting the stronger claim that all democracies, whether young or established, go into war with one another less frequently; while some do not.
Defining war Quantitative research on international wars usually defines war as a military conflict with more than 1000 killed in battle in one year. This is the definition used in the Correlates of War Project which has also supplied the data for many studies on war. It turns out that most of the military conflicts in question fall clearly above or below this threshold.
Some researchers have used different definitions. For example, Weart defines war as more than 200 battle deaths. Russett, when looking at Ancient Greece, only requires some real battle engagement, involving on both sides forces under state authorization.
Militarized Interstate Disputes (MIDs), in the Correlates of War Project classification, are lesser conflicts than wars. Such a conflict may be no more than military display of force with no battle deaths. MIDs and wars together are "militarized interstate conflicts" or MICs. MIDs include the conflicts that precede a war; the difference between MIDs and MICs may thus be less than it appears, though this remains an area of investigation.Douglas M. Gibler, Steven V. Miller, Erin K. Little, "An Analysis of the Militarized Interstate Dispute (MID) Dataset, 1816-2001: Research Note" International Studies Quarterly, Vol. 60, No. 4 (December 2016), pp. 719-730
Statistical analysis and concerns about degrees of freedom are the primary reasons for using MID's instead of actual wars. Wars are relatively rare. An average ratio of 30 MIDs to one war provides a richer statistical environment for analysis.
Monadic vs. dyadic peace