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General points
The theory is now widely studied with more than a hundred researchers having published many more articles. Many peer-reviewed studies mention in their introduction that most researchers accept the theory as an empirical fact, including many of those mentioned below.

Online reviews of the literature supporting the theory: A Lakatosian View of the Democratic Peace Research Program, Does Democracy Cause Peace?, Debate on the 'Democratic Peace', [http://poli.vub.ac.be/publi/orderbooks/myth/03Risse.pdf Democratic Peace – Warlike Democracies? A Social Constructivist Interpretation of the Liberal Argument]

Some fear that the the theory may used as an argument for wars against nondemocracies in order to spread democracy. However, studies, show that many attempts to spread democracy by force have eventually failed. Greater success in democratization has been achieved through diplomacy and support of internal democratic movements. Thus, the research may actually be an argument against such wars.

Evidence for less systematic violence between democracies
A review lists many studies finding no wars between liberal democracies and that this is statistically significant. Regarding possible exceptions, see User:Salix alba/History of conflict between democracies.

Militarized interstate disputes (MIDs) are lesser conflicts causing less than 1000 or even no battle deaths but including for example a military display of force. They also include the disputes later becoming wars. There have been more than 2000 MIDs since 1816, allowing more detailed statistical analyses than when looking at wars. A review lists many studies that have reported that democratic pairs of states are less likely to be involved in MIDs than other pairs of states. Another study finds that after both states have become democratic, there is a decreasing probability for MIDs within a year and this decreases almost to zero within five years. When examining the MIDs in more detail, one study finds that the inter-liberal disputes are less likely to involve third parties, the target of the hostility is less likely reciprocate, if the target reciprocates the response is usually proportional to the provocation, and the disputes are less likely to cause any loss of life. The most common action was "Seizure of Material or Personnel"

Many earlier papers found that democracies in general are as warlike as nondemocracies and only peaceful against other democracies. However, one paper notes that several recent papers have found democracies are slightly less involved in wars in general, initiate wars and MIDs less frequently than nondemocracies, and tend more frequently to seek negotiated resolutions. The tendency for this varies greatly between different democracies.

A review lists several studies showing the probability that disputes between states will be resolved peacefully is positively affected by the degree of democracy exhibited by the least democratic state involved in that dispute. Disputes between democratic states are significantly shorter than disputes involving at least one undemocratic state. Democratic states are more likely to be amenable to third party mediation when they are involved in disputes with each other.

In international crises that include the threat or use of military force, one study finds that if the parties are democracies, then relative military strength has no effect on who wins. This is different from when nondemocracies are involved. These results are the same also if the conflicting parties are formal allies. Similarly, a study of the behavior of states that joined ongoing militarized disputes reports that power is important only to autocracies: democracies do not seem to base their alignment on the power of the sides in the dispute.

A review lists several studies finding that democracies are more likely to ally with one another than with other states. Such alliances are likely to last longer than alliances involving nondemocracies.

One study argues that there is also a related but weaker peacefulness between oligarchies.

Evidence for less systematic violence in democracies
One study finds that the most democratic and the most authoritarian states have few civil wars, and intermediate regimes the most. The probability for a civil war is also increased by political change, regardless whether toward greater democracy or greater autocracy. Intermediate regimes continue to be the most prone to civil war, regardless of the time since the political change. In the long run, since intermediate regimes are less stable than autocracies, which in turn are less stable than democracies, durable democracy is the most probable end-point of the process of democratization.

One study finds that the most democratic nations have the least terrorism.

One study finds that genocide and politicide are rare in democracies. Another that democide is rare.

One study lists several other studies and states: "Repeatedly, democratic political systems have been found to decrease political bans, censorship, torture, disappearances and mass killing, doing so in a linear fashion across diverse measurements, methodologies, time periods, countries, and contexts." It concludes: "Across measures and methodological techniques, it is found that below a certain level, democracy has no impact on human rights violations, but above this level democracy influences repression in a negative and roughly linear manner." One study states that thirty years worth of statistical research has revealed that only two variables decrease human rights violations: political democracy and economic development. Of this democracy is more important and more easily created.

Evidence for that more participatory democracies have less systematic violence
One study finds that democracies with proportional representation are in general more peaceful regardless of the nature of the other party involved in a relationship.

Another study finds that proportional representation system and decentralized territorial autonomy is positively associated with lasting peace in postconflict societies.

Evidence for that it is democracy that causes the peace between democracies
Correlation is not causation. However, many studies, as those discussed in, , , supporting the theory have controlled for many possible alternative causes of the peace. Examples of factors controlled for are geographic distance, geographic contiguity, power status, alliance ties, militarization, economic wealth and economic growth, power ratio, and political stability. Several studies have also controlled for the possibility of reverse causality from peace to democracy ,,.

Several studies and reviews argue that the Realist criticisms, like that the external threat during the Cold War explain the peace, are flawed. This include the critical studies made by Layne, Spiro, Gowa, and Rosato. The critical realist studies are argued to have methodological problems and be contradicted by others which are better made. Also, an intuitive argument against the Cold War explanation is that such external threat did not prevent wars between Communist states, , ,.

One study finds and mentions several other studies finding that democracies conduct diplomacy differently and more conciliatory compared do nondemocracies.

The same study argues that the peacefulness appears and disappears rapidly when democracy appears and disappears. This makes it unlikely that variables that change more slowly are the explanation.

Possible ways in which democracy can cause peace between democracies
These theories have traditionally been categorized into two groups: explanations that focus on democratic norms and explanations that focus on democratic political structures.

One explanation belonging to the first group is that the democratic culture may make the leaders accustomed to negotiation and compromise. Another that a belief in human rights may make people in democracies reluctant to go to war, especially against other democracies. The decline in colonialism, also by democracies, may be related to a change in perception of non-European peoples and their rights.

One structural explanation notes that several studies show that democratic states are more likely than autocratic states to win the wars. One explanation is that democracies, for internal political and economic reasons, have greater resources. This might mean that democratic leaders are unlikely to select other democratic states as targets because they perceive them to be particularly formidable opponents. One study finds that interstate wars have important impacts on the fate of political regimes, and that the probability that a political leader will fall from power in the wake of a lost war is particularly high in democratic states.

As described in, several studies have argued that liberal leaders face institutionalized constraints that impede their capacity to mobilize the state’s resources for war without the consent of a broad spectrum of interests. Moreover, these constraints are readily apparent to other states and cannot be manipulated by leaders. Thus, democracies send credible signals to other states of an aversion to using force. These signals allow democratic states to avoid conflicts with one another, but they may attract aggression from nondemocratic states. Democracies may be pressured to respond to such aggression—perhaps even preemptively—through the use of force. As described in the same study, other studies have argued that when democratic leaders do choose to escalate international crises, their threats are taken as highly credible, since there must be a relatively large public opinion for these actions. In disputes between liberal states, the credibility of their bargaining signals allows them to negotiate a peaceful settlement before mobilization.

A game-theoretic explanation similar to the last two above is that the participation of the public and the open debate send clear and reliable information regarding the intentions of democracies to other states. In contrast, it is difficult to know the intentions of nondemocratic leaders, what effect concessions will have, and if promises will be kept. Thus there will be mistrust and unwillingness to make concessions if at least one of the parties in a dispute is a nondemocracy.

Several studies find that democracy, more trade causing greater economic interdependence, and membership in more intergovernmental organizations reduce the risk of war. This is often called the Kantian peace theory since it is similar to Kant's earlier theory about a perpetual peace. These variables positively affect each other but each has an independent pacifying effect. For example, democracy may empower economic interest groups that may be opposed to disruptive wars,. However, some recent studies find no effect from trade but only from democracy,.

Progressive research program
Imre Lakatos suggested that what he called a "progressive research program" is better than a "degenerative" when it is can explain the same phenomena as the "degenerative" one, but is also marked by growth and the discovery of important novel facts. In contrast, the supporters of the "degenerative" program do not make important new empirical discoveries, but instead mostly adjustments to their theory in order to defend it from competitors. On study argues that the democratic peace theory is now the "progressive" program in international relations. The theory can explain the empirical phenomena previously explained by the earlier dominant research program, realism in international relations. In addition, the initial discovery, that democracies do not make war on one another, has created a rapidly growing literature and a constantly growing list of novel empirical regularities, as noted above, ,.