Democratic peace theory is a theory which posits that democracies are hesitant to engage in armed conflict with other identified democracies. In contrast to theories explaining war engagement, it is a "theory of peace" outlining motives that dissuade state-sponsored violence.
Some theorists prefer terms such as "mutual democratic pacifism" or "inter-democracy nonaggression hypothesis" so as to clarify that a state of peace is not singular to democracies, but rather that it is easily sustained between democratic nations.
Among proponents of the democratic peace theory, several factors are held as motivating peace between democratic states:
Those who dispute this theory often do so on grounds that it conflates correlation with causation, and that the academic definitions of 'democracy' and 'war' can be manipulated so as to manufacture an artificial trend.
Research on the democratic peace theory has to define "democracy" and "peace" (or, more often, "war").
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 (1976) 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 (1983) 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 (1995) 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 one-party rule, and thus would not be counted under this definition (Ray 1995, p. 100).
Rummel (1997) 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."
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 (Gleditsch 1992).
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 (Doyle 1983, Russett 1993, Rummel 1997, Weart 1998). Rummel (1997) 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 (2002, 2005), 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 (Braumoeller 2004) 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 (Ray 2003) 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 (Ray 1998), (Ray 2003), (Hegre 2004), while some do not (Schwartz & Skinner 2002).
Quantitative research on international wars usually define 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 (Ray 1995, p. 103).
Some researchers have used different definitions. For example, Weart (1998) defines war as more than 200 battle deaths. Russett (1993, p. 50), 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; so the difference between MIDs and MICs may be less than it appears.
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.
Most research is regarding the dyadic peace, that democracies do not fight one another. Very few researchers have supported the monadic peace, that democracies are more peaceful in general. There are some recent papers that find a slight monadic effect. Müller and Wolff (2004), in listing them, agree "that democracies on average might be slightly, but not strongly, less warlike than other states," but general "monadic explanations is neither necessary nor convincing." They note that democracies have varied greatly in their belligerence against non-democracies.