'Liberal peace' theories argue that there is a strong tendency for domestic and international peace to follow when the large majority of individuals in a society have control over decisions in both political and economic issues. The theories assume common people have a self-interest in peace, since they normally can obtain material and non-material well-being only during peace. Hence, peace may be secured if narrow groups and would-be elites can be restrained through effective political institutions. Interstate trade serves as an analogous constraint on state leaders.
The dissertation studies theoretically and empirically the empirical evidence for the liberal peace both for domestic and interstate conflicts. For interstate conflicts, the investigation supports the liberal peace hypotheses: pairs of states that are democratic and/or trade extensively, have a low risk of militarized conflict. However, the dissertation points out there are limits to the liberal peace: the evidence for it is clearly strongest in relations between developed countries, and that trade reduces conflict mainly in symmetrical dyads.
Parallel results are found for domestic conflicts. Overall, democracies widely defined are no less civil war-prone than non-democracies. However, political systems that are consistently democratic along several dimensions, such as the degree of participation, the extent of constraints on the executive, and the extent to which the executive is popularly elected, have less internal conflicts than systems that mix democratic and non-democratic traits. Likewise, consolidated democracies are more peaceful than newly established democracies.
The domestic democratic peace is also contingent on economic development. Democracies are normally more consistent and more consolidated and stable in middle- and high-income countries than in low-income countries, and are therefore more effective in maintaining domestic peace.
The importance of development for the liberal peace is due to several factors. The dissertation highlights two of these: First, education and the absence of poverty strengthens citizens’ ability to constrain rulers that might benefit from war. Second, the increased mobility of assets associated with economic development does not favor forces that seek to gain control over them through the use of physical force. This both favors democratization and reduces the incentives for using military force to conquer territory.